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AVO CT160A Operating Instructions and Service Manual plus Inner

1. GL I E et ed d m ed e b 5 U OO J 4 GIL 200 3 Part No 5745 W39 14266 6 14268 4 15066 2 14269 4 20245 52 14267 1 40653 1 12049 5386 20908 11 40537 2 40538 A 12730 2 30006 W 14384 3 21124 D 10194 10184 10191 4 10158 4 10075 16 KNOB ASSEMBLY 15220 Description Cct Ref Special Screw Spring Washer Knob Carrier Knob Skirt Knob Washer Retaining Nut Spring Dowel Knob LAMP BOARD ASSEMBLY 20908 A Lamp 200V 15W RED ILP2 Resistor lOkn 1 HSC R9 Lamp Board Tagged MOVEMENT ASSEMBLY 40650 B Moulded Front Cover Moulded Case Rear Window Glass Swamp Bobbin Scale Plate Moving Coil Assembly Fixed Jewel Assembly Sprung Jewel Assembly Zero Adjuster Pivot Hair Spring MAIN INSTRUMENT VALVE PANEL ASSEMBLY 40947 40140 9 40140 13 40140 3 40140 4 40140 20 40140 12 40140 6 40140 7 40140 1 40140 14 40140 11 40140 10 40140 19 40140 16 40140 5 40140 8 40140 2 10281 1 10509 1 40166 3 14651 1 40157 C 20903 A 20968 1 14714 A 14264 2 40948 2 14745 1 11310 B 14732 2 Valve Holder UX6 Valve Holder Hivac 5 pin Valve Holder American UX7 Large Valve Holder American IX4 Valve Holder American UX7 Small Valve Holder Hivac 4 pin V
2. O C Q gt Rr Q 6 Q L Q lt gt c C C E FIG 2 BASIC CIRCUIT FOR THE TESTING OF RECTIFIERS AND SIGNAL DIODES 15 Figure 2 shows the basic circuit of the rectifier test The rectifier is loaded with a resistor RS RL and a reservoir capacitor C in parallel Sinusoidal voltage Va is applied of sufficient magnitude to operate the rectifier on the linear portion of its characteristic so that the combination should pass a rectified current equal to the maximum 10 load current for the valve The millivoltmeter M measures the voltage developed across a proportion of the load RL and is scaled by the appropriate range resistor RT so that the rated current through the load will deflect M to the middle of the good zone of the scale The proportionate deflection on the coloured scale denotes the state of the valve as before Switching of anode voltage meter range and load is ganged so that rectified currents of mA 5 mA 15 mA 30 mA 60 mA 120 mA and 180 mA per anode are available each anode of a full wave rectifier being tested separately mA and 5 mA ranges are suitable for signal diode testing 16 For the checking of inter electrode insulation figure 4 the unidirectional grid voltage Vg is applied through the meter M suitably loaded by a shunt s and high series resistor R across the electrode groups between which the insulation is to be measured
3. 2 01 D2 ELECTRODE SELECTOR SH CATHODE H H Valve Tester type 160A Circuit Diagram R 6 41 29 1 28 37 39 27 2 5 4 23 26 22 3 25 21 38 20 24 7 8 19 18 17 16 159 14 35 3411 33 10 32 31 30 401213 36 S SA SK1 SB SESD 5 1 511 SF 812 5 2 SJ3 5 4 SG1 SG6 SC 5 5 SH1 SH6 S MISC Rita FS2 RL1 ILP2 FS1 SKT1 11 12 D4 RL2 D3 RV5 RV3 D1 RV2 RV1 RV4 RL3 D2 RV6 C2 C1 M1 C3 MISC Schematic redrawn by SJ15 o Martin Forsberg Sweden 2010 08 25 SET la amp DR 5 1 d oc cecus gt o 5 o a 9600 12 625 117 fo HEATER VOLTS SD R29 R28 R27 R23 R22 R21 R20 R19 R18 R17 R16 R15 GRID1 SCREEN R36 ps CLOSED Vg X1 5 Va OPEN Vg X2 DIODE 4 SCREEN VOLTS SE DIODE 2 Cm UA NEST ANODE1 HICONTA 45 23796 5 ANODE 2 MAINS INPUT CIRCUIT SELECTOR SG 105 250 50 500Hz EN 02 R13 ELECTRODE SELECTOR SH CATHODE H H Valve Tester type 160A Circuit Diagram Redrawn R 6 4 S SA SK1 SB SESD 811 811 MISC Rita FS2 RL1 ILP2 FS1 SKT1 D4 RL2 D3 RV5 RV3 D1 11 72 Schematic redrawn corrected amp modified by Martin Forsberg Sweden 2010 08 25 List of changes D2 Changed from 66V RMS winding to 99V RMS winding R4 amp R41 Changed places in schematic 1 R37 R37 shown as R37A and R37B as per component list consisting of one 13 Ohm selected resistor each SH6 Ground connection for tags 2 5 WIRES
4. CT160 Euan has helped me with proof reading of the text and also supplied indepth information about components and mathematics It is quite long but really hope that you find it worthwhile to read through to the end There is more to come in the future with a look at the Dutch Military special edition of the AVO CT160 named AVO CT160A The AVO CT160A might have been used by others too but it has been referenced to as the Dutch Military versions in a few places and have found no other mentions of it Part 1 Anode Current Controls and replacing the CV140 valves with Silicon diodes In the AVO CT160 the anode current is measured by means of a Potentiometer which is a laboratory instrument for the precision measurement of an unknown voltage If you did Physics in the sixth form at school you would have encountered the Potentiometer in its simplest form it is comprised of a 1m length of Nichrome wire alongside a 1m wooden ruler driven by a 2V lead acid cell and calibrated using a galvanometer in series with Weston Standard cell to detect the null It is a common misconception that the CT160 is a Bridge it is not because a Bridge has four arms whereas the Potentiometer only has three In the CT160 the anode current flows through a 2000 sensing resistor the voltage drop produced across this resistor is then compared to a known voltage which is developed in a separate circuit comprised of a constant current flowing through nine switch
5. 28 33 34 36 CHAPTER 3 55 56 57 59 60 61 62 63 69 70 71 THE AVO gt VALVE TESTER TYPE 160 CONTENTS TECHNICAL DESCRIPTION Introduction Principles of operation Basic Circuits The Valve Holder Panel General Construction Mains Supply THE VALVE PANEL AND CONTROL UNIT The Valve Panel and Selector Switch Procedure for setting up Valve Base Connections Provision for new Valve Bases 2 The Control Unit and its Function The Mains Voltage Selector The Circuit Selector The Electrode Selector The Heater Voltage Switches The Anode and Screen Voltage Switches The Anode Current Controls The Negative Grid Volts Control The Set mA V Control Page OPERATING INSTRUCTIONS GENERAL PROCEDURE TOR 1 TESTING The connection of the instrument to a supply voltage Final setting of Mains Voltage Selector Panel Insulation checks with the valve cold Insulation checks with the valve hot Cathode to Heater insulation check Determination of valve condition from Static Characteristic Data relative goodness of valve with coloured comparison scale An 5 a Using recommended anode current A 5 b Using recommended negative grid voltage To check valve by direct reading of mutual mA V a Using recommended anode current 6 Using recommended negative grid voltage To check valves having a mutual conductance less than ima V Measu
6. Jcs screws see Fig 3 It should be noted that the bobbins if replaced should be positioned such that the flux which they produce is additive Operational Limits are as follows A Anode Overload Relay should operate on 100V short circuit B Screen Overload Relay should operate on 60V short circuit C The relay should not arc excessively on a 200V short circuit on anode or screen D The relay should not operate when checking a 180mA rectifier Before making any adjustments check that the lamp ILP1 is operative When the instrument is used solely on a 110V supply it may be preferable to replace ILPl with a 100V 15W Pigmy Lamp 9 Servicing the Valve Holder Panel and Adaptor Unit The Valve Holder Panel is connected electrically to the control panel by means of two S way side by side cables One of these cables embodies two thicker sections 16 012 for H and H leads Connections to tag boards on either unit are shown in Fig 2 The wiring of the valve holders on the panel is in the form of nine separate loops all pins comprising a loop and linking in roller 1 of the Roller Selector switch This form of 1oop connection is used likewise for pins 2 9 all nine circuits approximating in length and following a similar route around the panel These loops are further loaded with beads of ferroxcube which sufficiently damp the loop to prevent the valve under test breaking into parasitic oscillation A diagrammatic layout is
7. Page 7 of 17 AVO CT160 anode current measurements calibration resistors and the meter workings This text will endeavour to explain how the AVO CT160 works in certain areas especially the Anode Current controls the calibration resistors and the meter circuit It will explain how to replace the rectifier valves with Silicon diodes as AVO did in their later model the CT160A and also explain why there is a slight modification to the circuit as a consequence It will also explain how you can improve the calibration by making a further slight modification to the calibration circuit It will also explain how you can place a Silicon diode in the Anode voltage circuit and a small modification that this entails All of these modifications have actually been tried out on an AVO CT160 Serial No 4087 YF and the modification do not change the results in any way you get exactly the same results when comparisons to a standardised valve a CV455 are made both before and after the modifications were performed There is also a short description of the function of R14 and SW3 which are used in the SET ZERO position on the mA V dial The protection of the meter is also discussed and a comparison with AVO s other Valve Characteristic Meters is made This text is a collaboration between me Martin Forsberg and my friend Euan MacKenzie where have written most of the text and Euan have made most of the measurements and all of the modifications in his
8. Check mains connector No mains input Dial light bulb burnt out No dial light indication or meter deflection on SET setting of Circuit Selector Fuse blown No indication No anode volts of neter at valve pin current Replace Check mains Voltage Selec tor setting and replace 251 and or 52 Check that Links A amp tight and making firm contact Check that Links Ay amp No indication No anode volts at of meter 4 valve pin but current an screen volts protective present A are tight relay operates and making when testing firm tetrodes or contact pentodes 7 Relay Operates and Fails to Clear Should the relay operate due to a suspected faulty valve and fail to clear after switching off and on again with no valve in panel set Roller Selector switch to read 000 000 000 and remove top cap connecting lead Switch instrument off and on again If fault clears the most likely cause of the trouble is a short on the valve panel certain pin s being shorted out to earth by stray wire or solder or a breakdown in insulation If the fault still persists however check HT line for breakdown to earth between Roller Selector switch on Valve Panel and HT transformer on control unit 8 Adjustment of Protective Relay The relay should seldom require attention but if for any reason parts are replaced the adjustment is simple it only being necessary to position two 4BA
9. Checks insulation cathode heater to D 28 Cathode to Heater insulation check 62 Turn the CIRCUIT SELECTOR Switch to C H and the ELECTRODE SELECTOR Switch to C H Any insulation breakdown which occurs between heater hot and the cathode will be directly indicated on the insulation resistance scale of the meter It is not possible to state a rejection figure in M Q for a valve under test for such a fault will be of considerable importance in some circuits whilst in a few cases its presence has virtually no consequence at all The instrument is capable of giving the insulation between cathode and heater and the acceptance or rejection of the valve can only be determined when the operator has details of the circuit in which the valve is to be used those cases where these details are not known it is always better to reject a valve having an insulation resistance less than 2 It will be appreciated that there are many circuits in which an appreciable potential exists between heater and cathode DC amplifiers etc and the presence of heater to cathode insulation breakdown even of the order of many M 9 can often give rise to quite serious trouble Heater to cathode insulation breakdown either permanent or intermittent can also give rise to noise in valve amplifier circuits Determination of Valve condition from Static Characteristic Data 63 Normally a valve unless it is a diode or rectifier is checked by a comparison of
10. This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 8 of 17 turn the fine control past the 10 on the scale This is provided so you can see if the current is slightly higher than the maximum on that current range or if you should switch to a higher range When the maximum indicated anode current of 100mA is being measured in reality only SOMA is flowing through the 200 O anode current resistor because the valve works as a half wave rectifier for the applied AC voltage to it Now 50mA flowing through 200 O produces a voltage drop of 10V across the anode current sense resistor which is then compared using the Potentiometer to the voltage produced by the anode current control circuit In the coarse anode current control the voltage drop in each step of the rotary switch must equal exactly 1V since each step has a resistance of 80 Q then there must be a constant current of 12 5mA flowing through the circuit Now this value exceeds the manufacturer s maximum value of 9mA for the CV140 EB91 6AL5 valves which AVO used for rectification in the CT160 and is likely to lead to a shortened service life These particular valves were actually designed as FM detector diodes not as current rectifiers As you will have doubt noticed the maximum available screen voltage in the CT160 is 300V this is due to the valve manufacturer s PIV limit
11. amp COMPONENTS Moved for clarity RL1a MAINS INPUT 105 250V 50 500Hz A pq 9 41 29 1 28 37 39 27 2 5 41 23 26 22 3 25 21 38 20 24 7 8 19 18 17 16 159 14 35 3411 33 10 32 31 30 401213 36 SF 5 2 5 2 SJ3 514 SG1 SG6 SC 5 5 SH1 SH6 S RV2 RV1 RV4 RL3 D2 RV6 C2 C1 M1 C3 MISC 81 5 SA SET la amp DR 12 625 117 fo HEATER VOLTS SD R29 R28 R27 R37B R37A R23 R22 R21 R20 R19 R18 R17 R16 R15 ET 4 SCREEN R36 CLOSED Vg X1 OPEN Vg X2 DIODE 1 SCREEN VOLTS SE DIODE 2 CHR AIR ANODE 1 H CONT 5g TEST SET GAS ANODE 2 CIRCUIT SELECTOR SG 1 2 CIH 02 R13 ELECTRODE SELECTOR SH CATHODE H H Valve Tester type 160A Circuit Diagram Redrawn amp Corrected R 6 4 29 1 28 37 39 27 2 5 42 41 23 26 22 3 25 21 38 20 24 7 8 19 18 17 16 159 14 35 3411 33 10 32 31 30 401213 36 S SA SK1 SB SE SD 5 1 511 SF 5 2 5 2 SJ3 5 4 SG1 SG6 SC 5 5 SH1 SH6 S MISC Rita FS2 RL1 ILP2 FS1 SKT1 11 12 D4 RL2 D3 RV5 RV3 D1 RV2 RV1 05 RVARL3 D2 RV6 C2 C1 D6 D7 C4 M1 Schematic redrawn corrected amp modified by 51 5 sc 5 Martin Forsberg Sweden 2010 08 25 SET la amp DR roi cranes D2 Changed from 66V RMS winding to 99V RMS winding 5 1 5431 R4 amp R41 Changed places schematic 1 dod 10 5 Sr gf T i n Em miim em 2 R37 R37 shown as R37A and R37B as per component list o 5 con
12. 04yF 2 in series 0 02yF 200V wkg 450V wkg 2A 2A 6 3V 0 3A 200V 15W RED On Off Heater Volts Slope Switch Heater Volts Screen Volts Anode Volts Circuit Selector Electrode Selector Anode Current Mains Adjuster Grid Volts 240n 2400 2408 6002 3kn 15 814kn 406kn 2 2 2 t 4 2 2 10DE8 Silicon Diode 100 8 Silicon Diode 202ka x 10DE8 Silicon Diode 100kn 10DE8 Silicon M Diode 5 4 35 10k amp 138 100k2 2 41kf 1 22 82K I 305A 3250n including swamp HT Trans former LT Trans former 1 a 6 291283739272 5 23 26 22 25 01 3303 4 19 3848 72847 16 15 149 35 34 3310 32 31 30 40 12 13 36 A 5 SKI EJ E50 SJ su SF SL2 522 23 Ja 56 2566 5 5 SHI SH6 MISC ALI0 FS2ALIMLPAPSUSKIN _ MALAVA AVE t2 Misc 534 5 SET lo R26 R25 R24 ome R23 R22 R2 A20 AIG AIG RI AIO AIS jo ip zi 4 gt i ov D 5 m 8 sx B SELECTOR 52 D R36 Y CLOSED Vg X 1 se OPEN X2 SCREEN VOLTS SE 4 sa H CONT TEST LG SET GAS b LES et e 56 95 250 IT SELECT 05 250 CIRCUIT
13. CT160A have been available for check it is not known by me if the winding is still there or not The Silicon diodes also means that some circuits will have to be compensated since they have a lower voltage drop than the valves Modification B D amp E Current measuring range for rectifier valves increased up to 180mA To measure rectifier valves on the 180mA range AVO have modified the CT160A in the same way that the AVO Mk IV is designed with one extra resistor on the SET la amp DR switch for the measuring range and also a resistor R37 which is connected in parallell with the over current relay coil RL3 This extra resistor R37 is working as a shunt making less current run through the relay coil so it will not actuate as before on the old 120mA range and on the new 180mA range This is necessary so rectifiers with a higher current capability can be measured the relay would otherwise actuate to early The 500 resistor R12 on the D R circuit in the AVO CT160 have been replaced with two resistors R12 amp R13 at 1 KO in parallell making 500 O to be able to handle the current flowing through them at the higher current ranges for rectifiers This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 4 of 17 Modification F The Grid Volts control have had its range extended by a switch which makes it possible to double the voltage on th
14. V control to SET ZERO position and make any final adjustment to zero using fine ANODE CURRENT control See Note 3 iii Continue rotation of SET mA V control to expected value of mA V meter needle should rise iv The comparative goodness of the valve will now be shown by the position of meter needle on coloured scale This scale is divided into three zones and all valves coming within the green portion can be regarded as satisfactory Readings in the intermediate zone between the red and green sections denote 30 that the valve is not entirely satisfactory although it may be capable of working in some circuits at lowered efficiency whilst readings in the red zone indicate that the valve should be rejected or replaced Note 2 Valves having slope of less than mA V cannot be checked on the good replace scale and must be checked in the manner set out in paragraph 74 To check Valve by direct reading of Mutual Conductance mA V 72 a Using recommended anode current i Do not alter ANODE CURRENT controls but adjust NEG GRID VOLTS control until meter is balanced to zero 11 Slowly rotate SET mA V control to SET ZERO position and make any final adjustment to zero using fine ANODE CURRENT control See Note 3 ii Continue rotation of mA V control until meter needle reaches the calibration line in centre of good zone ImA V iv Read actual value of mutual conductance from SE
15. Vg S and R are such that the first meter indication is at 25 M Q full scale of course representing a dead short The meter is suitably scaled for direct reading between these limits This test serves for heater continuity and insulation measurements between anode and all other electrodes strapped and screen and all others with the valve cold With heater volts applied the normal cathode heater test is made whilst a further test of cathode and heater strapped rest takes care of sagging grids or filaments of directly heated valves Since a short circuit deflection is in fact a measure of the grid voltage this is used in checking the setting of the mains voltage of the instrument at position Set In this condition a short is put on the insulation test circuit and the mains selector is adjusted until the meter reads full scale deflection at which point the grid voltage and therefore all the other voltages working the instrument are correctly proportioned 2 00060000000 FIG 4 17 The full circuit figure 3 shows how all the above combinations are incorporated in a single circuit and selected by appropriate switch settings Despite the full range of test voltages available and the comparative complexity of the circuit the discreet use of ganged controls has reduced the operation to a simple and logical sequence 11 18 Figure 5 shows in diagrammatic form the panel marking and it will be seen there from that in addition to the con
