Acorn Electron Service Manual
Contents
1. Main PCB circuit diagram PLZ KEYBOARD CONNECTOR bo Di ba b3 CAPS LOCK 24 At Ata AB AS AO An A3 5 RETURN I8VAC REF K 5v 5y 5 R66 F FITTED AK FITTED RSP i NOT FITTED 4K ERXEE 4415 15 R7O OT FITTED 4x7 34 ga o 37 gam 40 e am 2 Ram WR RAS TAS EA RAZ RAS RAS LAS gaz aoo gt gt gt m NOT FITTED x6b2 KBOS CAPS LOCK 1C 17 NOT FITTED 1017 e Js ICA 4 74 500 gt YPP Y gt 235 BREDA 2 3 BASIC ROM 590 TOSHGA oo Inn 567 ON cB 74504 gt r lt u POZA AUREA AREA 74504 tavic VAC AS RETURN RETURN Ful EXPANSION CONNECTOR esr wor lt d gt 2 gt gt P PY Nga kun gt A aia Poa for PDS 4 PDS V 43 r07 E 0 gour 39 TRA Da 30 ef AIN 55 lt ov 2 NTSC 14 3 WIE PAL 17 7345M 0 51310 0 Ic 74504 NOTES X ITEMS ONLY FITTED IF 72415165 15 USED Im ICD POSITION ALL DIODES ARE 14 48 Sy 258 ISI 5 vec cas our 150 Ski CASBETTA CONNECTOR2. Electron Serv ice Manual Electron Service Manual Part no 0405001 I ssue no Date January 1987 O Copyright Acorn Computers Limited 1984 Neither the whole or any part of the information contained in or the product described in this manual may be adapted or reproduced in any material form except with the prior written approval of Acorn Computers Limited Acorn Computers The product described in this manual and products for use with it are subject to continuous development and improvement All information of a technical nature and particulars of the product and its use including the information and particulars in this manual are given by Acorn Computers in good faith However it is acknowledged that there may be errors or omissions in this manual A list of details of any amendments or revisions to this manual can be obtained upon request from Acorn Computers Technical Enquiries Acorn Computers welcome comments and suggestions relating to the product and this manual All correspondence should be addressed to Technical Enquiries Acorn Computers Limited Fulbourn Road Cherry Hinton Cambridge CB1 4JN All maintenance and service on the product must be carried out by Acorn Computers authorised dealers Acorn Computers can accept no liability whatsoever for any loss or damage caused by service or maintenance by unauthorised personnel This manual is intended only to assist the reader in the use of the product and therefor
3. EAS RC C 10K 4 De CAS IN 53 i MSE F45_ GBF sound off 162 2 Sar ros CONNECTOR BLUE I L Bs as eso Pr LIL e P Ros AN Wee 7 UHF Our GREEN POR a C SYNC IK jene SKB BLUE sP 369 aer I 2 aa IC1 pios CHANNEL umi 233 Din PAL SELECT bazz carla NTSC ov C24 oo f i LKS NTSC ONLY 33 BKAL sagwan E Di Cy SAAL 09 re Kr Main PCB layout Main PCB silk screen SOOO 000 0 OL 351933 SI m o e PA 0 el o ma 215 AO Oty 69u 834 98 A aa Caa a 256555 8 E zo v Eid l l n A SE HE NI 2 4 ie yS 9 M d y 200 w 8tH 99 Y e 5D Es Bs Bd bed fd olz zio LLO T SO alakos 3 3 21 921 v 99 Y pom Y eo Y E i rss 0001502 ES z 3190 on v azz 2 SIS NOYLDATA wv an 2861 yb doo g 222022201222 v 921 22222 52 0 E vo MT wom Greets BAW gt Sm o zz 28 ou Jesu Cd FR A vol t au 22 95 us gt FNS vor we 8 y 91 V A sy 25 v gt o g dd 42 i O 5 998 _ 1 es 5 Q ZX 50 o ES 8151 Loy O ino 23 y m 231 a i 5v Electron power supply board circuit diagram
4. Grey Optional 19V AC input power jack ran Keyboard matrix s gt 33V v a ca wid a EHI 51 N ya L 9S ov I 33443 AdO gt 27 2a CJ NI gt I ASt N W EA el 313730 Nan 133 At oie L oz 2v ua L zZ ev aa zz vv za 2 1 AO eal MY vaxa o zu L iv 2d gt vi Br gt ev Sav vd gt 21 Ow o A AS sav ei q N al amp m anosa Ser Electron parts lists Electron keyboard assembly Item Part No Description Qty Remarks 1 205 002 Bare PCB 1 2 201 089 Keyboard support plate 1 3 105 002 Assembly drawing 1 per batch 4 201 079 Keytop set 1 5 6 500 471 Resistor 470R 1 4W 10 carbon 1 RI 7 500 153 Resistor 15K 1 4W 10 carbon 4 R2 5 8 9 10 651 474 Capacitor 0 47uF disc ceramic 1 Cl 11 12 794 148 Diode IN4148 14 D1 14 13 799 001 LED TIL212 2 1 LD1 14 880 040 Sleeve 10mm long 15 886 003 Keyboard switch 56 16 17 882 972 Washer M3 shakeproof 2 18 882 121 Screw M3 X 6mm pan head posidrive 2 19 20 870 222 Ribbon cable assembly 1 21 22 886 000 Space Bar wire 1 23 886 001 Wire holder 2 24 886 002 Push rod 2 25 26 890 010 RTV silicon compound A R 27 28 29 30 Electron final assembly Item Part No Description Qty Remarks 1 005 000 A Final assembly drawing 1 per works order 2 005 000 PG Packaging assembly 1 per works order 3 105 000 Main PC