16. and their combined current through the meter at that point This will become more evident if you use Silicon diodes instead of CV140s since the difference in their forward voltage drops then becomes more apparent In the CV140 6AL5 EB91 valves the forward voltage is more dependent on the current flowing through the valve particularly as AVO were using the valves beyond their recommended current range than the forward voltage drop of a Silicon diode which is relatively constant in these circumstances The forward voltage drop curve is flatter for the valve than for the Silicon diode so it varies more when the current through the diodes is varied have seen two AVO CT160s which have been modified with a Silicon diode in the Anode circuit and the owners have not reported any adverse effect although they had not done the modifications themselves and had no information as to by whom or when the modification had been done have never seen any modification for the 66Vrms winding but my calculations and the fact that AVO used it on their other Valve Characteristic Meters show that it is a modification worth doing and calculations show that it will give a better calibration result and it will also make the calibration easier to perform without having to go back and forth between the different steps This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 1
17. for all load switch positions 1 The Valve Tester meter reading is approximately in centre of good scale 11 That the AvoMeter readings are within 10 of nominal value M Repeat test as in J with anode load connected to D2 on top cap board and electrode selector to D2 APPENDIX 1 SCHEDULE OF SPARE PARTS FOR AVO VALVE TESTER TYPE 160A AND ADAPTOR UNIT TYPE 160A 1 PROCEDURE FOR ORDERING SPARES Throughout the past decade AYO instruments have proved themselves to be unrivalled for versatility and reliability It is however inevitable that instruments will fail from time to time We are anxious that AVO instruments are repaired to the highest possible standard and we have therefore produced this Schedule of Spare Parts which will form a useful guide to the trained engineer who has the task of maintaining this instrument By following the procedure set out below delays will not occur due to unnecessary correspondence 1 State the part number of the items required also the quantity 2 State the serial number of the instrument This will be found on the front panel Overseas users of our instruments should send their requirements to our representatives on their territory If parts are required in Great Britain application should be made direct to AVO LIMITED ede Ite No 05 30 P Uri Gi to iH VALVE TESTE
18. is carried out in the manner already explained the ELECTRODE SELECTOR being used to select the diode or rectifier element the emission of which is indicated on the meter replace good scale When dealing with diodes or rectifiers the D and D positions of the ELECTRODE SELECTOR represent diode or rectifier anodes 1 and 2 respectively and correspond to figures 8 and 9 on the ROLLER SELECTOR SWITCH set up number 33 88 In the case of Triple Diodes since only two anode systems are normally catered for a special procedure is adopted in the set up figure At the position in the SELECTOR SWITCH number representing the third diode the symbol is included The first and second diodes being indicated by 8 and 9 respectively in the normal manner The valve should now be tested normally with the ROLLER SELECTOR SWITCH set to 0 where the 7 appears in the set up number This procedure will give emission figures for diodes 1 and 2 Now rotate the ROLLER SELECTOR SWITCH rollers so that the two rollers originally set at 8 and 9 are now set to 0 and set the pin marked in the Valve Data to 8 on the ROLLER SELECTOR SWITCH A further test with the ELECTRODE SELECTOR Switch at D will then give the emission of the third diode e g EABI is indicated in the data as 023 110 890 To test diodes 1 and 2 the set up on the ROLLER SELECTOR SWITCH will be 023 100 890 enabling these diodes to be tested in the normal manner To obtain the emission figure fo
19. meter is at its null position H Remove valve and external meter J Set the Neg Grid Volts control at 40 K Connect a resistance of 680kn 5 between grid and cathode sockets on the top cap connector panel M Set the Circuit Selector switch to position Gas The panel meter should indicate full scale deflection 20 N Disconnect the resistance and switch off 3 To Obtain Standard Figures for a Valve Using DC Supplies Using the recommended AvoMeters the valve should be connected as shown in Fig 1 unable to use the recommended meters ensure that those used are of sub standard accuracy the current meter having a maximum voltage drop of 100mV and preferably scaled 0 25mA and the voltmeters a sensitivity of 1000 n V If rectified AC is used for the HT supply it is essential that steps are taken to ensure that the supply circuit is adequately smoothed the Solartron Varipack is amp suitable source The bias supply should be obtained from a suitable battery Note polarity of connection The heater supply for the valve may be AC or DC but must be within 5 of the rated voltage A Set the Grid Bias voltmeter to read 9V B Adjust the HT supply to 200V then by means of successive adjustments of the bias and HT controls set the anode current at 16mA the anode voltmeter must read 200V Note the new grid bias reading C The standardised slope for the valve can now be obtained from The difference betwe
20. new valve to be plugged into an existing socket on the Valve Panel Blank adaptors are already available to accommodate non standard bases not incorporated on the existing Valve Panel The Control Unit and its Function 38 With the exception of the Roller Selector Switch and other features incorporated on the Valve Panel all the controls are situated on the Main Panel of the instrument By the manipulation of these controls and the use of the Valve Panel the following tests can be undertaken 1 Heater continuity 2 The measurement of insulation resistance between electrodes with the valve cold 3 The measurement of insulation resistance between Heater Cathode to all other electrodes strapped together with the valve heater at operating temperature 4 The direct indication of cathode to heater insulation with the valve heater hot 5 The direct indication of valve goodness on a coloured good replace scale for a complete range of applied H T and bias voltages 6 The direct indication of anode current and mutual conductance mA V at and pre determined combination of H T and bias voltages 7 The measurement of control grid current on a scale directly calibrated in 4A 8 The testing of half and full wave rectifiers under reservoir capacitor conditions with a range of DC loads which can be selected by means of a switch 9 The testing of signal diodes with suitable DC loads which can be selected by the oper
21. purposes in the TEST and GAS positions According to the AVO Patent No 606707 this is to ensure that no current is drawn through the diode formed by the grid and cathode when no anode current is flowing which could damage the valve s emission This means that it is very important to make sure that the Anode and Screen voltages are correct by means of the SET and then set the grid voltage correctly via RV3 The additional voltage from the 66V RMS winding means that there are two voltages that need to be correct on the transformer for the needle to read the calibration region This then works as an extra measure to ensure that the transformer voltages are correct Since the 66Vrms winding is part of the Anode Screen voltage windings you have thus ensured that they are also correct apart from a possibility of open circuit somewhere else in the transformer In the Mk IIl and Mk IV the additional voltage is also taken from the Anode Screen volts windings so you have the same function there In the Mk and Mk IV you have an extra potentiometer in the SET calibration circuit which means that you can adjust the deflection of the needle when you have made quite sure that the grid voltage control and Anode amp Screen voltages are correct However in the AVO CT160 you do not have such a potentiometer so it is even more important here that you have checked that the Anode Screen and Grid control voltages are correct apart from a
22. schematics to ground this has earlier been mentioned as necessary for shorting the charge buildup inside the Silicon diode when it is not conducting At the same time the winding which supplies this circuit has been changed to the 99V RMS winding this is necessary to ensure that the same current as in the AVO CT160 is driven through the circuit for the measuring purposes The calculation for the 99V RMS winding becomes 99V RMS equals 89 13V Mean the conversion factor being 1 1107 99V RMS 1 1107 89 13V Mean This voltage is half wave rectified which results in half the voltage after the rectification minus the voltage drop of the diode which can be assumed to be 0 7V 89 13V Mean 2 0 7V 43 87V When this voltage is divided by the current necessary in the measuring circuit which has earlier been shown to be 35 7754 4A the resulting calibration resistance will be 43 87 35 775 1 226 with a 1 1 22 MO resistor which ranges from 1 207 up to 1 232 MO you might end up just outside the range possible to adjust with the 20 KO potentiometer RV6 so it is best to use a resistor close to 1 22 MO This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 5 of 17 Modification H This modification of series resistor R6 has already been described in the earlier article on AVO CT160 modifications and Anode Cur
23. to understand is that the calibration resistors are not part of the actual measurement done on the valve in the TEST or GAS positions only in the other leakage and insulation tests The calibration resistors are not only there to give you a reading that tells you that your tester is still in working order compared to the last time it was calibrated and that you have chosen the correct mains voltage setting for the measurement that you intend to perform but also to deliver a current that is used for other measurement purposes Measurements may be performed when the needle is within the black area of the SET region but it should be close to the red line in the centre for the best correlation to the calibration performed previously However the current through the calibration resistors is also used to perform the insulation and leakage tests so in order for these to give the correct results the calibration current must also be correct The valve V1 B is supplied by the voltage from the 55Vrms winding as that voltage is in phase with the Anode and Screen voltage and the valve under test itself acts as a rectifier in the AVO CT160 and in the Mk III However in the Mk IV the Anode voltage is already rectified by a silicon diode The voltage from the 66V rms winding is only used to keep the grid negative during the half cycle where the anode of the valve is driven negative by the AC Anode voltage and is not used for any measurement
24. to 40 x 1 B Connect the Testmeter across RV2 Switch on adjust RV3 for a voltage reading of 20 8V Set NEG GRID VOLTS to 40 x 2 Adjust RVS for a reading of 41 6V Remove testmeter C Connect the Testmeter between G and C sockets on the top cap connector panel or if the panel has been disconnected for servicing to the 61 and C positions on the tag board at the back of the Unit D Check that at the 13 and 4 marks on the dial readings of 6 75V and 2 1V 5 are obtained If either or both readings are out of tolerance the dial should be adjusted mechanically to split the error If it is necessary to make an adjustment slacken the three counter sunk screws on the top of the dial which will then be free to move within the latitude of kidney shaped slots After adjustment re tighten screws and check readings The areas marked 0 5 15 and 40 should correspond within the indicated area to OV 2 6V 7 8V and 20 8V 5 respective ly E Switch off and remove the Testmeter SECTION 3 CHECKING THE SET mA V CONTROL Switch on and check that when the dial is advanced to its 10 5 and 2mA V positions readings of 52 5mV 105mV and 260mV 5 are obtained B If for any reason the relationship between the dial and the potentio meter has been upset the procedure setting the mA V Dial given in Section 6 should be adopted C Switch off and remove the Testmeter SECTION 4 CHECKING THE SET INDICATION A
25. what have been told by people who have either been employed at companies who serviced AVO Valve Testers or by people who were employed by AVO themselves there was only one FSD and that was at all other meters are broken or damaged meters and they probably suffer from a weakened magnet One more piece of information that speaks against the idea that there are two different meters is that there have not been any modifications done to the AVO Valve Testers to accommodate such a meter in the test circuit it would have been necessary to correct the shunt resistor and the rest of the circuit to get a correct reading A meter that shows the symptoms of a higher FSD current is most probably suffering from a weakened magnet and needs to be re magnetised If your meter for some reason needs a lower FSD current it is most probably suffering from a wrong adjustment of the magnetic shunt arm inside the meter or it could have been stored in a place where the meter has been exposed to a powerful magnetic field which have affected the magnet although this is not such a probable cause In this case it is also better to send the meter to Herts Meter Co Ltd than to fiddle around with it yourself as they are so easily damaged The magnetic shunt arm is situated on the magnet and looks like an L letter and is usually glued to the magnet and screwed tight on the left pillar holding the screw with the scale You need to open the meter remove the scale while not bend
26. 0 26 40140 27 40140 28 21564 A 21199 A 15626 2 14555 1 20903 D 15639 6 16981 1 16971 A 21561 C 15627 1 16911 1 14269 4 21563 A 16995 1 21585 15567 32 14747 1 15639 5 15639 6 15639 7 15639 8 Valve Panel Assembly Hinge Pin Valve Holder Acorn 5 pin Valve Holder Continental F8 Valve Holder B7A Valve Holder B8D Valve Holder B9D Valve Holder 5 pin in line Valve Holder 7 pin in line Valve Holder Nuvistor 5 pin Special Valve Holder Assembly Assembly for Flying Lead Valve a Socket Top Cap Escutcheon Assembly Plug 25 way Support Plate Contact Board Assembly Cover Assembly Plug Receptical Retaining Nut Partition Assembly Connector Retainer Valve Holder Assembly Lighthouse Label Ferroxcube Bead VALVE PANEL LEAD 21562 Socket 25 way Plug 25 way Cover Cover 20 E IND de at OO BD 00 PHB HHP EHH HHH HOW Hee ND S LIMUIO ONISICUVGNVIS TAWA 31435 A9 ZL l3 TlVHYd A 9 5 18 ATAVLIAS Alddfs 1 1 ATddNS L H Jo 2 pet ES Cte hy p Item i m STsseyO 4uouna4su C SAU SAU SAU Fig 4 Underside of Adaptor Unit Valve Panel COMPONENT LIST genns 1 10ko 500n 90n 5000 20kn 0
27. 1 18 Page 13 of 17 But remember the most important thing is that the voltages are correct on the secondary side of the transformers only after that should any modifications to the calibration circuit be performed Part 4 the Meter For the meter to give correct readings the movement must have the correct resistance of 3 2500 which includes the internal swamp resistor In addition it also needs to have the correct deflection at two particular points namely 22 2 and 27 Obviously it needs to give the correct reading across the whole scale but those points are used during calibration of the meter and tester The 22 2 point corresponds to the 1mA V mark on the green scale and the 27A point is the red centre line of the SET region If you can recommend that you remove the meter and test it to check that the FSD is correct and that it gives correct readings right across the scale Quite often these meters are known to have lost some of the flux in the magnets over the years so that they no longer give the correct readings The only company that know of that can re magnetise the meter is Herts Meter Co Ltd Unit 10 Berry Road Hatfield England AL10 8BJ but of course there may be other companies around Alternatively if you are competent and confident enough to tackle it yourself you can fit a pair of the modern Neodymium one of the rare earth series of elements disc magnets the ones Euan used were 108 x 3mm thic
28. 381 40465 34 40464 2 12049 29 12049 388 12049 389 12049 390 12049 391 12049 392 12049 393 12049 394 12049 395 12049 396 12049 397 12049 432 12049 911 NEGATIVE GRID VOLTAGE CONTROL Description Cct Ref Knob Screw 6BA Tin plated Coiled Washer 6BA Knob Carrier Spring Dowel Spindle Bush Hexagonal Locknut Friction Washer Washer 4BA Collar Hexagonal Locknut 4BA Tinplated Cover Instrument Head Screw 8BA Negative Grid Volts Dial Dial Flange Cheesehead Screw 4BA Tinplated Friction Disc Countersunk Screw 8BA Tinplated Clamp Plate Cursor Cheesehead Screw 6BA Tinplated Pillar Pillar Potentiometer Strap Potentiometer 10 RV2 MUTUAL CONDUCTANCE CONTROL ASSEMBLY 40465 A Dial Support Cover Slope Dial Dial Housing Cover Plate engraved Potentiometer 2 5 RV1 Drive Pin Drive Bracket Telephone Dial Assembly Resistor 22kn 2 HSC R14 Resistor 5000 1 HSC RS Resistor 702 1 HSC R2 Component Board ANODE CURRENT SWITCH ASSEMBLY 40464 B Anode Current Switch SJ Resistor 80n 24 HSC R15 R23 Resistor 2407 2 HSC R24 R26 Resistor 6000 2 HSC R27 Resistor 3kn 2 HSC R28 Resistor 15 2 HSC R29 Resistor 814kn 2 HSC R30 Resistor 406ka 2 HSC R31 Resistor 202ka 2 HSC R32 Resistor 100kn 2 HSC R33 Resistor 31 5k 2 HSC R34 Resistor 4 35kn 2 HSC R35 Resistor 1 22 2 HSC R40 Resistor 132 10 R37 2 in Parallel Selected 218 8 9
29. 4 P A CRANE SEM lt a 29 5 S 1 dn gt valo X EE nee Vn sau apt PL Nace Sica beh anu Abie ester peared Au xs 2 Ask 2 ya te 57 M 4 45 AM 4 A 55 RN 27 pads EA ORE SECTS FIG 7 THE LAYOUT OF THE INSTRUMENT 15 478 43 5 4 1 0000 So 959 SINS Nid DWAIH Nid UNOS Nise Summi uns Q 9e 40 E pA vea 5508 Nid Nvonisuv CZWS Nid 5 TIVWS X3ONI z BOW 1 120 SXN 3Ald NYJIYI NY 9358 Nid S b HSLLING 9XN NIGXISNVIIMANY SOV IVWNOILVNYSLNI O 9 p 9 oS v o o 48 N3A3S 68 NId ANIN asoq fo apisaapun SNOILOSNNOO Nid QUVGNVIS JO WVviuDvia General Construction 26 The instrument is designed in suitcase form for portability and ease of stowage It is of small size and constructed to comply with the requirements of the U K Govern ment Climatic Tests Specification K 114 When closed it is completely shower proof All components likely to requ
30. 5mA through the backing off circuit in this example by 100 However you will need to make a new scale for the anode current potentiometer and you will get a larger overlap on each range but it will still give the correct measurements for anode current The practical minimum value will be close to 900 as it was originally If for any reason you need to go lower than 800 then you will have to lower each 800 resistor and increase the current to maintain exactly 1V A typical reason for replacing it would be that the old one is open circuit or perhaps has become non linear due to wear or is otherwise damaged Making new scale is quite easy if you use a 360 protractor together with a multimeter to measure out each step of either 0 1V or 80 and mark them on the protractor then transferring them to a paper scale Or alternatively you can drill a hole through the centre of a fairly large protractor or a piece of PCB then fasten the potentiometer in the hole and using a large knob on the potentiometer and as you turn it mark each point on the protractor pcb which you can then transfer to a paper scale NB There is one case that has not been checked thoroughly so far and that is whether the gm measurements will be affected if the potentiometer is changed to any value other than 900 There is only a slight risk of that since each of the three 2400 resistors R24 to R26 are used to compensate for the anode current control resistances thi
31. 9 8yA 11 CHAPTER 3 FINAL TEST PROCEDURE CONTENTS TEST EQUIPMENT REQUIRED SECTION 1 Page 12 FINAL TEST DETAIL SECTION 2 Page 12 NOTE When all repairs have been carried out and the instrument is re assembled carry out the following final test detail SECTION 1 TEST EQUIPMENT REQUIRED AVO Electronic Testmeter or equivalent mean DC Valve Voltmeter B Model 7 or Model 8 AvoMeter C Valves CV428 CV 491 and US2 D Resistor 1 megohm 1 B Resistor 680kohm 2 5 SECTION 2 FINAL TEST DETAIL See WARNING on page 4 A Apply a 500V megger test between the mains input and frame B Apply a known AC 50 c s voltage in the range 200 250V to the instrument and with Circuit Selector at Set 4 set mains voltage selector until needle on panel registers as near as possible to the centre of the mark Check that the selector settings show 5V of the actual mains voltage It is most important to ensure that the meter reading in the Set position is maintained at the centre of the mark on the scale for all subsequent tests C Set the circuit and electrode selector switches as given in the table below and connect a 1 megohm 1 resistor across the electrodes on top cap board as detailed under the heading Condition Check that a leakage of 1 megohm 10 is indicated on the panel meter in each case Circuit Selector Electrode Selector Condition Switch Switch A R Al Resistor connected across
32. AVO VALVE TESTER Type 160A AND ADAPTOR UNIT Type 160A 1 OPERATING S VALVE DATA MAINTENANCE MANUAL AVO Ltd AVOCET HOUSE DOVER KENT ENGLAND Page 1 of 17 AVO CT160A Operating Instructions and Service Manual This AVO CT160A manual is the result of combining and editing two different sets of photocopies of AVO CT160A manuals and also from information from the original AVO CT160 manuals This AVO CT160A manual has been kept as original as possible even though some photocopies of the AVO CT160A pages have been replaced with AVO CT160 pages where the text did not differ in any point but where the quality of the CT160 scanned pages was much better than the CT160A scanned pages All figures and photos have been kept in their original shape although most have had to be edited due to the poor quality of the original photocopies The schematics at the end of the manual have been extended with three new sets where the first is an unaltered original redrawn schematic the second schematic is a redrawn and corrected schematic and the third is a redrawn corrected and modified schematic The second schematic has been corrected in the following areas D2 Changed from 66V RMS winding to 99V RMS winding calculating the current in the circuit shows that the 99V RMS winding is necessary for the circuit to work correctly R4 amp R41 Changed places in schematic the component list on page 17 of the Service Manual and also the component l
33. Al and and of the following S H C A2 or G S R Al Resistor connected across S and any of the following H C or G C H R Al Resistor connected across C H or H and any of the following A1 A2 S or G C H C H Resistor connected across C and either H or H NOTE in positions C H R and C Hcheck that heater volts appear across H and D Check operation of the overload cut out with 1 Anode Cathode short at an anode voltage of 100 Connect Short across A1 and C top cap board 12 E F G H J K 11 A screen cathode short at a screen voltage of 60 Connect short across S and C on top cap board Check the following unloaded anode volts with the Testmeter on appropriate AC range connected across Al and C on top cap board Nominal Volts 60 100 150 250 400 Actual AC Volts 66 110 165 275 440 Limits 2 6 Check the following unloaded heater volts with the Testmeter on appropriate AC range connected across H and H on top cap board Nominal Volts Actual AC Volts ev eii Insert a CV428 in the appropriate socket set the instrument to the correct electrode selector switch to Al and obtain a balance then check that 1 By varying the grid voltage an anode current of 100mA is obtainable 11 The valve does not oscillate NOTE The meter reading should not vary appreciably when the hand is placed near or on the insulated anode lead 111 By varying sc
34. B SC SL Mains Input Board Assembly Front Panel Heater Volts Switch SD Mutual Conductance Control Assembly For details see page 18 Anode Current Potentiometer 909 RV4 Anode Current Switch Assembly SJ For details see page 18 Handle Bush for Item 19 Hex Stiff Nut for Items 19 20 End Frame Component Board Assembly For details see page 17 L Shape Mounting Bracket Cover for Mains Voltage Panel Mains Lead 8 0 PVC 21 16 8 S OO PHO gto HR EAS HHH Ui PU E OS WIS amp Ge Part No 13701 9 PVC100 11310 A 20901 5 20899 D 20908 40466 14013 1 40650 14267 1 5745 W39 14670 2 20245 52 21589 A 12049 385 12049 403 12049 3584 12049 402 12048 1042 12049 237 12049 400 12049 404 12049 14709 B 15581 60 12049 876 10770 9 15832 9 12049 244 12049 432 12049 1043 12049 380 12049 40466 24 14655 2 14548 4 14650 A 10466 2 14647 2 11832 5 14648 2 14644 2 14643 2 14642 2 14646 2 20885 A 14645 2 20885 B 13658 2 Description Cct Ref Movement Mounting Pillar 5 Way Lead 24ins per instrument Tag Board Assembly LT Transformer Assembly T2 HT Transformer Assembly T1 Lamp Board Assembly For details see page 19 Relay Assembly For details see page 17 Modification Record Plate Movement and Case Assembly For details M1 see page 19 30uA 32500 includ
35. By finished mean that have soon finished cleaning up all the pages While AVO was changing the current range for Diodes and Rectifiers by adding the 180mA range they also changed which voltages that are used for testing Diodes and Rectifiers In the AVO CT160 they used the windings for 40V 75V and 125V to cover the ranges from 1mA up to 120mA but in the AVO CT160A these windings have been changed to 40V 75V 150V and 200V The two higher voltages are used in conjunction with the added Switch SJ1 which connects resistor R37 in parallel with relay coil RL3 for over current protection From reading the component list could also see that AVO used two resistors to make up the resistance of R37 which is stated as 13 O each This will then make R37 to have a resistance value of 6 5 O With coil resistance close to 5 which Euan MacKenzie has helped me to measure for RLS this means that you will have roughly a 57 decrease in sensitivity in RL3 as the current is divided between the two resistors making up R37 and the relay coil The maths for this is simply 13 Q in parallel with 13 Q gives you 6 5 This then gives the current through the relay RL3 to be 6 5 Q 6 5 Q 5 0 6 5 11 5 56 5 This figure of 57 corresponds fantastically well with what did in their Mk IV tester on the current ranges for Diode and Rectifier testing on the ranges 60mA 120mA and 180mA Here AVO used two 3 9 resistors in parallel with
36. ELECTOR Switch set up as explained in Chapter 2 Section 35 2 Set the HEATER VOLTAGE Switches to the correct value for the valve under test and insert the valve into its appropriate holder making any necessary connection to top or side caps Note Where given heater voltages in parenthesis should be employed 3 Set the CIRCUIT SELECTOR Switch to H CONT and the ELECTRODE SELECTOR Switch at C H The meter should now indicate a short thus indicating heater continuity 4 Set the CIRCUIT SELECTOR Switch at A R and S R in turn using each of these settings in conjunction with successive settings of the ELECTRODE SELECTOR Switch at A and Dj The meter will now indicate any insulation breakdown between electrodes 58 5 Table 2 sets out the manner in which insulation checks are made 27 TABLE 2 Circuit Selector Electrode Selector Switch Position Switch Position Insulation Check A R Checks insulation anode 1 to screen filament cathode anode 2 and grid A R Checks insulation anode 2 to screen filament cathode anode 1 and grid A R D Checks insulation D to screen filament cathode anode 1 and grid A R Checks insulation to screen filament cathode anode and grid Ay Checks insulation screen to filament cathode S R and grid 59 Study of the table set out above will show that all normally expected insulation breakdo
37. OF THE LINKS ON THE VALVE PANEL OF THE INSTRUMENT 98 These links in the and circuits respectively enable a load to be inserted into the anode circuit of the valve under test By removing the shorting links and inserting the appropriate resistor or other load across the terminals which it is desired to include in the circuit it is possible to obtain certain dynamic figures for the valve or electrode system under test 99 The links also enable a suitable dc Moving Coil Ammeter with low millivolt drop to be inserted in series with the anode of the valve under test Variations in the settings of the ANODE CURRENT controls will not materially affect the readings of the external meter which will read 0 5 of the actual current flowing i e the external meter reading must be multiplied by 2 for true anode current Note 6 Beware of high voltages on shorting links Checking Tuning Indicators TI 100 Tuning indicators Magic Eyes are tested with the controls set to the figures in the data table the SCREEN SWITCH being used to obtain target volts the appropriate anode load being inserted in accordance with the value shown in the remarks column 35 At the approximate bias given in the table the triode section should be at cut off and the eye fully closed Reducing the grid bias to zero the eye should open fully and the value of anode current should be approximately that appearing in the table where given In the case o
38. R 1604 ADAPTOR UNIT 1604 1 Part No 40944 E 40945 40947 14740 3 14746 7 11237 E 40944 8 40944 D 14740 4 13714 5 40374 2 40507 1 20970 20970 14635 3 11596 1 11727 11727 5 16957 2 11982 1 20909 1 14822 3 40471 12049 89 12049 88 12239 6 50010 14 13698 1 20911 1 13657 1 16961 40949 2 14822 2 40465 14558 2 40464 B 13845 3 13846 5 N82 20888 1 21589 8 14274 1 14627 2 17087 1 SCHEDULE OF SPARE PARTS MAIN ASSEMBLY 40946 A Description Cct Ref Case Assembly for details see below Chassis Assembly for details see below Vaive Panel Assembly for details see page 19 Metal Foot for Bottom Case Identity Label Lead CASE ASSEMBLY 40944 E Top Case Assembly Bottom Case Assembly Foot For Top Case Washer for Item 3 Gasket for Valve Panel Assembly Seal Between Case and Chassis Hinge Assenbly Hinge Assembly Release Block for Hinge Assemblv Rubber Foot Handie Sub Assembly End Cap for Item 11 Negative Grid Volts Control CHASSIS ASSEMBLY 40945 Mains Input Cover Plate Grommet For Item 1 Electrode Selector Switch SH Circuit Selector Switch SG Mains Adjustor Assembly 5 1 amp SKT1 Fuse Holder Fuse Holder Cap 2 Amp Fuse 51 FS2 Indicator Lamp 6 3V 0 3A ILP1 Indicator Lamp Assembly complete Screen Volts Anode Volt Switch SE SF Toggle Switch SA S
39. SERVICING NOTES SECTION 2 Page 5 VOLTAGE CHECKS WITH NO VALVE UNDER TEST SECTION 3 Page 8 SECTION 1 TEST EQUIPMENT REQUIRED A AVO Electronic Testmeter or equivalent dc mean valve voltmeter B Valve CV491 Standardised for Mutual Conductance at 16mA anode current C Model 7 or Model 8 AvoMeters 3 D Power Valve capable of passing 100mA anode current CV428 E Resistor 680ka 5 SECTION 2 FAULT FINDING AND SERVICING NOTES See WARNING on Page 4 NOTE All measurements and tolerances stated do not include those of the testing instrument and where necessary these should be ascertained particularly before commencement of the calibration procedure Where possible the recommended instruments should be employed 1 500c s AC Supply Operation amp its Relation to Servicing Whilst the instrument is suitable for use on 50 to 500 c s AC supplies service and calibration should normally be carried out using a 220 230V 50c s supply The following features play a vital part in the correct operation of the instrument on a 500 c s supply A The two electrostatic screens 51 and S2 on the transformers prevent spurious mA V readings and care must be taken when replacing a transformer to ensure that these screens are connected as shown in the Circuit Diagram See Fig 4 B The separate cable forms lying side by side across the instrument ensure that the grid circuit and its associated wiring is kept well apart from th
40. Standardise the Testmeter at 47V DC Set the panel control as follows CIRCUIT SELECTOR TO SET and ELECTRODE SELECTOR to B Connect the Testmeter across RV2 and switch on C A reading of 47V should now be obtained whilst the meter on the instrument panel should indicate within the v zone If voltage reading is correct but panel meter indication is outside 7s zone adjust RV6 as required D Switch off and remove Testmeter SECTION 5 la CALIBRATION CHECK A Open the link on the valve base panel and insert a Model 7 AvoMeter set to a suitable DC range into the circuit B Set up the instrument and place under test any power valve capable of passing 100mA anode current E G CV428 C Set the Anode Current controls to 100mA 90mA and 10mA switch on and with the instrument set to its test position allow the valve to warm up D Set the panel meter pointer to zero by means of the Neg Grid Volts control The external meter should indicate between 47 5mA and 52 5mA 0 5 x indicated value on Anode Current control 5 the panel instrument indicating zero If required repeat this test at any other settings of Anode Current controls Switch off and remove the meter SECTION 6 SETTING THE mA V DIAL A With the link open and the SET mA V dial at rest set RV1 at its maximum anti clockwise position viewed from the front panel and adjust friction tight the locking nuts o
41. T mA V dial This figure can be compared with that given in Data Manual 73 b Using recommended negative grid voltage Do not alter NEG GRID VOLTS control but adjust ANODE CURRENT controls until meter is balanced to zero ii Slowly rotate SET mA V control to SET ZERO position and make any final adjustment to zero using fine ANODE CURRENT control See Note 3 11 Continue rotation of SET mA V control until meter needle reaches the calibration line in centre of good zone marked ImA V iv Read actual value of mutual conductance from SET mA V dial This figure can be compared with that given in the Data Manual To check Valves having a Mutual Conductance less than 1mA V 74 Since the SET mA V dial is not calibrated below 1mA V it is not possible to check on the coloured comparison scale valves having an expected mutual conductance less than ImA V Such valves are checked by direct measurement of mutual conductance using the procedure set out in paragraphs 72 or 73 with the exception that the mA V dial is rotated to the lmA V position and the actual value for mutual conductance being less than ImA V is read on the meter scale calibrated 0 1 1mA V 75 For valves with more than one electrode assembly having set up for any difference in electrode voltages repeat above test with ELECTRODE SELECTOR at A gt See also comments under Testing of Specific Valve Types Note 3 Certain valves require an exceptional
42. X4 16 American six pin UX 6 Hivac Deaf Aid five pin Sm 5 British seven pin B7 All Glass nine pin B9G Pencil Tubes 5 Wires in Line Hivac Deaf Aid four pin Sm 4 Flying lead 7 Wires in Line 9 Special base for type 2C39 corn Nuvistor F8 B8D 19 CHAPTER 2 THE VALVE PANEL AND CONTROL UNIT The Valve Panel and Selector Switch 28 The Instrument Valve Panels between them comprise 32 Valve Holders including the following types English 4 5 pin English 7 pin English 9 pin Philips 8 pin side contact B7G B8A B8G American Loctal B9G B9A Mazda Octal International Octal B3G Hivac 4 pin miniature Hivac5 pin miniature American 4 pin UX American 5 pin UX American 6 pin UX American small 7 pin UX American medium 7 pin UX two disc seal and special flying lead Valve Holders See Figs 8 and 9 for diagram of standard pin connections 29 In the case of the flying lead and disc seal valves see Fig 9 the pin numbering sequence corresponding to the set up Data is printed on the panel adjacent to the appropriate socket It is assumed that all flying lead valves will be inserted into the appropriate Valve Holder with the envelope pointing downwards and the wire connections uppermost corresponding to the normal method of designating valve pin numbers looking into the valve pins 30 All Valve Holders are wired with their corresponding pins
43. alve Holder American UX5 Valve Holder Mazda Octal Yalve Holder British 9 pin Valve Holder 876 Valve Holder B8A Valve Holder 8 pin side contact Valve Holder B9A Valve Holder B amp G Valve Holder British 4 5 Pin Valve Holder International Octal Valve Holder British 7 pin Valve Holder B9G Valve Holder B3G Escutcheon Grommet Roller Selector Switch Top Cap Escutcheon Assembly Valve Holder Segment Ceramic Valve Holder Spring Assembly for Item 24 Collar for Item 24 Valve Holder Panel Warning Label Tag Board Assembly Plug Block 6 de bed fot pad TO BO ded ed ed del e ed 00 00 2 ded de ded 000 30 gt IN Pune Part No 14746 6 11673 3 14747 1 13561 1 14739 2 14745 2 50008 63 15639 5 10511 6 16995 1 16973 1 Description Cct Ref Identity Label Pillar for Item 29 Ferroxcube Bead Insulating Plug Special Screw Warning Label Washer for Item 24 Socket 25 way Cap for Item 38 Connector Retainer for Item 39 Socket ADAPTOR UNIT TYPE 160 1 Part No 40953 A 40703 Q 40952 B 21562 A 11237 E Case Assembly Valve Panel Assembly for Details see Below Valve Panel Lead for Details see Below Top Cap Lead ADAPTOR UNIT VALVE PANEL ASSEMBLY 40052 B 40952 A 17008 2 40140 23 40140 21 40140 15 40140 22 40140 25 4014
44. and remove the meter Screen Voltages A B C D E Connect the AvoMeter ensuring that the mains On Off switch is in the Off position and set to its AC voltage range between S and C on the top cap selector panel Short circuit the diode rectifier 1 Switch on and rotate the screen voltage switch through successive posi tions the external meter being set to the appropriate range as required The meter readings obtained should be 1 1 x the voltage indicated by the Screen Voltage switch 2 461i Due allowance must be made for the limits of accuracy of the measuring instrument for each particular reading Switch off and remove the meter CHAPTER 2 CALIBRATION CONTENTS TEST EQUIPMENT REQUIRED SECTION 1 Page 10 CHECKING THE NEG GRID VOLTS CONTROL SECTION 2 Page 10 CHECKING THE SET mA V CONTROL SECTION 3 Page 10 CHECKING THE SET INDICATION SECTION 4 Page 10 Ia CALIBRATION CHECK SECTION 5 Page 11 SETTING THE mA V DIAL SECTION 6 Page 11 THE INDICATING METER SECTION 7 Page 11 SECTION 1 TEST EQUIPMENT REQUIRED Avo Electronic Testmeter or equivalent DC mean valve voltmeter NOTE The Electronic Testmeter should be standardised at the appropriate voltages before making any adjustment mentioned in the following paragraphs SECTION 2 CHECKING THE NEGATIVE GRID VOLTS CONTROL A Set the Mains ON OFF switch to OFF CIRCUIT SELECTOR to TEST ELECTRODE SELECTOR to and NEG GRID VOLTS