5. integrated circuit contains an extensive and powerful machine operating system and an extremely powerful and fast BASIC interpreter The interpreter includes a 6502 assembler which enables BASIC statements to be freely mixed with 6502 assembly language The standard television output is 625 line 50Hz interlaced fully encoded PAL modulated on UHF channel 36 The display modes provide user definable characters in addition to the standard upper and lower case alphanumeric font Graphics may be freely mixed with text Text characters can be positioned not only on for example a 40 X 32 grid but at any intermediate position in graphics modes Separate or overlapping text and graphics windows can be easily user defined over any area of the display Each of these windows may be filled and scrolled separately The Electron is able to support the following modes 640 X 256 two colour graphics and 80 X 32 text 20K 1 320 X 256 four colour graphics and 40 X 32 text 20K 2 160 X 256 sixteen colour graphics and 20 X 32 text 20K 3 80 X 25 two colour text 16K 4 320 X 256 two colour graphics and 40 X 32 text 10K 5 160 X 256 four colour graphics and 20 X 32 text 10K 6 40 X 25 two colour text 8K The installed RAM is divided between the high resolution graphics display the user s program and machine operating system variables The MOS requires 3 5K of RAM 3 2 Machine operating system A 32K byte ROM is used for both the MOS and t
6. Computers Limited Fulbourn Road Cherry Hinton Cambridge CB1 4JN England A I ON m im m im mm m m m Fm P m P moi On AA LAN UY Y LA DL Fi SER SD
7. pins 3 and 11 of IC 11 IC11 is a 74574 A 74LS74 inserted in this position can cause the circuit to fail Check that there are signals from pins 9 and 6 of IC 11 4 4336MHz and also signals from pins 5 7 7kHz approx and 9 of IC 12 Check that L1 has not gone open circuit and that C21 has not short circuited and check Q7 Failing all this test the logic circuit formed by IC 14 IC 15 IC 16 IC10 and resistors R50 to R57 d If the RGB picture is distorted and out of synchronisation then LK3 must be altered to invert CSYNC LK3 must be set to suit whichever RGB monitor is being used with the computer 8 8 Cassette interface First test that the 16MHz clock is arriving at pin 2 of IC9 Then test that the divided by 13 clock 812ns period is leaving pin 11 of IC9 and is arriving at pin 55 of the ULA IC 1 If it is not then IC9 is not working Check the power supply to IC9 pin 16 5V pin 8 ground Check the fixed voltage connections to the LM324 IC 13 pin 4 5V pin 11 5V To test the cassette output if the Electron will accept commands then SAVE a program or section of memory from the ROM is best and check that there is a synthesised sine wave signal from pin 50 of the ULA of around 1 8V peak to peak To test the cassette input press BREAK and measure the quiescent voltage of each of the three amplifier outputs pin 7 pin 8 and pin 14 of IC 13 This voltage should not be greater than 100mV If any of these pins is at too high
8. 20 to 70 C Humidity 95 relative humidity at 55 C 4 Disassembly and assembly To service the Electron disconnect the power supply lead and undo the four fixing screws underneath two recessed in the front corners and two in the cutaway portion on the back edge The assembly diagram is given in the Appendix When you split the computer the keyboard is attached to one half and the two PCBs are in the other These two halves are connected to each other by a short piece of ribbon cable The ribbon cable is attached to the main PCB by a socket which should be unplugged before carrying out repairs Note however that the Electron will not run without the keyboard connected The keyboard can be removed from the top case by undoing the five philips screws which hold it in place The bottom case contains two PCBs the smaller one being the power supply board This is connected to the main PCB by a socket and two faston tabs which are marked 18V AC and lead only to the expansion port The socket connects three wires to the main PCB red 5V blue SV black ground The loudspeaker is also connected to the main PCB via a socket All of these sockets and wires may be unplugged to allow removal of the PCBs from the bottom case The main PCB is held in place by four philips screws the power supply PCB by three 5 Circuit description 5 1 General The microprocessor used in the Electron is a 6502A with the clock signals provided by a 16MHz cr
9. B assembly 1 4 105 002 Keyboard assembly 1 5 6 7 201 080 Case top moulding 1 8 201 081 Case base moulding 1 9 201 112 Case label 1 10 332 002 PSU PCB assembly 1 11 12 890 010 RTV silicon compound A R 13 201 123 Self adhesive foam pad 1 14 201 203 Speaker 15 883 306 Screw M3 X 6 12 16 882 673 Screw No 4 X 5 16 flange head posidrive 4 41 18 19 20 21 22 23 24 25 26 27 28 29 30 890 001 201 248 201 308 Rubber foot Expansion connector cover Registration label Electron main PCB Item XO Roa 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 42 Part No 205 000 105 000 502 100 508 240 502 470 502 680 502 101 502 471 502 681 502 821 502 102 502 122 502 152 502 222 502 182 502 332 502 392 502 472 502 562 502 822 502 103 502 393 502 563 502 104 502 124 502 154 502 224 502 274 502 824 685 330 631 039 685 470 685 101 Description Bare PCB Assembly drawing Resistor 10R 14W 5 carbon Resistor 24R 4W 10 carbon Resistor 47R YW 5 carbon Resistor 68R YW 5 carbon Resistor 100R 14W 5 carbon Resistor 470R 4W 5 carbon Resistor 680R W 5 carbon Resistor 820R W 5 carbon Resistor IK 14W 5 carbon Resistor IK2R 4W 5 carbon Resistor 1K5R W 5 carbon Resistor 2K2R W 5 carbon Resistor 1K8 1 W 5 carbon Resistor 3K3R W 5 carbon Resistor 3K9R W 5 carbon R