45. anel to match the supply voltage as accurately as possible Set the Circuit Selector to Test and the Electrode Selector to and proceed to check the relevant electrode voltages as follows 1 Heater Voltages A Connect the AvoMeter switched to its AC voltage ranges between H and H sockets on top cap connector panel B Switch on and rotate the Heater Voltage switch through the full range of values the external meter being set to the appropriate voltage range as required 2 3 C D The heater voltage reading on the meter should conform to the voltage linits shown in the following table Due allowance must be made for the limits of accuracy of the measuring instrument for each particular reading Switch off and remove the meter Anode Voltages A B C D Connect the AvoMeter ensuring that the mains On Off switch is in the Off position and set to its AC voltage range between A arid C sockets on the top cap selector panel Switch on and rotate the Anode Voltage switch through successive posi tions the meter being set to the appropriate range as required The meter readings should be 1 1 x the voltage indicated by the Anode Voltage switch 2 464 Due allowance must be made for the limits of pro ed of the measuring instrument for each particular reading e g with the Anode Voltage switch set to 100 the actual voltage reading should be 110V 2 6 Switch off
46. anode control circuit 9 x 800 900 8100 thus the new value of 1744 8 810 934 80 You could if you prefer use a small variable resistor in series with a larger resistor so that you can adjust the mean current to precisely 12 5mA Alternatively you might choose to This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 9 of 17 use a combination of resistors either in parallel eg 1 1k 6 2k or in series eg 9100 240 to adjust the total current If you wanted to measure the current you could insert a current meter in series with the circuit but you would then have to adjust the circuit for the additional resistance introduced by your current meter as it will also produce a voltage drop It is therefore easier to measure the voltage drop across each resistor with a digital multimeter as that will have high input resistance typically which will not affect the circuit as much as the current measurement would do quite apart from that you do not need break the circuit for voltage measurements This also means that if you need to replace the 900 anode current potentiometer with another value say 1000 since 900 will be hard to find nowadays you can do that but then you will need to reduce the series resistor R6 by the additional resistance in the new potentiometer in order to keep the current at 12
47. ator 10 The testing of the separate sections of multiple valves the non operative section of the valve under test being maintained at reasonable working electrode voltages 11 The ability to derive data from which the characteristic curves Ia Vg Ia Va Vgj Vgo etc can be drawn with a range of applied electrode voltages cor responding to DC operating conditions 12 The testing of valves with suitable loads included in the anode circuit together with the ability to read the required electrode current on a separate external meter The instrument is therefore suitable for making tests on non standard and specialised types of valves not catered for in normal circuit arrangements The function of the various controls is as follows The Mains Voltage Selector 39 The instrument has been designed to operate from supplies of 50 500 5 over the following voltage ranges 105 120V 175 190V 195 210 215 230V 235 250V 40 Access to the Voltage Selector Panel can be gained by turning a thumb screw and lifting the transparent cover The Coarse Voltage Selector is marked 110 180 200 220 and 240 whilst a Fine Voltage Selector Arm is marked 5 0 5 10 The setting on this Fine Voltage Selector must be added to the voltage marked under the socket into which 23 the selector pin has been inserted For example if it is desired to operate the instrument on a 230V AC supply the Coarse Selector Pin should be s
48. been in contact with So as long as your meter has an internal resistance somewhere between 3 0000 and 3 5000 and an FSD current at and does not stick while moving along the scale you should be able to adjust the shunt resistor accordingly to have the meter work as designed in your tester Adjusting the shunt resistor is simple only use the rule for resistors in parallel connection to calculate the new shunt resistor Use the This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 17 of 17 wanted internal resistance of 2 452 830 and your meter internal resistance to calculate the new shunt resistor value An example lets say your meter has an internal resistance of 3 1800 then the new shunt resistor should have a resistance of Rshunt 1 1 2 452 830 1 3 1800 10 726 50 The shunt resistor makes sure that exactly 39 75 is flowing through the meter and shunt resistor which is necessary for the metering circuit to work properly NOTE Never ever adjust the resistors inside the AVO Valve tester around the metering circuit if you are not absolutely sure of what you are doing Changing one value will almost always affect all other values for all other ranges Never change the Anode circuit measuring resistor at 2000 as that affects all of the resistors on all ranges as that resistor is the resistor used for measurement pur
49. crewed into the 220V socket and the Fine Selector Arm turned to its 10 position 41 The Fine Voltage Selector also allows minor adjustments to be made to the input tappings on the mains transformer to compensate for voltage variations of the mains The instrument should be switched off whilst adjustments are made to the Coarse MAINS VOLTAGE SELECTOR NOTE If the instrument is intended for use on 110v the red warning lamp LP2 should be replaced with a 110v version The Circuit Selector 42 This is an eight position switch which determines the type of test to be undertaken on the instrument All the necessary internal circuit connections are made automatically to satisfy the test conditions required whilst internal test circuits unnecessary to the particular test in hand are removed from the valve 43 The switch position SET enables final mains voltage adjustment to be made At the H CONT position the meter indicates helater continuity At the positions A R and S R and used in conjunction with the ELECTRODE SELECTOR Switch it is possible to check leakage between electrodes with the valve heater cold 44 At the position CH R again using the ELECTRODE SELECTOR Switch the valve is automatically checked for electrode leakage between cathode and heater strapped and all other electrodes with heater voltage applied 45 At the position C H the valve is automatically checked for cathode to heater insulation with heater voltage ap
50. ctrode voltages Such conditions are catered for by calibrating the backing off controls in terms of anode current milliamps Thus when the standing anode current has been backed off to zero as shown by the meter indication not calibrated in anode current the reading of the backing off controls gives the anode current for the valve under test at a particular combination of electrode voltages This figure can be compared with expected anode current for the conditions employed to determine the valve s suitability for the function it has to perform 29 67 It is obvious that this arrangement also enables complete valve characteristics of anode current related to electrode voltages to be plotted it merely being necessary to record the anode current obtained at a series of electrode voltage settings either anode screen or grid and plotting the mutual characteristics I aVg Ia Va etc from the data thus obtained 68 The detailed instructions for making the measurement outlined above having completed the inter electrode insulation checks are as follows 69 Set ANODE VOLTS SCREEN VOLTS NEG GRID VOLTS and ANODE CURRENT controls to the value indicated in Valve Data then set CIRCUIT SELECTOR to TEST and ELECTRODE SELECTOR to A Note Should the protective relay operate switch off and check for incorrect setting of the ROLLER SELECTOR Switch or electrode voltages If these are correct and the relay continues to b
51. currents The difference between these voltage drops will be taken up by RV3 as it has enough resistance for this adjustment A quick calculation of this follows 55V RMS is equivalent to 55 1 1107 49 52V Mean which half wave rectified becomes 49 52 2 24 76V Mean DC For SET Vg calibration purposes AVO state in their calibration procedure that a voltage of 20 8V Mean DC should be present across the Grid Volts control RV2 this then means that the diode in the CV140 This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 10 of 17 plus the SET Vg potentiometer has a total voltage drop given by 24 76V 20 8V 3 96 Mean DC If we replace the CV140 diode with a Silicon diode with a forward voltage drop of 0 7V that leaves 3 96V 0 7V 3 261 mean DC to be accommodated by the potentiometer The current through this circuit is 20 8V 1937 90 10 733mA the total resistance of the grid voltage control circuit is 1937 90 leave that as a calculation exercise for you to perform by yourself Or you can cheat and look at the appendix at the end of this message This voltage drop and current through the circuit then necessitate a resistance for RV3 of approximately 3040 which is well inside the 5000 value of RV3 Part 2 Gm measurements resistor R14 and switch SW3 on the mA V control When the mA V control is in its re
52. e HT wiring to prevent the transference of energy from one circuit to the other at high mains frequencies If at any time it is necessary to displace wiring within the instrument great care must be taken to ensure that it is replaced in its original position C The 0 02uF C1 and 0 02uF C2 capacitors prevent spurious readings on insulation ranges when the instrument is used at high mains frequencies 2 Check Accuracy of Instrument Before commencing servicing the instrument should be checked as follows A Ensure that the mains On Off switch is in the Off position B Connect the instrument to an AC voltage supply of 200 250V 50 c s of known magnitude C Set the instrument voltage adjustment to its appropriate position D Switch on noting that the panel indicator is illuminated E Set the Mains Voltage Selector fine control such that the meter pointer lies as near as possible to the centre of the zone F Connect the AvoMeter in series with the A link G Using the CV491 standardised in accordance with Para 3 with 200V DC anode volts check that 1 For 16mA anode current the negative grid volts indication is within 5 of the standardised value 11 The slope mA V is within 7 of the standardised value A reading of 8mA on the external instrument will be equivalent to a DC current of 16mA through the valve this is the value normally indicated by the anode current controls when the
53. e AVO CT160A Operating Instructions and calibration Instructions Mr Yutaka Matsuzaka have published some information on his website http www6 wind ne jp yutak avo cti60 index htm about calibration of the CT160A and also a comparison of the CT160 and CT160A have also been given photocopies of another version of the CT160A manual by Frank Philipse which he received from Wim De Grotte these copies have recently been published on Frank Philipse s website http frank pocnet net instruments AVO index html Some of the modifications that did to the CT160A have already been mentioned in the article CT160 anode current measurements calibration resistors and the meter workings on the UK Vintage Radio Repair and Restoration Discussion Forum which also follows this text But the differences will be described in more detail in this article The AVO CT160A has been referenced to as the Dutch Military versions in a few places and have found no other mentions of it Please also understand that have not had access to an AVO CT160A myself for this article only to second hand pictures and information plus photocopies of AVO manuals However the modifications are not complicated once you understand how an AVO CT160 and an AVO Mk IV works under the hood Any errors are my own but have made my best to check everything before publishing it here Part 1 Modifications done by AVO AVO have updated the AVO CT160A to work more like
54. e scale in the X1 position you have 0 40V and in the X2 position you instead have 0 80V This modification has been done by introducing a new winding on the Anode Screen volts transformer This winding now have three taps ground 55V RMS and 120V RMS The switch SL1 amp 2 for the X1 and X2 range switches between the 55V RMS and the 120V RMS windings plus it also inserts or shorts resistor R39 which compensates the gm measuring voltage circuit When the 120V RMS winding is selected resistor R4 and potentiometer RV5 is connected for calibration purposes of the 0 80V range in the same manner as is used for the 0 40V range The biggest change to the circuit is that the negative voltage which earlier was used from an out of phase winding of 66V RMS has been removed all together This is then contradicting AVOs text in the patent that this voltage is necessary for the valve not to form a diode between the grid or screen to the cathode when the valve under test is not conducting Maybe this is a trade off for some reason maybe it was more expensive to have it there and have it changed in step with the grid volts switch or maybe AVO found that it was not necessary to have that voltage there Modification G Here AVO have changed the calibration resistor circuit introducing a potentiometer RV6 for calibration purposes and also changed resistor R3 to 1 22 MO AVO also changed the circuit by introducing a resistor R4 wrongly named R41 in the original
55. ead on a mean reading dc voltmeter K being a constant The above relation holds equally well for screen grid or pentode valves which would follow the general form rd Ea m la Es Ea 9 Thus with an applied rms anode and or screen voltage equal to 1 1 x and a mean value of half wave rectified bias voltage equal to 0 5x Vg dc then the valve will read a mean dc anode current equal to one half of the dc anode current taken from the static characteristics if Va dc and Vg dc were the applied dc test volts This relationship holds for all practical purposes over the full characteristic and is the basis of operation of the VT160 enabling accurate testing of valves at any point on their characteristic with simple and small apparatus This accuracy is just as necessary on the simple go no go type of instrument as on a complete characteristic meter as it may be necessary to set the test point anywhere on the characteristic to correspond to required working conditions Further in the absence of any printed or predetermined test figure it must be possible to determine test conditions directly from manufacturers published curves or data Basic Circuitry 10 The principles of operation of the main function of the tester the comparative testing of mutual conductance lie in the application of anode screen grid and heater 8 voltages corresponding to the working point of the valve and backing off
56. ectrode voltages may be made after having measured the mutual conductance of a valve This measurement should not be made where an apparent fault in the valve has previously caused the protective relay to operate possibly due to softness 77 The CIRCUIT SELECTOR switch should be set to GAS thus inserting the meter into the grid circuit of the valve where it records grid current directly in uA The meter is limited to read a maximum of 100uA but it is not possible to state the value at which a valve becomes useless due to the presence of gas The point at which gas current reaches a value great enough to affect the successful employment of the valve depends very much upon the circuitry in conjunction with which it is to be employed e g it is possible for appreciable grid current to flow in the secondary of an R F transformer connected between grid and earth but if resistance capacity coupling is used in the circuit the same magnitude of grid current may produce appreciable voltage across the bias resistor thus completely upsetting the normal functions of the circuit Checking Power Rectifiers 78 The testing of rectifying valves should ideally be associated with the requirements of the circuit in which they are to operate In most cases throughout the AVO Valve Data Manual the figure quoted denotes the standard emission per anode to be expected from the type of valve under test 79 The procedure for testing a rectifying valve is exac
57. ed on the control panel Ascertain the voltage of the mains supply which must of course be 50 500c s and set the MAINS VOLTAGE SELECTOR panel as described in Chapter 2 Connect the mains lead of the instrument to the power supply ensuring that the red wire is connected to line the black or blue wire to neutral and the green wire to earth Set the MAINS Switch on the panel to its position and observe that the panel indicator lamp is illuminated The valve to be tested should NOT be inserted at this stage Final setting of Mains Voltage Selector Panel 56 Having allowed a few moments for the instrument to warm up set the CIRCUIT SELECTOR Switch to the position SET Set the fine voltage adjustment control so that the meter needle lies in the black zone If this cannot be done the coarse control will require adjustment and should be moved to the next higher tapping if the needle is too far to the right of the mains adjustment zone and in a similar manner it should be moved to the next lower tapping if the meter needle is to the left of the mains setting zone Once the mains voltage tapping has been correctly set provided that extensive mains fluctuations do not occur test voltages are automatically correct throughout the instrument Insulation checks with the valve cold 57 1 The Valve Data Manual supplied with the instrument or the Valve Manu facturer s Data should now be consulted and the ROLLER S
58. ed resistors R15 to 23 and one variable resistor RV4 which are labelled the ANODE CURRENT controls The anode current controls perform the same action as the backing off controls do in the Mk Ill and Mk IV they effectively balance out the large standing anode current which is flowing through the 2000 resistor in the anode circuit This makes it possible for the meter to show the small current change produced by the mA V control when the mA V measurement is being performed In the CT160 the anode current controls are designed in such a way that you have to read the anode current which is flowing through the valve on the scales provided on the controls not on the meter Whereas in the AVO Mk III and Mk IV the backing off controls are not provided with any scales and the anode current is indicated on the meter The anode current controls or backing off circuit act as a power supply which is designed to produce exactly one volt for each step from OmA OV 90mA 9V on the rotary switch or coarse anode current control plus the variable OmA OV 10mA 1V from the fine anode current control in fact the maximum on the fine anode current control is actually 11 25mA or 1 125V Thus makes it possible to balance out a maximum of 10 125V which corresponds to an anode current of 101 25mA The small extra voltage on the fine anode current control is intended to produce a small overlap on each range since it is possible to
59. en the two anode current readings i e lmA over the difference between the two grid voltage readings Ve Vg The result will generally be between 4 and SmA V See Final Test Procedure For greater accuracy it is suggested that readings of grid voltage be plotted against value of anode current between 10 and 20mA and the slope taken from the curve at 16mA The valve should now be labelled as follows 200V DC 16 DC 9 9 9 99 9 9 9 8 9 1 1 9 CV 491 The valve should be re standardised daily when in use 6 4 Construction The main instrument comprises two units in a hinged transit case the lid of which is not detachable Electrical connection between the two units is effected by means of two 5 way side by side cables The adaptor box is connected to the main instrument by means of a 25 way connector 5 Removal of the Instrument From its Case See WARNING on Page 4 To facilitate servicing or calibration of the instrument it is necessary to remove both sections from the casing this being accomplished by the removal of four hexagonal headed bolts which form the feet of the control unit from the underside of the case The control panel will then be released The valve panel can be withdrawn from its section of the case by the removal of eight fixing screws around its periphery 6 Simple Faults SYMPTOMS POSSIBLE FAULT ACTION No dial light indication
60. ent of 27 passing through the resistance of the meter causes a voltage drop of 87 75mV 3 2500 27 87 75mV This means that the current flowing through the meter shunt R9 is 8 775 87 75mV 10 8 775 The total current flowing through the meter circuit is therefore 35 775 27 8 75544 the red line This current will require a voltage 47 223 volts across the calibration resistors amp 35 775 x 1 32 47 223V add this to the voltage drop across the meter 87 75mV and you therefore require a total voltage of 47 3108V In the calibration part of the AVO CT160 Service manual AVO simply state that with the Grid voltage link connected you should have 47V DC across the grid volts potentiometer which is exactly the same as taking the measurement across the calibration resistors and meter since they are connected in parallel So we did the calculation correctly only backwards and arrived at the correct voltage drop although AVO did not stipulate any decimal places here but 47 3V is the correct voltage from the calculations NB it is another common misconception with the CT160 that R3 R4 comprise a matched pair of 1 32MO resistors what meant was that the values of amp R4 should be matched so as to give an overall resistance of 1 32MQ NOTE There is no reason why a Silicon diode could not also be placed in the Anode voltage circuit somewhere after the Anode voltage selector switc
61. f an AVO Mk IV Valve tester Some more that you wanted to know about the inner workings of an AVO Valve tester All that you wanted to know about the inner workings of an AVO CT160 Valve tester AVO CT160 anode current measurements calibration resistors and the meter workings CT160A Special edition of the AVO CT160 comparison have included adapted versions of the CT160A and CT160 texts above in this document The work of editing and testing the corrections and modifications in the AVO CT160A manual and writing of the articles above has been performed by Martin Forsberg Sweden and Euan MacKenzie Australia Sweden 2010 11 18 This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 2 of 17 The following texts are adaptations done to the texts published by me Martin Forsberg and Euan MacKenzie at the UK Vintage Radio Repair and Restoration Discussion Forum and contain important information about the AVO CT160A Valve Tester as well as important information on the 160 Valve Tester which the CT160A is based on AVO CT160A Special edition of the AVO CT160 comparison This text will endeavour to explain how the AVO CT160A works in certain areas in comparison to the well known AVO CT160 Some of the information comes from Mr Yutaka Matsuzaka who kindly sent me a copy of th
62. f double sensitivity indicators giving multiple images responding to different sensitivities two sets of data where possible are given the first set referring to the more sensitive indication Checking Gas Filled Rectifiers GR 101 Gaseous rectifiers are tested in conjunction with a load resistor of suitable wattage connected across the link terminals the value of resistance being given in K Q in the remarks column This type of rectifier is not tested on the rectifier or diode test circuits but with the CIRCUIT SELECTOR set to TEST the appropriate voltage and representative anode current figures being given in the valve data columns Full wave examples of this class of valve are of course tested at the ELECTRODE SELECTOR positions and A a suitable resistor being connected across each of the two anode links The maximum loading on these rectifiers must be limited to 100mA per anode to avoid damage to the instrument Checking Cold Cathode Rectifiers CCR 102 Cold cathode rectifiers designated by the symbol CCR can be tested in a similar manner the anode voltage approximate anode current and load resistance being given in the data columns COPYRIGHT No information or diagrams in whole or in part may be copied or reproduced without the prior permission in writing of Avo Limited 36 MAINTENANCE INFORMATION MAINTENANCE INFORMATION CONTENTS TEST EQUIPMENT REQUIRED SECTION 1 Page 5 FAULT FINDING amp
63. f the U shaped stirrup B Connect the Testmeter set to a suitable range across RS Switch on and advance the SET mA V dial to a reading of 5 Rotate the RV spindle further by means of the stirrup in a clock wise direction until the Testmeter gives a reading of 105 C If this reading is achieved without further clockwise advancement of the stirrup or if its procurement necessitates an anti clockwise move ment of the stirrup then investigate the accuracy of R1 R2 R5 and RV1 D The locking nuts on the stirrup should now be tightened and the reading of 105 on the voltmeter checked Again check that the DC millivolts developed across R5 at the 2mA V and 10mA V settings of the dial are 260mV and 52 5mV 3 E Check that the dial can now be rotated to its lmA V position and that the motion is eventually arrested by the stop screw on the dial and not by the stop at the end of the potentiometer track Switch off and remove the Testmeter SECTION 7 THE INDICATING METER This is a self contained unit which may be withdrawn from the control panel by the removal of two 2BA screws When used in the instrument as an anode current null indicator the meter has a full scale deflection for approximately 10mA not critical When removed from the instrument the meter has a full scale deflec tion of and internal resistance of 3 2502 When shunted by R9 only see Circuit Diagram the meter has a full scale deflection of 3
64. grid region thus making full use of the emissive capabilities of the cathode and failing emission may well prevent oscillation taking place As a subsequent test therefore it is helpful to note the anode current at the rated test figures with the normal heater voltage applied and then to decrease the heater voltage by about 15 for a short period It may be necessary to operate the HEATER VOLTAGE TOGGLE SWITCH to give the necessary decrease in voltage In the case of a valve with failing emission the decrease in cathode temperature will result in an excessive decrease in the anode current considerably greater than the percentage decrease in heater voltage Such a result would suggest that the valve will not oscillate satisfactorily Conversely a negligible or small decrease in anode current or of the same order as the percentage change in heater volts will show that the valve is developing its full emission at the rated heater voltage and provided that the circuit conditions are correct it should oscillate normally 97 Frequency Changers Employing Separate Electrode Assemblies for oscillator and mixer functions are designated by TH triode hexode and triode pentodes The separate sections of this type of valve are not interdependent as in the case of the phantom cathode types mentioned in the previous paragraph and they must therefore be tested in two separate sections as a pentode or triode respectively THE USE
65. h This is exactly what was done by AVO in their Mk IV the circuits are more or less identical in every other aspect Two minor points though firstly it is difficult to access the anode voltage selector switch without having to remove a lot of the mechanical structure so the easiest place to fit the diode is at one of the tags for the anode current relay coil RL3 It is the middle row of the six tags and the anode coil has a much lower resistance than the two other coils between 5 and 60 Secondly as discovered it is necessary to fit a 100 resistor to earth after the diode in order to avoid any spurious voltage readings presumably due to charge stored in the reverse capacitance of the diode In the Mk and Mk IV there is an extra potentiometer for adjustment of the SET current so it is possible to put the needle exactly on the red centre line of the SET region While did the calculations for the 160 realised that it would be beneficial if you were to put such a potentiometer in the CT160 s 66V rms circuit too as it would then give you the same means of adjustment that is available in the Mk Mk IV the Mk III is more or less the same tester as the CT160 apart from some minor differences in both electrical and mechanical design Without this potentiometer in the 66Vrms circuit there is inevitably a compromise between the negative Grid voltage calibration and the calibration of the SET
66. ht hand position 1 4 80 Table 1 gives details of the 32 available heater voltages Note Where given heater voltages in parenthesis should be employed 24 TABLE OUTER RING INNER RING On load heater volts ap On load heater volts ap pearing at valve base with pearing at valve base with toggle switch set to position toggle switch set to position 625 1 25 1 4 2 5 3 4 4 5 5 5 7 6 3 7 5 10 11 12 6 13 15 16 18 20 23 25 28 30 35 40 45 48 55 70 80 117 The Anode and Screen Voltage Switches 50 These switches enable the requisite electrode voltages to be applied to screen and anodes of valves for the purpose of carrying out mutual characteristic measurements They are calibrated in the equivalent dc voltage settings and therefore no account need be taken of the actual value of AC voltage which appears at the electrodes of the valve for as already explained in Chapter 1 the actual voltage will differ from the equivalent dc value marked at the switch position The Anode Current Controls 51 This is a dual control comprising two knobs the first being continuously variable and calibrated from 0 10mA the second having an inner and outer set of calibration figures Only the outer set of figures marked in steps of 10mA from 0 90mA apply when anode current is being measured These controls enable the expected anode current from the valve under test to be set upon the i
67. ifier Where multiple electrode assemblies are concerned test figures for each assembly are given In case a valve is en countered that does not appear in the manual the base connections and manufacturer s or other recommended test data can be directly set up on the controls without any calculation or complexity KEY TO FIGURE 7 Roller Selector Switch Disc Seal Valve Holders Top Cap Connector Panel Set mA V Control Anode Current Controls Electrode Selector Circuit Selector Mains Voltage Selector Coarse Adjustment Mains Voltage Selector Fine Adjustment Indicator Lamp Fuses Screen Voltage Switch Mains on off Switch AC Mains Input Socket Anode Voltage Switch Heater Voltage Switches Negative Grid Volts Control Panel Indicating Meter Anode Links 7 4 O J tA gt 14 C JL Tc NL 797991 p TR E 244 SES n v 2 se me r 1 3 EEO MEO 23 09 MOORE vrbe Ine D ERES E AR Wn CODE S ON 5 Aue MEE n E O beac 2 E C d d Y 225 3 NOT Aa Jm ne 7 Pp YA duse a lt X
68. in parallel i e all pins numbered are wired together all pins numbered 2 and so on This wiring combination is associated with the well known AVO MULTI WAY ROLLER SELECTOR SWITCH which enables any one of the nine standard pin numbers to be connected to any one of the electrode test circuits in the instrument thus enabling any electrode combination to be set up for all Valve Holders 31 It will be seen Fig 10 that the Selector Switch comprises nine thumb rollers numbered from left to right 1 9 This numbering appears on the moulded escutcheon immediately behind the rollers and corresponds to the valve pins in the order of their standard pin numbering Thus valves with any number of pin connections up to nine can be accommodated To cater for Top Cap and other external valve connections a socket panel has been provided with nine sockets marked D1 D2 A2 Al S G H short lead is provided which is fitted with a plug for insertion into the panel whilst the remote end of the lead is fitted with a universal connection clip to cater for all types of external valve connections socket panel is fitted with two links marked A2 and to which reference is made in section 98 32 Rotation of the rollers of the Selector Switch will reveal that each roller can be set in any one of ten positions the appropriate setting being indicated in the window opening at the front of the escutcheon The ten positions of each ro
69. ing swamp KNOB ASSEMBLY 15220 Knob Special Screw Spring Washer Knob Carrier Spring Dowel COMPONENT BOARD ASSEMBLY 21589 Conponent Board Tagged Resistor 10kn 2 HSC R8 Capacitor 0 02uF 2007 working C2 Resistor 330kn 2 HSC R7 Capacitor 0 04yF 2 in series C1 Resistor 910n 1 HSC R6 Resistor lOkn 10 R36 Resistor 8ka 58 R11 Capacitor 450V working C3 Electrolytic Resistor 1 2 5 R12 13 Resistor 2000 2 5 R10 Silicon Diode Type 100 8 MR1 4 Resistor 82ko 10 R41 Potentiometer 5002 RV3 5 Potentiometer 20 10 RV6 Resistor 100ka 103 R38 Resistor 1 22 0 2 R3 Resistor 2 41kn 1 HSC R39 Resistor 2 34kn 1 HSC R1 Resistor 2202 10 R4 RELAY ASSEMBLY 40446 A Relay Board Tagged Nylock Anchor Plate 4BA Contact Contact Screw Contact Armature Spring Split Pin Spring Spring Retainer Armature Pole Piece Contact Spring Wound Bobbin RL3 Armature Retainer Wound Bobbin RL1 2 Anchor Plate 17 BENEN 8 PRE HHH NRHN Item 1000 40 U1 WN CO tn gt WW Part No 14267 1 5 745 W 39 14670 2 20245 52 14711 2 14275 2 15908 2 14681 3 W 3 14669 2 N 77 14630 1 867 14559 2 13920 2 A 1510 14632 4 5 869 13917 1 14560 1 740 14710 2 13843 10 14557 1 14558 1 14666 2 14630 1 20969 1 20902 1 20897 1 14662 1 14659 2 14660 2 12049 387 12049 382 12049
70. ing the needle which might break it off from the moving coil if you are not very careful while removing it You will then have to loosen the magnetic shunt from the glue and at the same time make sure that you do not get any of the glue residue inside the meter as it will disturb the movement of the moving coil Moving the magnetic shunt up and down a miniscule amount will adjust the magnetic flux in the magnet circuit and that will result in a higher or lower FSD current necessary for the moving coil to reach the FSD point But you might have a meter that has a too strong magnet anyway and this might not be enough to get the correct reading so I still recommend you to have the meter sent to Herts Meter Co Ltd for a proper adjustment and refurbishing The internal resistance of the AVO meters also differ between different meters myself have seen the internal resistance of the meter ranging from 3 1800 up to 3 3040 but when these meters where shunted with the internal shunt of the corresponding AVO Valve Tester where they came from the total resistance was very close to the necessary resistance of 2 4530 3 2500 in parallel with the shunt at 10 0000 is 2 452 83Q It then looks like AVO selected the shunt resistance to match each meter instead of correcting the internal resistance of the meter as long as the meter had the correct FSD current at This is only an observation have made and not something that could verify with the sources have
71. ire replacement or adjustment in service are conveniently located on sub assembly boards on the outer framework of the assembly and are immediately to hand on removing the case whilst the open framework construction used reduces weight to a minimum and ensures a maximum of accessibility Mains Supply 27 Special attention to design details has rendered an instrument suitable for operation on AC mains from 50 500 c s 10 over the following voltage ranges 105 120V 175 250 FIG 9 DIAGRAM OF CONNECTIONS FOR SPECIAL VALVE HOLDERS Showing pin connections viewed from above 17 BODL SAND 0 WHE ER urit veneni n UN tapa m ai aI DIAGRAM OF SPECIAL VALVE HOLDERS FITTED TO THE AVO VALVE TESTER TYPE 160 bases are viewed from lop of valve panel 5 6 LARGE LUG SMALL LUG FLYING LEAD ADAPTOR 5 NUVISTOR wo DH A t N 4 Ww N KEY TO FIGURE 10 Valve Holders International Octal A08 Disc seal British four and five pin B4 or B5 American loctal B8B or B8G Miniature nine pin noval or B9A British nine pin B9 Philips P type 8SC Miniature seven pin B7G Philips eight pin locking B8A Mazda Octal M08 Miniature three pin B3G American five pin UX5 American Small seven Sm7 American Medium seven pin UX7 American four pin U
72. ist on the page before the original schematic shows this as the correct order as well as an internal component placement comparison between a CT160A and a CT160 R37 amp R37 shown as R37A and R37B as per component list on page 18 of the Service Manual consisting of one 13 O selected resistor each SH6 Ground connection for tags 2 5 was missing from original CT160A schematic but can be found in later CT160 schematics and also in the original CT160A amp CT160 testers WIRES amp COMPONENTS Moved for clarity The third schematic has been modified in the following areas All of the corrections above from the second schematic plus D5 R42 amp R38 Anode voltage rectification added components used D5 D1 R42 R38 100 10 1 W this modification has been tested and proved to work in an original CT160 and is also used by AVO in the AVO Mk IV D6 07 amp C4 Meter protection circuit added D6 07 D1 C4 8uF 63VDC Non polarized preferably Polypropylene this modification has been tested and proved to work in an original AVO CT160 and is also used by AVO in the AVO Mk IV If you want to read more about the inner workings of the AVO Valve Testers including the AVO CT160 and AVO CT160A recommend that you visit the UK Vintage Radio Repair and Restoration Discussion Forum here http www vintage radio net forum index php and read the threads Everything you wanted to know about the inner workings o
73. its actual mutual conductance with the rated figure The broad procedure for obtaining this figure consists 1 Applying to the valve the recommended electrode voltages ii Backing off to zero the standing anode current thus produced 11 Applying a small incremental signal to the grid of the valve iv Assessing the mutual conductance and consequently the goodness of the valve from the resultant rise in anode current 64 Provision is made to test for mutual conductance under two conditions 1 Where the measurement is made at a predetermined fixed value of grid bias the resulting anode current being balanced out ii Where the measurement is made at the predetermined optimum value of anode current the grid bias being adjusted to give a balance 65 In either of these cases the determination of goodness can be made by a A comparison of the mutual conductance of the valve on a percentage basis with its rated figure the comparative goodness factor being indicated on a coloured good replace scale b A direct numerical determination of the valve s mutual conductance in mA V which can then be compared with the rated figure 66 In certain circumstances where for example the valve is used as an oscillator or output valve working at peak emission more useful information than is given by the usual mutual conductance figure can be gained from the anode current obtained for a given set of ele