10. a voltage then replace the LM324 To test the input LO A D a program from a cassette which is known to work and check that a signal is arriving at pin 59 of the ULA with an amplitude of between 500mV and 2V 8 9 Keyboard The PIT system will test the keyboard A manual check is possible using an ohm meter and the keyboard circuit diagram Check for shorts dry joints and lifted tracks Replace any faulty keys Check that both ends of the keyboard connector cable are making good contact that the connector on the Electron end of the cable is covering all the pins 1t could be inserted with an offset and miss some of the pins and that the cable is not damaged in any way Check that the diodes and resistors are correctly fitted and are of the correct value 8 10 Sound Tf the Electron will accept commands then give a SOUND command and check that there is a signal from pin 62 of the ULA IC1 If not then replace the ULA with a known good one and try again Check all the connections in the simple amplifier circuit to the loudspeaker especially transistors Q3 and Q4 see circuit diagram Appendix Diagnostic flowcharts Measure voltage output from maina power adapter with adapter plugged into Electron Is voltage greater than Replace mains power adapter 17 RMS Remove connector frem PCB connect 100 2 5W resistor across red and black leads and measure voltage across the resistor Is voltag
11. age should be between 0 5V and 2V peak to peak SOUND 0 P Delivers a tone to the speaker from an internal oscillator POR power on reset On power up the RC circuit connected to this input provides a ve edge to reset parts of the ULA and to cause the ULA to activate the RST line which resets the processor CSYNC This output is part of the CRT control circuitry and is low during either a horizontal synchronisation period or during a vertical synchronisation period and is high at all other times RED GREEN BLUE These TTL outputs are active high and give the current pixel colour HS This output is part of the CRT control circuitry and is low during a horizontal synchronisation period and high at all other times CLOCK IN 16MHz clock input 13 IN Divided by 13 clock input RAMO RAMS Data lines to and from the RAM Data flow is controlled by RAS CAS and WE WE The write enable line at logic 0 selects a write operation to the RAM and at logic 1 selects a read operation from the RAM RAS The row address strobe latches the row address into the RAM on a ve edge and also controls refresh CAS The column address strobe latches the column address into the RAM on a ve edge RAO RA7 RAM address lines which send both row and column addresses to the RAM KBDO KBD3 Direct inputs from the keyboard CA PS LOCK Output which controls the CAPS LOCK LED RST Master reset for the 6502 Enabled on power up and when
12. am 31 Main PCB layout 33 Main PCB silk screen 35 Electron power supply board circuit diagram 37 Keyboard matrix 39 Electron parts lists 41 1 Introduction This manual is intended to provide the information required to diagnose and repair faults on the Electron microcomputer which was designed by Acorn Computers Limited of Cambridge England The information contained in this manual is aimed at service engineers and Acorn dealers who will be servicing the Electron microcomputer on behalf of Acorn Computers Limited 2 Packaging and installation The Electron microcomputer is supplied in a two part moulded polystyrene packing which is further packaged within a cardboard sleeve Supplied with the microcomputer is a User Guide a book called Start Programming with the Electron an Introductory Cassette package a UHF TV lead and a mains power adapter The mains supply for UK models is 240V AC 50Hz Power is delivered to the microcomputer from a separate mains power adapter which has a built in square pin plug This plug cannot be changed and if it is unsuitable for the socket outlet available then an adapter must be used The output from the mains power adapter is 19V AC 50Hz at 14 watts The lead on the mains power adapter plugs into the microcomputer in a socket on the right side This socket is labelled 19V A C POWER IN on the bottom of the machine Do not use the microcomputer in conditions of extreme heat cold humidity or dust o