74. k but smaller diameters would be just as suitable it s the thickness that is the more critical parameter on the outside of the existing ferromagnetic poles Fortunately it is easy to determine the correct polarity set up a mid scale deflection on the meter with a suitable external circuit eg a 1MO resistor in series with a variable transistor power supply then with a very firm grip on the disc magnet slowly approach the pole piece If the indicated current rises it s the correct pole if it falls then it s the wrong pole Final trimming to give the exact FSD value of is done via the magnetic shunt which is located under the RHS needle stop It is a hinged plated steel arm which is mounted on a neoprene bush it shorts out some of the magnetic flux across the pole gap It is or it should be cemented to the existing ferromagnet you will need to break the cement in order to adjust the shunt and then re cement it when the adjustment is correct If you need to change your meter for example if the moving coil is burnt out you will have to ensure that you find a meter with an internal resistance lower than the stipulated 3 2500 so that you have some leeway to correct it upwards AVO state below 1 6000 in their Service manual but have never found one below 2 4000 in of all of those that have tested If you have to replace the meter with one which has some other value of current and internal resistance then you will
75. ll 8 12V electrolytic this is not recommended as there is insufficient polarising voltage so they rapidly become leaky Note there is nothing magic about the value of 8uF just remember it was a standard value the industry at the time The time constant is 3 2500 10kO x 8uF 20ms so using a modern 10pF will not make any discernible difference The diodes can be the same Silicon diodes as used elsewhere in this document such as the BYW96E as these will work just as well here There have been some suggestions that Silicon diodes do not work in the CT160 meter circuit additionally it has also been alleged that using these diodes upsets the measurements can assure you that there is no problem in making these modifications to either an Mk III or the CT160 which are more or less the same construction Both and many other people have tried this modification and it has been successfully used for many years without causing any discernible reduction in the meter reading AVO themselves made these modifications to their later models of the Mk IV and also the VCM163 Admittedly the early models of the Mk IV did not have this meter protection the circuit then only shows one Silicon diode in one direction and no capacitor across the meter either However they introduced the capacitor modification in April 1960 and the two diodes across the meter in October 1960 see the AVO Mk IV Service Manual p38 This document is a collab
76. ll of the CT160 Mk Mk IV family are equally important as they provide the same function in all of these testers letting a predetermined current flow through the meter so the meter needle indicates either on or very close to the red SET line on the meter scale This will ensure that the setting for the transformer primary and therefore the secondary voltage is correct so that all the subsequent measurement will also be correct In addition the leakage and insulation tests also rely on this current through the calibration resistors This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 11 of 17 In the 160 and also the Mk Mk IV the same current is supposed to flow through the meter at the centre of the SET calibration region on the meter scale The centre of this area is usually marked by a red line which is aligned with the 90mA line or 90 of FSD However on some meters this red colour may have faded completely so you have to use either the centre of the black area as a reference or use the 90mA line Alternatively you could test your meter to check that it is giving the correct deflection compared to the scale then you can mark your own point on the scale where the needle shows 90 of FSD What is it we really we wish to accomplish with the calibration resistorsO The important thing
77. ll the other components of course otherwise you will not get the correct reading on the meter All this means that you need the calibration resistors for two purposes one is to ensure that the voltage Settings are correct so that the Anode Screen and Grid voltage controls will deliver the correct voltages in the TEST and GAS positions and also to ensure that the current at the other measurement positions is correct This dual purpose means that you need these calibration resistors to have the correct resistance for the measurements to be correct This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 12 of 17 There is really only one value of these calibration resistors that will work as you need the correct current through the meter By performing the calculation backwards and comparing the result with the AVO CT160 Service Manual it will hopefully give you an understanding of the necessity for the required resistance in the calibration resistors Let us first check the calculations with the values given by AVO for their calibration resistors The meter resistance is 3 2500 nominally and the meter shunt R9 is a resistor of 10kO these are connected in parallel and have a total resistance of 2452 80 The current through the meter when the needle is aligned with the calibration mark at 90 of FSD corresponds to 27 This curr
78. ll wave rectifiers each anode of the valve is rated independently and the setting of the ANODE CURRENT control should indicate half the total value of the current which the valve would be expected to deliver in a full wave rectifier circuit e g a valve rated at a maximum of 120mA would be tested with each anode at the 60 position on the ANODE CURRENT control The load rating given in the Valve Data Manual is the load per anode Checking Signal Diodes 85 Signal Diodes are checked in exactly the same manner as rectifiers except that the right hand ANODE CURRENT control is always set to 1 or 5 according to the anode current figure given in the Data Where Valve Data does not give a current figure for a diode it is always checked with the right hand ANODE CURRENT control set to its ImA position INSTRUCTIONS FOR TESTING SPECIFIC VALVE TYPES 86 The functions of a valve as distinct from the type number given to it by its manufacturer is indicated by a symbol in the form of letters appearing at the right of the test data given in AVO publications e g a half wave rectifier is marked R whilst full wave rectifier is designated by RR Ina similar manner diode valves are shown by the letter D the number of diode elements being indicated by the number of Ds e g DDD refers to a triple diode Multiple Diodes and Rectifiers D DD DDD R RR 87 The testing of Multiple Diodes and Rectifiers
79. ller are marked as under 1 2 3 4 5 6 7 8 9 0 C H H 5 2 DI D2 20 33 The numbers are provided in order that the switch can be rapidly set up from the code numbers given in the Valve Data Manual the corresponding electrode denominations being shown by the letter which appears in the escutcheon window immediately below the number thus amp Corresponds to Cathode or any other earthy electrode normally connected to cathode e g G3 Corresponds to Heater normally earthy or connected to negative L T in the case of a battery valve _ Corresponds to the other Heater connections or centre tap Corresponds to Control Grid Corresponds to Screen Grid or G2 To Oa Corresponds to the normal Anode of single or multiple valves In the case of an oscillator mixer valve represents the oscillator Anode Corresponds to the second Anode of double valves and in the case of oscillator mixer valves the mixer Anode 8 Corresponds to the first Diode Anode of half and full wave Signal Diode and Rectifier valves Diode and Rectifier Amplifier combinations p Corresponds to the second Diode Anode of Signal Diode and Rectifier valves Diode and Rectifier Amplifier combinations Corresponds to dis connected valve pin or to pin on which an internal electrode 13 anchored Such pins are marked in Manufacturer s literature This switch position leaves the particular valve pi
80. ls so that the voltage across each 800 resistor will still be exactly 1V If you place a standard Silicon diode in place of each valve diode remember the CV140 is a double diode so one Silicon diode will be needed for each diode then you will have to increase the series resistor R6 to approximately 935 O in the anode control circuit The calculation for this is as follows the anode current control circuit is supplied from a transformer winding of 50V rms which is equivalent to a mean voltage of 45 02V the ratio between rms and mean voltages being 1 1107 for a sinusoidal waveform which gives us 50V RMS 1 1107 45 02V Mean Now the approximate forward voltage drop for a Silicon diode is 0 7V however for a CV140 it is 2 2V as measured at the manufacturer s maximum current of 9mA so the value of R6 must be increased The anode current control circuit requires an exact current of 12 5mA derived from the 1V volt drop across each 800 resistor 1V 800 0 0125A or 12 5mA After rectification by the Silicon diode you then have a voltage of 45 02V 2 0 7V 21 81V DC mean The 45 02V has to be divided by 2 because it is a half wave rectifier the mean voltage following half wave rectification is obviously halved and the figure of 0 7 Volt comes from the forward voltage drop of the Silicon diode The total resistance in the circuit will then have to be 21 81V 12 5mA 1744 80 from this we must subtract the existing resistors in the
81. ly long period to reach working temperature the symptoms being a continual rise of anode current when the SET mAJV dial is at the SET ZERO more sensitive position Slope measurements should not be taken until a condition of stability has been reached 31 Note 4 When checking certain valves of the CV 138 type back emission sometimes occurs between the anode and the suppressor grid which is normally connected to cathode This condition which is caused by local overheating of the anode would not affect the operation of the valve under normal circuit conditions although it could give rise to doubt as to the goodness of the valve when checked on a valve testing instrument The phenomenon shows itself as an apparent gradual fall in anode current as the valve heats up and the only manner in which this effect can be overcome is to lower the power dissipation of the valve by reducing the anode voltage Note E in the Valve Data Manual relates to the above conditions and a note appears against those valve types concerned together with an alternative set of test data Note 5 In literature issued by American Valve Manufacturers the term transconductance is used in place of mutual conductance Transconductance given in micromhos divided by 1 000 gives mA V Transconductance Micromhos 1 000 i mA V Measurement of Grid Current 76 The measurement of grid current at a desired set of el
82. n dis connected from any circuit Some instruments will be found to be fitted with rollers marked 2 which can regarded as being synonymous with 9 gt Procedure for Setting up Valve Base Connections 34 The procedure for setting up a valve ready for test is as follows From some suitable source i e the AVO Valve Data Manual Valve Manufacturer s Data Leaflet or any other Manual of Valve Data determine the pin basing connections for the valve Rotate the roller of the SELECTOR SWITCH until the code number or electrode letter combination appears in the window reading from left to right in accordance with the standard pin numbering sequence see Fig 8 When a valve has less than 9 pins the free rollers on the right of the set up combination corresponds to non existent valve electrodes and should be set at 9 Insert the valve into its appropriate holder following the sequence laid down in the general procedure for testing a valve section 55 and by means of the lead provided connect any top cap or side connection on the valve to its appropriate socket on the socket panel Note that the Loctal Valve Holder which has only 8 normal electrodes has its centre spigot connected to the ninth roller corresponding to Pin No 9 to accommodate valves which have a connection made to this spigot 35 The accompanying examples show how to correlate the pin basing data and the equivalent code number for various valves in comm
83. n stators nine of which are 13 each connected to an electrode test circuit the tenth one being open circuited The rotors are in the form of edge operated rollers each having the nine electrode denominations marked in symbols round their periphery the operative selection appearing in a window Thus any valve holder with pins up to nine can be set up to any electrode combination the open circuit connection serving for valves with internally connected pins 24 The problem of self oscillation that can occur with high slope valves at random high frequency due to the inter valve holder wiring has been virtually eliminated by wiring the panel in connection loops of approximately similar length and configuration so that a valve would tend to oscillate at a frequency dependent on the line thus formed These wiring loops are then closed on themselves via a connector loaded to give high loss and thus lower the Q of the line so that oscillation is virtually impossible 25 A manual is provided with a line of data for each valve likely to be encountered giving the mutual conductance and operating voltages The data given comprises the pin combinations in the order of their standard numbering and in the form in which they appear in the roller selector switch window top cap or side contact connection if any heater volts anode volts screen volts negative grid volts operating anode current and mutual conductance or load current in the case ofa rect
84. n voltages on which the loading might vary from a fraction of a mA to over 100 mA dependent on the type of valve being tested and the nature of the test being performed Such a requirement could of course be met by the provision of a number of regulated power supplies which would render the instrument cumbersome and expensive whilst a large amount of metering would not only mean additional expense but also make the instrument difficult to use and would not entirely overcome the problem 3 It can be shown however that if alternating electrode voltages are applied in their correct proportions an amplifying valve can by virtue of its property of self rectification be caused to give DC anode and screen currents which for all practical purposes bear a constant relationship to those obtained from its DC static characteristics 4 This immediately simplifies the problems of power supply to the valve under test The design of transformers to give negligible regulation errors over the range of secondary currents involved is comparatively simple whilst the range of electrode voltages may be simply provided by a predetermined secondary tappings selected by calibrated switches thus minimising to a very large extent problems of size weight and cost and eliminating the necessity for separate metering 5 A slight difficulty occurs in the supply of the variable negative grid bias voltage which would normally consist of an alternating voltage of sui
85. need to recalculate all of the resistors in the measuring circuits in order to get it to work correctly that is too much work to do here However I have been given such a schematic of an AVO Mk III which will come back to in the future after have checked it thoroughly Another method is to install an OP AMP meter amplifier and a new meter movement If all resistors need to be recalculated in the measuring circuit the total resistance in the measuring circuit will be altered and that will make differences to the measurements as it then necessitates a change of the Anode measuring resistor resistance and the whole Valve tester will now have an altered internal resistance from the view of the valve There is also a superb webpage which shows you how to put a small magnet on the meter magnet to help it to get back to the correct flux so it will have the correct deflection again on Mr Yutaka Matsuzaka s website here http www6 wind ne jp yutak avo ct160 mk4 2 htm Mr Yutaka s main site can be found here http www6 wind ne jp yutak avo ct160 index htm There is a lot of other interesting information on calibration of the AVO CT160 there too The elusive AVO CT160A which does incorporate Silicon diodes and also has an additional switch which doubles the Grid voltage range is shown in some small detail there If you use Google Translate you can read most of the information on Mr Yutaka s webpage translated into English for instance This doc
86. nstrument and also serve as a means of final adjust ment prior to making slope measurement mA V tests These controls do in effect back off the anode current passed by the valve and to prevent overloading they should be set to the expected figure before the CIRCUIT SELECTOR switch is set to the position TEST 25 52 When checking diodes and rectifiers the inner range of figures around the right hand switch become operative enabling the operator to select the required load which is normally 1mA per anode in the case of signal diodes and from 5 120mA per anode for high vacuum power rectifiers The Negative Grid Volts Control 53 This is a continuously variable control calibrated 0 40 and marked NEG GRID VOLTS which enables the initial negative bias at which a test is made to be set at any value between 0 and minus 40 volts A times two switch enables the voltage indicated to be doubled The Set mA V Control 54 The mA V control marked 1 20mA V enables the rapid checking of the operative goodness of a valve on a replace good scale on the moving coil indicator or alternatively the direct measurement of mutual conductance in mA V 26 CHAPTER 3 OPERATING INSTRUCTIONS amp GENERAL PROCEDURE FOR TESTING A VALVE The Connection of the instrument to a supply voltage 55 Remove the mains supply lead from its storage position in the lid of the instrument and connect the cable termination to the socket provid
87. of 330V now referred to as Repetitive Reverse Maximum Fitting suitable Silicon diodes will remove this limitation so if you do wish to utilise a higher screen voltage you could disconnect the 125V wire from the screen voltage selector switch as this is the odd one out in the sense that 125V is not available on the anode voltage selector switch Then move the higher voltages back one tag on the screen switch wafer and then connect the last tag to the 400V tap on the HT transformer AVO marked the taps on their HT transformer with the mean voltage so it is labelled as H400 instead of using the customary rms value ie 440V The screen selector options will then be exactly the same as on the anode voltage selector As for suitable Silicon diodes don t fit any old diodes that just happen to be lying in your scrap box it is well worth fitting soft recovery diodes which are designed to minimise circuit switching oscillations for example Philips BYW96E which is rated at 3A and 1kV Vrrm Admittedly we don t require a forward current of 3A however the lower current soft recovery diodes don t seem to have a sufficiently high enough Vrrm 800V would be sufficient If you are going to replace the valves in the CT160 with Silicon diodes you will have to alter the series resistance R6 730 O according to the legend but in reality 750 O was used which is used to obtain the correct current flowing through the anode current contro