13. display then the PAL modulator is faulty and should be replaced If the colour burst part of the waveforms is missing then the fault lies in the chrominance circuitry see below If any other part of the waveforms is incorrect make link 4 temporarily with a pair of pliers perhaps and test the composite video output with the oscilloscope The waveforms should be similar to those indicated above but of reduced amplitude 1 V peak to peak If the composite output waveforms are good with link 4 made including colour burst but the UHF output does not work then the fault lies in the UHF luminance circuitry Q5 D1 D2 D3 C13 and associated resistors Check that diodes D1 D2 and D3 are inserted the correct way round the PCB is marked If the video output waveforms are bad then the video luminance circuitry Q8 and associated resistors is faulty and possibly chrominance circuitry also see below A composite colour monitor can be used to test the video output with link 4 made If the composite colour monitor works in black and white only then the chrominance circuitry is faulty see below c If the RGB works and the UHF works in black and white only then the chrominance circuitry is faulty Test pin 6 of IC8 with a frequency counter The measured frequency must be 17 7345MHz 200Hz and can be adjusted using VC1 If there is no signal on pin 6 then debug as for the oscillator in section 8 2 Check that the 17 7345MHz is reaching
14. e SV 0 25V Remove resistar and measure current drawn by main PCB from 5V supply Is current 800mA 100 8 1 Power supply Replace power supply PCB Look for and correct short circuit on main PCB Is voltage zero Measure voltage across red and black leads Is current zero Look for and correct open circuit on main PCB Check for device which is hot and replace it Is current greater than TOOMA Current is less than 700 look for broken tracks Check power supply 8 1 tracks and connections Find broken track between IC8 and IC1 Check passive components and or replace IC8 Is Is Is 16MHz 16MH2 5V across 4V peak to peak 4 peak to peak pin 14 and pin 7 on pin 49 on pin 8 of IC8 IC1 8 8 2 Oscillator Are pins 9 43 and 48 at 5V pins 51 and 68 ground Is 16MHz clock input present at pin 49 Substitute known good ULA Does Electron work correctly now Check power supply 8 1 and resistors R5 and R58 look for broken tracks Check oscillator 8 2 and look for broken tracks Check that RAS CAS ROM and amp out are wobbling move on to next stage of debugging 8 4 8 4 CPU Is pin 8 of IC3 at 5V pins 1 and 28 ground Check power supply 8 1 a
15. e Acorn Computers shall not be liable for any loss or damage whatsoever arising from the use of any information or particulars in or any error or omission in this manual or any incorrect use of the product This manual is for the sole use of Acorn Computers authorised dealers and must only be used by them in connection with the product described within This manual remains the property of Acorn Computers at all times and must be returned to them immediately upon the termination of the dealer s appointment First published 1984 Published by Acorn Computers Limited Typeset by Bateman Typesetters Cambridge Contents 1 Introduction 1 2 Packaging and installation 2 3 Specification 3 3 1 Acorn Electron microcomputer 3 3 2 Machine operating system 3 3 3 BASIC 4 3 4 Power supply 4 3 5 Cassette interface 4 3 6 Video outputs 4 Modulated output 4 Composite video 5 Colour monitor 5 3 7 Audio output 5 3 8 Environment 5 4 Disassembly and assembly 6 5 Circuit description 7 5 1 General 7 5 2 PCB links 7 6 The uncommitted logic array ULA 8 7 Electron test equipment 9 8 Fault finding inside a dead Electron 10 8 1 Power supply 10 8 2 Oscillator 10 8 3 ULA 10 8 4 CPU 11 8 5 ROM 11 8 6 DRAMs 11 8 7 Video 11 8 8 Cassette interface 13 8 9 Keyboard 14 8 10 Sound 14 Appendix Diagnostic flowcharts 15 Final assembly 25 Electron block diagram 27 Functional block diagram of the Electron ULA 29 Main PCB circuit diagr
16. e middle of the screen on the oscilloscope and test the 5V supply across the blue and black leads the trace should deflect in the opposite direction The 5V supply should be tested with a 47 ohm resistive load and keep the 10 ohm load across the 5V red and black leads Any fault on the 5V line will show up as a cassette fault Now remove both resistors and test the current drawn by the main PCB from the 5V supply The board should draw 700 900 mA from the 5V supply If the 5V current is zero then at the same time measure the voltage If the voltage is 5V then the main PCB has gone open circuit so look at the connectors and tracks If the voltage is then the power supply has cut out due to overload and there is a short circuit on the main PCB The power supply is short circuit protected and the power supply lead must be removed from the Electron for at least 15 seconds for the power supply supply board to reset If the current from the 5V is higher than it should be then measure the voltage If the voltage is greater than 5 25V then replace the power supply PCB If the voltage is in spec 4 75 to 5 25V then one or more of the ICs or other components on the main PCB is short circuiting Disconnect the power supply and feel which of the components is abnormally hot The 5V supply rails are laid out in a ring around the board and so would have to break in two places for any section to be isolated Now measure the 5V current whic
17. esistor 4K7R W 5 carbon Resistor 5K6R 5 carbon Resistor 8K2R W 5 carbon Resistor 10K YW 5 carbon Resistor 39K W 5 carbon Resistor 56K W 5 carbon Resistor 100K 4W 5 carbon Resistor 120K 4W 5 carbon Resistor 150K 4W 5 carbon Resistor 220K W 5 carbon Resistor 270K 4W 5 carbon Resistor 820K 4W carbon Capacitor 33p plate ceramic Capacitor 39p plate ceramic Capacitor 47p plate ceramic Capacitor 100p plate ceramic Qty NY DH Q Ra NON y Remarks 1 per batch R65 66 R5 58 R61 R21 45 46 47 48 49 R17 R13 38 40 53 R51 56 57 R1 2 3 4 50 R16 22 32 34 37 39 63 R54 41 R19 R12 24 33 52 R26 R8 55 71 R14 31 42 R10 11 R15 R25 27 R18 28 64 R20 R6 7 R23 R29 30 R35 44 R36 R9 R43 C4 6 C18 C21 C3 5 685 151 685 471 633 082 630 150 629 002 634 004 680 001 685 333 651 224 613 100 622 330 624 100 622 100 622 470 699 004 611 470 794 148 780 183 780 213 780 239 780 309 741 004 741 074 742 000 742 074 742 086 743 169 770 324 704 864 706 502 201 620 201 621 800 852 800 094 800 050 800 070 800 002 800 003 800 611 800 200 800 168 810 001 860 005 820 177 Capacitor 150p plate ceramic Capacitor 470p plate ceramic Capacitor 820p plate ceramic Capacitor 1n5 plate ceramic Capacitor 2n2 plate ceramic Capacitor 4n7 plate ceramic Capacitor 47n 33n decoupler Capacito
18. h should be approximately 25mA If the 5V current is zero then either there is a short circuit or the LM324 is disconnected because of a broken track 8 2 Oscillator Check with the oscilloscope that a 16MHz 4V peak to peak clock pulse is reaching the ULA pin 49 of IC 1 If this signal is good then the oscillator is working If not check pin 8 of IC8 there could be a track broken between the two Next check that there is 5V across pin 14 ve and pin 7 ground of IC8 If the oscillator is not working then it could be the 74504 IC8 the crystal or any of the passive components around them 8 3 ULA The ULA is tested before being installed in the Electron As it is easily removed the best way to check it is to use the machine with a known good one a good ULA must always be kept for Electron fault diagnosis The method of removal and insertion is as follows 10 The ULA is IC 1 on the PCB and is held in place by a square metal clip To remove the ULA insert a small screwdriver between the front left corner of the clip and the plastic ULA holder as shown in the diagram below being extremely careful not to let it touch the PCB tracks which are very easily damaged Figure 2 Twist the screwdriver until that corner unclips and then repeat the procedure for the front right corner The clip should then spring off altogether and the ULA can be removed by turning the Electron upside down Insert the good ULA making sure that the n
19. he BASIC interpreter The MOS software controls all input output devices using a well defined interface The MOS supports the following interrupts Event timer used as elapsed time clock Vertical sync Keyboard and keyboard buffer Serial interface input and output and buffers Many of the operating system calls are vectored to enable the user to change them if required 3 3 BASIC The BASIC interpreter is a fast implementation very close to Microsoft standard but with numerous powerful extensions Long variable names Integer floating point and string variables Multi dimension integer floating point and string arrays Extensive support for string handling IF THEN ELSE REPEAT UNTIL Multi line integer floating point and string functions Procedures Local variables Full recursion on all functions and procedures Effective error trapping and handling Cassette loading and saving of programs and data Full support for the extensive colour graphics facilities Easy control of the built in music generation circuits Built in 6502 mnemonic assembler enabling BASIC and assembler to be mixed or pure assembly language programs to be produced 3 4 Power supply Transformer Maximum AC input 264V AC Minimum AC input 216V AC Rating 14 watts Supply frequency 47 to 63Hz Power supply module Maximum AC input 20 9V RMS Minimum AC inpu