88. on use 21 Valve Type Roller selector Switch Code Base Diagram C Osram indirectly 4 2 3 1 0 0 0 0 heated triode G H C British 5 pin base Osram 050 full wave 0 2 0 8 0 903 0 rectifier directly heated H DI D2 H International Octal Base Mullard Pen 4 in 0 4 5 2 3 1 6 0 0 directly heated output S H C A pentode Bricish 7 pin base Brimar 6K8 indirectly 0 2 7 5 4 6 3 1 0 heated frequency 2 5 G A H C changer Top Cap GI International Octal Base Mullard TDD2A battery 6 8 2 3 9 0 0 0 0 double diode triode DI H H D2 British 5 pin base Top Cap GI Mullard EF50 indirect 2 5 6 1 O 1 4 0 3 ly heated HF pentod 5 A C C G H B9G base Note 36 When the SELECTOR SWITCH setting is derived from manufacturers Data and a pin connection is shown as I C internally connected the roller appertaining to this pin should be set to 2 Where a pin connection is an electrode normally connected to cathode e g G3 the roller corresponding to this pin should be set to 22 Provision for New Valve Bases 37 Although the Valve Panel caters for all valves in common use the possibility has not been overlooked that new valves and corresponding new bases may appear on the market from time to time Should such a situation arise plug in adaptors will become available commercially which will enable the
89. oration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 16 of 17 Part 6 What to do when FSD current is not or the needle sticks on the AVO meter If you are unlucky and have a meter that does not have an FSD current that is either lower or higher or your needle sticks at certain points while moving over the scale you need to have that meter serviced before you do any calibration or use your tester It is very common that the meter needs a higher current at the FSD point and across the whole scale and this is due to that the magnet has lost some magnetic flux over the years If your meter needs a higher current or sticks you can send it to the only company that know of that can re magnetise the meter which is Herts Meter Co Ltd Unit 10 Berry Road Hatfield England AL10 8BJ but of course there may be other companies around Herts Meter Co Ltd Will also clean your meter fix the balance and the needle jewel bearings and treat the glass with antistatic spray and also re glue the glass if necessary A meter that has been refurbished by Herts Meter Co Ltd is like new when you have it back from them There is some information on the Internet which says that there are two types of meters in the AVO Valve Testers one at 30A FSD and one at and with different internal resistance than the stated 3 2500 To the best of my knowledge and from
90. plied For this test the ELECTRODE SELECTOR is set to C H 46 With the CIRCUIT SELECTOR switch set to TEST and in conjunction with the ELECTRODE SELECTOR ANODE CURRENT and other relevant controls the valve is tested for its normal characteristics the majority of the information being obtained from the setting of controls the meter being used as a form of null indicator 47 At the position GAS the meter is connected in series with the control grid connection and directly indicates gas current in The Electrode Selector 48 This switch used in conjunction with the CIRCUIT SELECTOR enables the meter to be associated with the anode circuit under test with the exception of test position C H This latter setting is used only in conjunction with the CIRCUIT SELECTOR set to C H for the measurement of cathode heater insulation Triodes pentodes and multiple grid valves are checked with the ELECTRODE SELECTOR set to or whilst diodes and rectifiers are checked at positions D and Dp The Heater Voltage Switches 49 Heater voltages are selected by means of two switches the first being a simple toggle switch marked gt 625 117 and 1 4 80 the second being an 18 way switch sur rounded by two sets of calibration figures The outer set of voltages on the latter can be selected when the toggle switch is in its left hand position 625 117 whilst the inner set of voltages can be selected when the toggle switch is in its rig
91. ploying the normal oscillator section as a phantom cathode for the mixer section are not very satisfactorily tested in two sections since the nature of the valve construction is such that each section is dependent upon the other for its correct operation For test purposes therefore this valve is shown connected as a pentode or triode for which where possible anode current and or mutual conductance figures are given Such valves are indicated by the letter in the type column 94 An exception to this type of valve is the Octode designated by O in the type column which as will be seen from the Data is normally tested as if it had two separate electrode assemblies separate data being given for each In this case the oscillator section is tested with the ELECTRODE SELECTOR Switch at and the mixer section at A 34 95 As a further test to ensure the probability of such a valve oscillating satisfactorily an indication of failing emission will probably give the most useful results When a valve is perfect its cathode will develop its full emission at the rated heater voltage and any change in the cathode temperature will result at the most in a corresponding percentage change in emission If however the emission of the cathode is failing then an increase or decrease in the cathode temperature will result in a higher percentage change in emission 96 When a valve is oscillating it tends to run into the positive
92. poses Appendix The calculation of the total resistance of the Grid voltage and mA V control circuit The meter has a total internal resistance of 3 2500 which is in parallel with the shunt resistor R9 which has a resistance of 10kQ results in a total resistance of 2 452 80 The two calibration resistors have a combined resistance of 1 32 MO add these together and you have a total resistance of 1 322 4530 in this leg The other leg of the circuit has the mA V control which has a total resistance of 70Q in parallel with RV1 2 5000 plus the 2 3400 in series which equal 2408 10 This resistance is in parallel with the Grid volts potentiometer RV2 which has a resistance of 10 which incidentally is a special law not log this makes the total resistance of 1940 75 Ohm for this leg of the circuit When you put this leg in parallel with the other leg 1 322 you get a total resistance of 1937 90 Q E D This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 AVO VALVE TESTER Type 160A AND ADAPTOR UNIT Type 160A 1 OPERATING S VALVE DATA MAINTENANCE MANUAL AVO Ltd AVOCET HOUSE DOVER KENT ENGLAND 1 V091 AdAL LINN Pu 091 UALSAL AA TVA OAV FHL ar pO um t ames t i CHAPTER 1 1 2 9 10 21 22 25 26 27 CHAPTER 2
93. r the third diode the SELECTOR Switch should be altered to read 023 180 000 and the ELECTRODE SELECTOR set to position D Diodes and Rectifiers combined with other Electrode Assemblies DT DDT DP DDP DTP 89 Combined diode and amplifying valves are represented in the type column of the Data by and DDT for diode triodes and double diode triodes whilst DP and DDP indicate diode pentodes and double diode pentodes 90 The testing of the separate sections of each valve is carried out in rotation the amplifying sections being tested first with the CIRCUIT SELECTOR at TEST and the ELECTRODE SELECTOR at position A 91 The rotation of the ELECTRODE SELECTOR to the D or D position will automatically set the instrument in readiness for testing one or both diodes with the right hand ANODE CURRENT control set to 1 on the inner scale Double Triodes and Double Pentodes TT PP 92 Double Triodes or Double Pentodes are indicated by the letters each section being tested in the normal manner the selection of each assembly being made by the rotation of the ELECTRODE SELECTOR Switch to A and corresponding to the ROLLER SELECTOR SWITCH numbers 6 and 7 When double valves are used in Class B or balanced circuits a close match of the characteristics of both halves is essential Frequency Changers TP 93 Frequency Changers of the Heptode and Hexode classes em
94. reen voltage the anode current varies accordingly Insert a CV491 12AU7 strapped as single triode and standardised for mutual conductance at 16mA anode current with 200V applied to the anode connect the AvoMeter on 10mA AC range in place of the Al link Then check that SELECTOR SWITCH SET TO 641 226 413 FOR STANDARDIZATION PURPOSES 1 For 16mA anode current on the VT160A the AvoMeter reading is 8 11 Negative grid volts is 5 of the standard figure 111 Mutual conductance is 7 of the standard figure NOTE The mutual conductance figure for the CV491 will normally be in the order of 4mA per volt at an anode current of 16mA The tolerance specified for measurement of mutual conductance on the applies only to this point on the mutual conductance control Check that with the anode current set up to 8mA on the external meter the Valve Tester meter indicates a nominal figure of 16mA Remove the CV491 and with the bias control set at 40 connect a 680kn 5 resistor between G and C connections on top cap board SET grid volts SWITCH TO X2 Check that with the circuit selector switch set to Gas the Valve Tester meter shows f s d within 20 Insert a U52 in the appropriate socket set the circuit selector switch to test and electrode selector switch to D1 Insert the AvoMeter on appropriate DC current range in series with the load from the top cap board D1 to the rectifier anode Check that
95. rement of Grid Current Checking Power Rectifiers Checking Signal Diodes Instructions for testing specific valve types Multiple Diodes and Rectifiers D DD DDD R Diodes and Rectifiers combined with other electrode assemblies DT DDT DP DDP DTP Double Triodes and Double Pentodes TT PP Frequency Changers H TH O TP The use of the Links on the Valve Panel of the instrument Checking Tuning Indicators TI 3 m Checking Gasfilled Rectifiers GR Checking Cold Cathode Rectifiers CCR 5 CHAPTER 1 TECHNICAL DESCRIPTION Introduction 1 Whilst good bad testing on a semi production basis will undoubtedly be the major use for this tester it is certain that the instrument will find considerable use in laboratories and service departments where engineers and skilled personnel will be available and where more precise details of valve performance can be used to advantage To this end additional facilities on the tester enable Ia Va Ia Vs and la Vg characteristics to be plotted over a wide range of voltages these being readily available from the calibrated panel controls Principles of Operation 2 The tester is basically designed to check the valve according to its static charac teristics which would normally require the provision of the requisite range of variable DC supplies The difficulty lies in the regulation problems involved in the supply of the wide range of DC anode and scree
96. rent control in the UK Vintage Radio Repair and Restoration Discussion Forum and only confirms our calculations there The current through the Anode Current controls needs to be exactly 12 5mA and when the valve is replaced with a Silicon diode with a much lower voltage drop the difference in voltage drop will have to be compensated for with an increase in resistor R6 Modification I AVO also changed the circuit by introducing a resistor R38 in the Screen Volts control to ground this has earlier been mentioned as necessary for shorting the charge buildup inside the Silicon diode when it is not conducting As has earlier been mentioned it is possible to insert a Silicon diode in the Anode Volts circuit just after the Anode Volts switch before the Electrode Selector switch plus an extra resistor of 100 KO 10 at 1 Watt this resistor needs to be there for the same reason as R38 above New Valve Sockets The extra Valve Holder panel that is connected via a cable to the Valve Holder panel on the CT160A box have been equipped with the following valve sockets Pencil Tubes 5 Wires in Line Flying Lead 7 Wires in Line B7A B9D Special Base for 2C39 Acorn Nuvistor F8 and B8D This finishes the current comparison between the AVO CT160 and the AVO CT160A Part 2 Modifications done by AVO to the over current relay protection Here is some more new information as have studied the soon finished AVO CT160A manual in some more depth
97. rtin Forsberg amp Euan MacKenzie 2010 11 18 Page 15 of 17 Part 5 Meter protection Since the circuit of the AVO CT160 is more or less the same as the Mk III and not very different from the later AVO Mk IV it is possible to protect the meter by the same means used in these other AVO testers You can fit the exactly the same circuit as used by AVO in the Mk IV to protect the meter somewhat without affecting the readings in any way You will have to use Silicon diodes across the meter not Germanium diodes as those have a too low voltage drop which will affect the meter reading making it non linear As in the AVO Mk IV you fit two Silicon diodes one in each direction across the meter and also a non polarised capacitor With AVO s microammeter removed a measured current of 100mA was set up the anode circuit a 4 load on 400V and both ANODE CURRENT CONTROLS were deliberately left on zero With the protection circuit installed using a DMM the maximum measured voltage across the meter terminals was 0 27V with the DMM set to current instead the corresponding current through the meter was 864A However without the protection diodes the maximum voltage was 3 03V and the current was 0 30 a ten times overload You can use an 8uF capacitor preferably a polypropylene type or failing that a non polarised electrolytic just be sure to use a good long life model with very low leakage originally used a sma
98. s should be investigated further before can recommend changing the potentiometer for another value but my guess is that the change doesn t matter as it is the voltage delivered between the two points that form the backing off circuit is what the measurement is compared with and since that is unchanged so then is the current and resistance in that circuit path However there also the possibility to put a potentiometer with a higher value in and then shorting out the last part of the track above 900 and then make a new scale to fit the new potentiometer then this will work just as well as the old one except that the new scale will be more cramped For the anode current control measurements to be accurate you must ensure that the remainder of the components in the AVO CT160 are within tolerance and also that the tester is calibrated however for the anode current controls in themselves to be accurate you must ensure that a voltage of 1V Mean DC is developed across each of the 800 resistors The other diodes in the CV140 valves V1 amp V2 can also be replaced with Silicon diodes and fortunately the value used for the SET Vg potentiometer RV3 is large enough so that nothing needs to be changed or added there Each rectifier in the CV140 has a forward voltage drop of approximately 2 2 2 9 V at the currents involved which range from close to 11 to up to 14mA through each diode whereas a Silicon diode has close to 0 7V at these
99. scale is zoned in three colours green from 130 to 70 indicating a good value white from 70 to 50 representing a failing valve and red below 50 indicative of a reject Thus the operating procedure after backing off the initial anode current is to set control dVg calibrated in mA V to a maximum of 20 to the rated mutual conductance and note the deflection on the coloured scale of M to determine valve goodness 12 This arrangement which gives an incremental grid voltage inversely proportional to slope avoids errors otherwise likely to occur on high slope valves which often exhibit marked curvature of characteristic 13 The stopper diode D3 shown in the screen supply circuit in figure is to prevent erroneous results and possibly valve overheating and damage that can occur when testing certain beam tetrode valves with alternating anode and screen voltages 14 As the applied electrode voltages approach zero during a portion of the operative cycle the focusing of the electrode beam is to some extent upset with the result that the screen current decreases and rapidly becomes negative with consequent rapid and continuously increasing anode current The rectifier whilst presenting negligibly low forward impedance to normal screen current by virtue of its high reverse impedance successfully prevents the flow of reverse screen current C Rs RL lt gt E lt gt gt Mo
100. shown in Fig 2 Ferroxcube is also used on leads feeding the selector switch as a precaution against IF oscillation The Adaptor Unit Type 160A 1 is similar in construction to the valveholder panel fitted to the main instrument except that it caters for the more unusual type of valves i e those with B7A BSA and Nuvistor valve bases etc The Unit is connected to the nain instrument by means of a 25 way cable which is stored in a compartment within the Adaptor Unit when not required When not in use the Operating Valve Data and Maintenance Manual is housed in the removable lid of the Adaptor Unit and is held in position by means of a retaining strip Where it is necessary to replace valve holders these with the exception of the B8B are fitted to the panel with nuts and bolts and are thus easily remov able Care should be taken to replace all wire in its original position 10 Removal and Replacement of Knobs and Setting of Knob Skirts To remove any knob remove 6Ba screw and spring washer To remove knob spindle and skirt release locking pin The switch nut is now accessible To adjust skirt slacken lock nut rotate skirt to desired position and re tighten lock nut Reverse procedure to replace SECTION 5 VOLTAGE CHECKS WITH NO VALVE UNDER TEST Connect instrument to known 220 230V 50 c s supply ensuring that the mains On Off switch is in the Off position and adjust coarse and fine settings of the mains voltage selector p
101. sisting of one 13 Ohm selected resistor each 709 SH6 Ground connection for tags 2 5 D5 R42 amp R38 Anode voltage rectification added D5 D1 R42 R38 100 KOhm 10 3 Watt D6 D7 amp C4 Meter protection circuit added D6 D7 D1 C4 8uF 63VDC Non polarized R29 R28 R27 WIRES amp COMPONENTS Moved for clarity 440V 12 625 117 fo HEATER VOLTS SD R23 R22 R21 R20 R19 R18 R17 R16 R15 2207 pu S GRID1 SCREEN 8 CIRCUIT R11 SELECTOR R36 ps CLOSED Vg X1 5 Va OPEN Vg X2 DIODE 4 SCREEN VOLTS SE DIODE 2 Cm UA NEST ANODE1 H CONWNA 4 5 29 8 5 ANODE 2 MAINS INPUT CIRCUIT SELECTOR SG 105 250 50 500Hz EN 02 R13 ELECTRODE SELECTOR SH CATHODE H H Valve Tester type 160A Circuit Diagram Redrawn Corrected amp Modified
102. sting position the mA V potentiometer RV1 is turned to a position where the wiper which is connected to common ground is connecting the positive side of the negative grid voltage supply to the common ground and at the same time shorting R5 This is done so that the extra voltage from the mA V control which is added during the gm measurements will be zero when the large standing Anode current is balanced out which in itself is a necessary prerequisite for the gm measurements to be performed When the large standing current has been balanced out by the Anode Current controls or backed off the gm measurement can commence When the mA V dial is in its resting position ie turned fully anti clockwise by the spring mechanism the switch SW3 is closed and R14 is shorted When the mA V dial is moved to the SET ZERO region the switch SW3 is opened and the resistor R14 22 kQ is inserted in the Anode Current circuit this results in a very small voltage drop from the current then flowing through the circuit usually below 1 For example a current of 1 will result in a maximum voltage drop of 22mV which corresponds to a standing Anode current of 0 22mA If the balancing out of the Anode Current controls has been exact then no current will flow However if this is not the case any residual current flowing in the circuit can now be balanced out by fine adjustment of the Anode Current control thus enabling the balancing out procedure