20. ly to IC2 pin 28 5V pin 14 ground Check that the output enable pin 22 is wobbling is not fixed at one logic level and also the address lines 8 6 DRAMs Check the power supply on pin 8 5V and pin 16 ground of ICs 4 5 6 and 7 Check that the row and column address strobes RAS pin 4 and CAS pin 15 are wobbling see figure 1 in section 5 1 If they are not then the DRAMs may be destroyed and there is a track broken assuming RAS and CAS are present at the ULA 8 7 Video Look at the displays from the three monitors RGB UHF and composite and see which of the following a b c or d best describes them a If none of the monitors operates RGB UHF and composite then there is no signal coming from the ULA Replace the ULA and test again 11 b If the RGB works but the UHF doesn t then test the UHF modulator input voltage waveform using the oscilloscope Set the oscilloscope to 50mV per division 10 microseconds per division auto trigger and attach the probe to the wire shown in the diagram Figure 3 Press BREAK on the computer and check that the PAL waveform looks something like this 3 05V Colour burst 2 9V 2 75V 2 6V Figure 4 Black PAL voltage waveform Type in the following COLOUR129 CLS and press RETURN 12 The waveform should now look like this 3 3V Figure 5 White PAL voltage waveform If these waveforms are correct and the UHF monitor does not give a
21. nd PCB tracks ls clock input i pedem Check oscillator r m 37 8 2 and PCB MODE 6 oo Is Check keyboard reset line key at logic 1 check ULA RST 5V line for power on reset Check PCB tracks and ULA 8 3 replace 6502 unlikely Are IRQ NMI and R W wobbling 8 5 ROM Is 5V across pin 28 and pin 14 of IC2 Are output enable and address lines wobbling Check power supply 8 1 and PCB tracks Check ULA 8 3 and PCB tracks 19 Check power supply 8 1 and PCB tracks pin 8 and pin 16 of each of ICs 4 5 6 and 7 Are RAS and CAS webbling Check ULA 8 3 and PCB tracks Is there a screen display albeit an incorrect one eg a pattern of dots of any colour Faulty DRAM or broken track repiace 8 6 DRAMs 20 1 any of the monitors giving a correct display Does RGB work and UHF not work at all Test UHF modulator input voltage waveform Does RGB work and UHF Check work only in black and white chrominance circuitry 8 7 Video Check ULA 8 3 and PCB tracks Replace good modulator Is composite video waveform umina good circuitry Check video luminance circuitry 21 Check oscillat
22. nks on the Electron PCB most of which are hard wired in the correct position The action of these links can be ascertained from the circuit diagram see Appendix but two are of special interest LK3 controls the action of CSYNC LK3 is set to ground as standard and setting LK3 to 5V causes CSYNC to be inverted The correct setting will depend on the type of RGB monitor in use LK4 is normally open circuit If it is made it adds the chrominance component to the composite video output This is useful when debugging the video circuitry see chapter 8 and also of course when using a colour composite monitor 7 6 The uncommitted logic array ULA The ULA IC 1 coordinates every function of the computer It is contained in a quad in line package pin being marked at the top left of the chip and the other pins following consecutively anti clockwise around the package For the method of removal of ULA from its holder see 8 3 It has 68 pins in all Each pin as labelled on the circuit diagram see Appendix functions as follows CAS OUT Pseudo sinusoidal output to cassette circuitry 1 8V peak to peak CAS RC This is an input output connected to an RC circuit The RC is used to detect a continuous period of high tone on the cassette input CAS MO This output drives the transistor which operates the tape motor relay CAS IN This is the input from the cassette circuitry which goes to the ULA cassette deserialiser The input volt
23. nosis inside the Electron assuming that any part of the machine could be faulty If you know roughly the area in which you believe a fault lies then you can go straight to that part of the analysis This chapter should be studied in conjunction with the circuit diagram and block diagram of the machine and flowcharts which are given in the Appendix Obviously with some machines fault diagnosis will not be possible 8 1 Power supply Using an AC voltmeter measure the output from the mains power adapter when it is plugged into a working Electron The measurement can be made most easily across the two grey leads where they connect to the power supply PCB The voltage should be at least 17V RMS If not then replace the mains power adapter Next remove the power supply connector from the right side of the Electron s main PCB Connect a 10 ohm 2 5W resistor across the red and black leads the 5V supply and measure the voltage using an oscilloscope all measurements on the power supply should be made with a resistive load drawing around 500mA from the 5V side The voltage should be between 4 75 and 5 25 volts and a maximum of 50mV noise is permissible If the mains power adapter works but the 5V supply is dead or the voltage is out of spec then replace the power supply PCB and test again The power supply PCB is easy to work on and can be repaired if you wish see circuit diagram If the 5V voltage is correct then adjust the zero volts trace to th