103. t flow through each branch which Ohm s law dictates that the resistances must be the same as the voltage drop across the resistance and meter is the same when they are connected in parallel When you are going to make the measurements on the AVO CT160 meter recommend that you use a small 1 2V NiMH battery a few 1 resistors and three potentiometers preferably 10 turn models for easier adjustment one switch plus a DMM or an AVO 8 and some cables with crocodile clips fora quick hook up of all components to check the meter movement The meter in the CT160 has a total internal resistance of approximately 32500 So to protect the meter from overload by a too high current you will need to have a total resistance in the measuring circuit that will only allow a maximum current close to to be drawn With a standard 1 2V NiMH that means a total resistance of 40kO when the meter resistance is subtracted that leaves 36 7500 With the following resistors in series R1 at 18kO plus R2 at 15kO plus a trim potentiometer RV1 with a resistance of 10kO you can vary the resistance from 33kO up to 43 kO which means that the current can be varied from 26 to Then you put another 10 10 turn potentiometer in series with this so that you can make fine adjustments to the current through the meter to check the FSD value It is best to use a trim potentiometer for RV1 so you don t accidentally change the current to a too high a
104. t to the instrument 20 The final position of the CIRCUIT SELECTOR marked GAS places the meter M shunted as a microammeter in series with a resistor in the grid circuit of the valve being tested and allows the direct measurement of grid current in 12 21 Further examination of the circuit diagram will show the inclusion of a safety relay with windings RL2 and RL3 inthe screen and anode circuits respectively Overloads due to conditions of valve failure or misuse associated with either or both of these circuits will trip the relay This connects a high resistance lamp in series with the transformer primary windings assisted by a hold off winding RLI at the same time making a red warning light visible through the meter scale This places the whole instrument in a safety condition Normal working cannot be restored until the instrument has been switched off the fault removed and the instrument switched on again The Valve Holder Panel 22 The valve holder panel by means of which the valve is connected to the test circuit comprise a total of 32 valve bases covering all valves likely to be encountered in normal use including disc seal and wire ended valves The holders are wired with their correspond ing sockets according to standard numbering in parallel FIG 6 INSTRUMENT CHASSIS 23 The wire connection loops thus formed are connected to the wiper rotors of the multi way selector switch Associated with these rotors are te
105. table magnitude applied in anti phase to the anode voltage Since rectification occurs at the anode and screen and the grid should pass no current it will be readily seen that during the half cycle where the anode and screen are passing no current a positive half cycle of considerable magnitude is applied to the grid with the result that the latter can pass damaging current and in certain circuit conditions phase changes can occur that disturb the 180 relationship between the anode and grid voltages during the operative half cycle 6 Since no current is taken from the grid voltage supply however and the voltages involved are not very high the inclusion of a simple half wave rectifier without smoothing between the transformer winding and the variable grid volts supply will s suppress the positive half cycle whilst still maintaining the sinusoidal form of the operative negative half cycle K 7 Using the simple expression for the anode current of a triode Ia Ea wpEg a and transforming this for ac operation on the positive half cycle of applied anode volts we have K o QU X oUm 1 Ja mean Ea sin ote Eg sin ot Ra QU 0 0 8 Deriving this in terms of rms applied voltages and remembering that anode current flows only on the positive half cycle and will be read on a mean reading dc meter we K RMS DC Ia mean Ea uEg x2 2Ra 1 1 Eg dc is the applied Bu sine wave dc as r
106. the later AVO Mk IV in many areas but not all Apart from the looks on the front panels and the extra valve holder panel connected via the new contact on the original valve holder panel most modifications are easy to describe and understand if you understand how the original AVO CT160 works Modifications that AVO have done to the AVO CT160A are A All valves have been replaced with Silicon diodes D1 D4 making it necessary for further modifications described below to compensate for different voltage drop than valves and also for different behaviour compared to valves B The range for current capabilities for rectifier valves have been extended up to 180mA this simple modification will be described below in the part describing the over current relay modification as they are closely connected R40 modifications for over current relay RL3 C Since the valves have been disposed of the 6 3V marked as 5 8V in the AVO CT160 schematic winding for the heaters have been removed from the schematic for the Heater voltage transformer D The 500 O resistor in the D R circuit have been replaced with two 1 KO resistor to handle the higher current range R12 amp R13 This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 3 of 17 E The overcurrent relay winding RL3 have been shunted with a resistor for the 120mA and 180mA ranges an e
107. the relay coil which has a resistance close to 1 5 an AVO Mk IV this will also mean a 57 decrease in current sensitivity The maths for this is simply This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 6 of 17 3 9 in parallel with 3 9 gives you 1 95 This then gives the current through the relay coil RL3 to be 1 95 1 95 1 5 1 95 3 45 56 5 If you check the calculations above and take all decimals into account you will actually see that these two calculations give exactly the same result the factor between the numbers is exactly three and one third 3 333 That was what meant with the fantastically well correspondance between the Mk IV and the AVO CT160A here in this circuit Is this just a coincidence or something that AVO planned My guess is that it is planned but the resistance for the resistors and RL3 will of course vary between each tester but in the AVO CT160A service manual AVO says that these resistors will be selected which probably means that they are selected to work in each AVO CT160A tester being built AVO stated the tolerances for these resistors to be 10 in the AVO Mk IV and 20 in the AVO CT160A This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18
108. tly the same as that for a valve with one or more grids to the end of the insulation checks 80 From this point onwards before setting the CIRCUIT SELECTOR to the position TEST the right hand ANODE CURRENT control switch reading on the inner set of figures should be set to the load current given for the valve in the Valve Data 32 Manual Set CIRCUIT SELECTOR to TEST and the ELECTRODE SELECTOR either to D in the case of a half wave rectifier or Dj and D for a full wave rectifier 81 Having correctly set up the controls as explained above the goodness factor of the valve under test will be shown on the coloured replace good scale of the meter 82 The inner ring of figures on the ANODE CURRENT Control relate to load currents are marked Diodes and Rectifiers the figures marked being the emission in mA expected per anode of the valve under test 83 The setting of this control can either be determined from the tabulated data given or can alternatively be related to the total current that a valve is required to deliver Thus on a piece of equipment where the total drain on the rectifier is SOmA then a rectifier load setting of 60 will be an adequate test of the valve s emissive state assuming that it is a half wave rectifier Alternatively if the valve is a new one the maker s rating for maximum load current can be used as the basis for the setting of the ANODE CURRENT switch 84 In the case of fu
109. to be more precise This will ensure that when the gm measurement is performed when the mA V dial is turned into the gm measurement zone the measurement will be more exact You will also be able to get a more precise measurement from the Anode Current controls after this fine adjustment have been performed This fine adjustment procedure in no way effects the large standing current flowing through the valve during the testing as the large Anode current is still flowing as long as the tester is in the TEST position as only in this position the Anode voltage is applied to the valve under test So releasing the dial and letting it go back to the resting position still leaves the large standing Anode current flowing through the valve the only difference in the position of the mA V dial is how much the grid voltage is changed from zero to 0 260mV this is the extra voltage necessary for the gm measurement to be performed Part 3 Calibration resistors To make sure that it is possible to calibrate the CT160 and this also applies to the AVO Mk III and Mk IV you will need to ensure that all the components are within the tolerance levels that AVO specified not only the calibration resistors but it is obviously worthless to have the correct value for the calibration resistors if the rest of the tester is not within tolerance especially the meter anode current controls and 2000 resistor in the anode circuit The calibration resistors in a
110. to zero the standing anode current thus obtained A small incremental bias is applied to the valve and the change in anode current thus obtained is a measure of the mutual conductance of the valve The figure is then compared with the correct mutual conductance to give comparative goodness on a coloured scale Va D3 lt gt c 009000 FIG 1 BASIC CIRCUIT FOR CHECKING MUTUAL CHARACTERISTICS 11 Figure 1 shows the fundamental circuit used in this measurement With the requisite electrode voltages applied to the valve the half wave anode current causes a voltage drop in the resistor RL which is sufficiently low resistance not to influence the characteristics This voltage is backed off by a voltage of similar form from the Control Vb The voltage difference across the two arms of the bridge thus formed is shown on the DC millivoltmeter M When this difference is zero the voltage Vb is measure of the anode current in RL Ia V and the control Vb is thus calibrated in mA anode current RL A small change in bias is then applied to the valve from control dVg which causes an increased voltage drop in RL which unbalances the bridge This unbalance is shown on M and is a measure of the mutual conductance of the valve For a deflection on M of RL millivolts then the mutual conductance of the valve in mA V is volts In practice dVg the f s d of M is 130 RL millivolts and the
111. trols supplying the appropriate electrode voltages only three controls are really involved in a measurement The CIRCUIT SELECTOR rotates through the various insulation checks to position TEST at which point the circuit is operative for mutual conductance testing by backing off with the ANODE CURRENT controls and by setting control SET mA V at which point the meter shows the valve goodness Separate electrode systems of dual or multiple valves are measured by setting the ELECTRODE SELECTOR to Al or 2 With this switch to D1 or D2 the circuit is ready for rectifier or signal diode testing with the CIRCUIT SELECTOR at TEST The selection of load current is made by rotating the anode current control also separately scaled in rectifier load current to the appropriate position valve goodness being immediately shown on the same coloured meter scale MA 001066 MICI 5555 2 aM maar Ne 5 2 1 MAINS 1 05 230v 50 300 CIRCUIT T SELECTOR FIG 5 19 The rotary control SET mA V is of the spring return type and once a test has been made automatically returns to its start position at which point the measuring circuit is shunted to a safety condition Thus if a subsequent test is carelessly attempted with circuit wrongly set or if for instance gassy valve is tested this will be shown up before the circuit is put in a sensitive condition for mutual conductance testing and no damage will resul
112. ument is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Martin Forsberg amp Euan MacKenzie 2010 11 18 Page 14 of 17 have included a PDF file for making meter movement measurements of the CT160 FSD moving coil meter the schematic shows a simple means of measuring the FSD and the internal resistance of the meter Before you make any measurements in this circuit you should short the two connections where the CT160 meter will be tested so you can set the maximum current in the circuit with the DMM AVO connected in series with the circuit This will protect the CT160 meter from overload if you have set the potentiometers too low which could result in a too high current flowing through the circuit possibly destroying the CT160 meter The meter FSD can be checked with the switch SW1 in the open position and the internal resistance of the meter can be checked with switch SW1 closed This is done by first having switch SW1 open and setting the FSD of the meter and then by closing SW1 and adjusting the potentiometer across the meter until the meter shows 50 of FSD with the needle pointing straight up then open switch SW1 again and measure the resistance of the potentiometer which will correspond exactly to the internal resistance of the meter This will give you the exact figure for the internal resistance since the meter and potentiometer work as a Current splitter letting exactly 50 of the curren
113. uzz when the instrument is switched on again the valve is probably soft gassy and the test should proceed no further If upon removing the offending valve the relay continues to operate the instrument should be switched off When switched on again the instrument should function normally To check relative goodness of Valve in conjunction with Coloured Comparison Scale 70 a Using recommended anode current i Do not alter ANODE CURRENT controls but adjust NEG GRID VOLTS control until meter is balanced to zero 1 Slowly rotate SET mA V control to SET ZERO position and make any final adjustment to zero using fine ANODE CURRENT control See Note 3 iii Continue rotation of SET mA V control to expected value of mA V meter needle should rise iv The comparative goodness of the valve will now be shown by the position of meter needle on coloured scale This scale is divided into three zones and all valves coming within the green portion can be regarded as satisfactory Readings in the intermediate zone between the red and green sections are not entirely satisfactory although the valve may be capable of working in some circuits at lowered efficiency whilst readings in the red zone indicate that the valve should be rejected or replaced 71 b Using recommended negative grid voltage i Do not alter NEG GRID VOLTS control but adjust ANODE CURRENT controls until meter is balanced to zero ii Slowly rotate SET mA
114. value and use regular 10 turn potentiometers for RV2 amp If you wish you could use a locking nut on these adjusted so that there is just sufficient friction to prevent any accidental rotation when you let go of the shaft Now you should short the points where the CT160 meter will be connected later and set both of the 10 kO 10 turn potentiometers to their minimum values and connect the DMM in the position shown in the schematic Set the DMM to its minimum current range which is usually 200A then adjust RV1 the first 10 10 turn potentiometer so that the current is 2 Now try RV2 the second 10 10 tum potentiometer to see that you can vary the current below and then leave it in a position below If this works ok you can remove the short and install the CT160 meter note which connection should go to the electrode closest to the battery the positive one and which should go to the DMM side of the connection the negative one By adjusting the second 10 kO 10 turn potentiometer and looking at the meter you can adjust the FSD point and then check the DMM AVO for the current drawn If you can t obtain FSD with RV1 set at 32uA then you will need to adjust your meter either by having it sent to a professional company to be re magnetised or by trying to add extra magnets as is described earlier This document is a collaboration between Martin forsberg Sweden and Euan MacKenzie Australia Copyright Ma
115. wns are covered with the exception of cathode to grid which case is covered in a later check Thus a reading on the meter of 1 M when the SELECTOR Switch is set to A R and the ELECTRODE SELECTOR switch is set to can only indicate a breakdown from anode 1 to grid provided that breakdowns are not indicated in any other insulation test with heater either hot or cold It is therefore apparent that it is possible to deduce between which electrodes a breakdown is occurring although this information is normally never required for in general any appreciable inter electrode breakdown will render the valve useless Insulation checks with the valve hot 60 All the tests referred to in section 58 were carried out with the valve heater cold The CIRCUIT SELECTOR Switch should now be set to CH R and a short time allowed to elapse to enable the valve to reach working temperature With the ELECTRODE SELECTOR Switch set to Aj and D in turn any deflection will denote in M Q the amount of insulation breakdown which occurs with cathode and heater strapped together to any other electrode 61 Table 3 sets out below the manner in which these insulation checks are made TABLE 3 Circuit Selector Switch Position CH R CH R CH R Electrode Selector Switch Position Switch Postion CHR Insulation Check Checks insulation cathode and heater to 5 Checks insulation cathode and heater to Dj
116. xtra switch wafer for the SET la amp DR switch have been added for this modification R37 amp SJ1 unfortunately in my opinion re numbered by AVO to SJ1 as it would logically have been SJ5 as SJ1 SJ4 are already present in the AVO CT160 F The grid volts control have been modified with a switch for the capability of doubled voltage up to 80V the circuit have also been modified more details of this in the text below R4 R39 RV5 and SL1 amp 2 have been introduced plus a new winding on the Anode amp Screen volts transformer G The calibration resistor circuit have been modified due to the modification done to the grid Volts control this modification is also described below D2 RV6 R41 and R3 plus 99V RMS winding from Anode Screen volts transformer old R4 removed from circuit H The Anode Current controls have had the series resistor changed to compensate for the current through the circuit when a Silicon diode have been inserted R6 The Silicon diode in the Screen volts circuit have made it necessary to introduce an extra resistor to ground R38 In all other aspects the modification status of the AVO CT160A corresponds to the last modification status of the AVO CT160 up to resistor R36 for the Grid to Cathode connection Modification A amp C Replacing the valves with Silicon diodes meant that the extra heater winding is not necessary any more and it has been removed from the schematics but since no actual AVO

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