24. or on pin 2 IC9 8 2 and and 13 clock on divider circuit pin 55 look for broken IC1 PCB tracks peak to peak on pin 50 IC1 when SAVE is used Replace ULA Is 5 on Check 5V from power supply blue IC Ne ee and black leads on pin 11 and look for i broken PCB tracks Is quiescent voltage of pins 7 8 and 14 of IC13 lt 100mV Replace LM324 8 8 Cassette interface 22 Is signal present on pin 62 of IC1 when SOUND command is issued Replace ULA Check amplifier circuit formed by Q3 and Q4 8 10 Sound 23 Final assembly 25 Electron block diagram Expansion connector Audio cassette 1 0 Data address Control Power UHF RGB Comp video 1200 baud LE Comp video PER Chroma amp IN 82K x8 ROM Filter 4 x 64K x 1 RAM Data ab Ar Address ULA master controller Audio cassette interface Chroma mat uP 6502 Sound generation amplifier Switch mode Integral power supply speaker L Keyboard IN Mains adapter 240 AC Functional block diagram of the Electron ULA RAM ADDRESS ROM R W Interrupt Mer NM control generator OUT gt CAS RAS CAS OUT control z CSYNC HS POR 13 IN SOUND O P R G CAS MO B CASIN Cassette in RAM RAM DATA data interface WE PD Keyboard interface
25. otches cut out of the three corners are in the correct position inside the carrier and that the little metal spring in the remaining corner is not bent down under the ULA When the ULA is seated correctly replace the clip by pressing it on to the socket with the fingers Check the power supply pins 9 43 48 5V pins 51 68 ground see circuit diagram Check that the clock output is present on pin 60 In modes 4 5 and 6 this will be varying between 1MHz and 2MHz In modes 0 1 2 and 3 there is no clock output at all for some of the time see chapter 5 Check that the ROM line pin 61 is wobbling is not fixed at one logic level Do this also for the two RAM address strobes RAS and CAS pin 52 and pin 53 If RAS is fixed at logic O then the DRAMs are very rapidly destroyed For a description of ULA function see chapter 6 8 4 CPU Check the power supply to IC3 pins 1 and 21 ground pin 8 5V Test the clock input pin 37 which should be varying between IMHz and 2MHz in modes 4 5 and 6 and also stopped for some of the time in modes 0 1 2 and 3 see chapter 5 Check that the IRQ pin 4 NMI pin 6 and R W pin 34 lines are wobbling are not fixed at one logic level If these lines are fixed then test the reset line pin 40 which should be 5V If it is held low then there could be a short on the PCB or the ULA could be faulty Press BREAK while monitoring the reset and check that it goes to OV 8 5 ROM Check the power supp
26. r 33n ceramic multilayer Capacitor 220n ceramic disc Capacitor lu 35V tantalum Capacitor 33u 16V electrolytic axial Capacitor 10u 35V electrolytic axial Capacitor 10u 16V electrolytic axial Capacitor 47u 16V electrolytic Capacitor variable 5 5 40p Capacitor 47u 16V Tantalum Diode IN4148 Transistor BC 183L Transistor BC 213L Transistor BC239 Transistor BC309 Integrated circuit 74504 Integrated circuit 74574 Integrated circuit 741 500 Integrated circuit 74LS74 Integrated circuit 74LS86 Integrated circuit 74LS169A Integrated circuit LM324 Integrated circuit 4164 Integrated circuit 6502A Integrated 12CO21 Integrated circuit OS BASIC 3 way 0 156 SIL lock right angle 3 way 0 1 SIL right angle 2 way SIL pin Shunt 6 pin DIN socket 7 pin DIN socket Video connector Faston tab IC socket 68 way Relay Coil 33uH Crystal 17 7345MHz NOR RNR e oo ax Re NFR WN N NR NR Roa a L gt C13 C17 C20 22 C26 C15 C14 16 A C12 C10 19 C8 9 Q3 Q4 Q1 2 6 7 Q5 8 8 11 IC10 16 IC12 IC14 15 9 IC13 IC4 5 6 7 IC3 IC1 IC2 PL3 LK3 SPKR LK3 E SK2 SK3 18 VACIN RETURN IC1 RL1 2 98 820 160 Crystal 16MHz 1 X1 99 100 101 880 040 Sleeving A R Use with item 102 102 825 000 UHF modulator UM1233 E36 1 SK4 103 104 105 106 107 108 109 110 44 NOTES NOTES amp COMPUTER Acorn
27. r in places subject to vibration Do not block ventilation under or behind the computer Ensure that no foreign objects are inserted through any openings in the microcomputer 3 Specification 3 1 Acorn Electron microcomputer A fast powerful self contained computer system generating high resolution colour graphics and capable of synthesising music or noise The computer is contained in a rigid injection moulded thermoplastic case The computer provides the following facilities A 56 key full travel QWERTY keyboard with ten user definable function keys two key rollover and auto repeat The internal loudspeaker is driven from a music synthesis circuit with envelope control A modulated 625 line PAL A UHF colour television signal channel E36 for connection to a normal domestic television aerial socket is available through a phono connector A 6 pin DIN RGB connector supplies output for use with a colour monitor A phono connector supplies a video output to drive a black and white monitor A standard audio cassette recorder can be used to record computer programs and data files at 1200 baud using the CUTS standard tones The cassette recorder is under automatic motor control and is connected to the computer via a 7 pin DIN connector An interrupt driven elapsed time clock enables real time control and timing of user responses The unit uses a 2MHz 6502 and includes 32K of read write Random Access Memory A 32K Read Only Memory ROM
28. t 17 1V RMS Maximum current 5V rail 1 5A 5V rail 100mA 3 5 Cassette interface Output impedance Less than 1k ohms Input impedance Greater than 100k ohms Output level Nominal 200mV peak to peak 70mV RMS Dynamic input range Nominal 50mV to 5V peak to peak 25 to 15dB OdB 350mV RMS Motor control By miniature relay within computer Contact rating 1A at 24V DC Baud rate 1200 baud using standard CUTS tones Connector 7 pin DIN socket 3 6 Video outputs Modulated output Standard 625 line PAL A UHF colour television signal Channel E36 Vision carrier Nominal 591 25MHz RF output 1 0 to 2 5mV 6db bandwidth 15 to 20MHz RF output impedance 75 ohms Connector Phono Composite video Output level Output impedance Option Connector Colour monitor RGB signals CSYNC signal Connector 3 7 Audio output Loudspeaker Impedance Rating Output power from computer Nominal 1V peak to peak Nominal 75 ohms Chrominance information wire selectable allows composite PAL monitors to be used Phono TTL type levels TTL type level ve ve going link selectable 6 pin DIN 1 5 diameter 16 ohms 0 2 watt Nominal 0 1 watt maximum Note Sound circuit is largely independent of loudspeaker impedance 3 8 Environment Air temperature Humidity Storage conditions System on 0 to 35 C System off 20 to 70 C System on 85 relative humidity at 35 C System off 95 relative humidity at 55 C Air temperature
29. the BREAK key is pressed ROM ROM select output which enables data to be read from the ROM AO 15 Processor address lines which take memory address information from the 6502 PDO Processor data lines which send data to and receive data from the 6502 NMI The ULA is responsible for all RAM accesses and uses the NMI input to determine whether the processor should have full IMHZ access to the RAM regardless of screen mode PHI OUT This output provides the processor with its clock The processor works at one of three speeds 2MHz 1MHz and stopped It is derived from the 16MHz input IRQ This output is used by the ULA to interrupt the processor when certain events within the ULA have occurred R W This is an input through which the processor tells the ULA the direction of data on the data bus logic 0 for processor write and logic for processor read A functional block diagram of the ULA is given in the Appendix 7 Electron test equipment Two test systems are available for the Electron and can be purchased from Acorn Computers Limited The PIT test production inspection test can sometimes identify the area of the Electron in which the problem lies The Electron Watchdog is a soak testing device which is used to show up intermittent faults Full operating instructions are supplied with the equipment 8 Fault finding inside a dead Electron This chapter will take you step by step through fault diag
30. ystal oscillator 1C8 in conjunction with divider circuitry in the ULA IC1 The 6502 runs at three speeds depending on what is being accessed 2MHz The processor will run at 2MHz during an access cycle to the ROM This is because the ROM is being accessed only by the processor which has a maximum speed of 2MHz 1MHz The processor will run at 1MHz during an access cycle to the RAM The processor and the video are competing for access to the RAM and the RAM can only support a bandwidth of 2MHz In screen modes 4 5 and 6 this is no problem since the screen only needs IMHz access to the RAM Stopped In screen modes 0 1 2 and 3 the screen uses all the available memory time slots during the display period The processor is denied RAM access during 40 microseconds of each 64 microseconds of the 256 lines in 312 which is the display period and it is made to wait for RAM access until the end of the period Random Access read write Memory on the Electron is provided by four 64Kbit dynamic memory devices ICs 4 to 7 giving 32Kbytes in all This means that to read a byte from memory requires two accesses IC4 contains bit 0 and bit 1 IC5 contains bit 2 and bit 3 IC6 contains bit 4 and bit 5 and IC7 contains bit 6 and bit 7 The mechanism for doing this is handled by the ULA The timing diagram for the DRAMs is shown below Bay Fa p3 p EG Figure 1 DRAM timing diagram Timing in nanoseconds 5 2 PCB links There are several li
Download Pdf Manuals
Related Search