`94 TWIN AMP SERVICE MANUAL
Contents
1. 94 TWIN AMP p n 02 4809 000 120V ELECTRONICS SERVICE MANUAL CONTENTS Notices Specifications Parts lists PCB assembly Chassis assembly Cabinet assembly End item Schematics diagrams Fender Musical Instruments Corp 7975 North Hayden Road Scottsdale AZ 85258 94 TWIN AMP This is the model name for warranty claims SERVICE MANUAL JUNE 1995 REVA TYPE PR266 IMPORTANT NOTICE The information contained herein is CONFIDENTIAL and PROPRIETARY to Fender Musical Instruments Corp It is disclosed solely for use by qualified technicians for purposes of equipment maintenance and service It is not to be disclosed to others without the expressed permission of Fender Musical Instruments Co All specifications subject to change without notice For warranty repair service only Fender specified part numbers are to be used It is recommended they also be used for post warranty maintenance and repair Parts marked with an asterisk indicate the required use of that specific part This is necessary for RELIABILITY and SAFETY requirements DO NOT USE A SUBSTITUTE A coded naming convention is used in the description of certain parts The codes and what they mean are as follows CAPACITOR CODES HARDWARE CODES CAP AE Aluminum Electrolytic BLX Black Oxide CAP CA Ceramic Axial CR Chrome Plated CAP CD Ceramic Disk HWH Hex Washer Head CAP MPF Metalized Polyester Film M Machine Screw CAP MY Mylar NI Nickel2. PART 028895 006260 031017 047800 037036 031871 031647 024952 028889 041527 028714 031899 PART 047775 047768 94 TWIN AMP CHASSIS ASSEMBLY CONT DESCRIPTION REFERENCE DESIGNATION TUBE SOCKET 8 PIN XV9 10 11 12 TUBE SOCKET 9 PIN XV1 2 3 4 5 6 7 8 TUBE VACUUM 5881 6L6WGC V9 10 11 12 WASHER LCK INTL 3 5X 681X 032 J14 16 WSHR FLAT 284X1 2ZI R92 94 BIAS BALANCE POTS WSHR FLAT 380X 630 FIBER J14 15 16 WSHR FLAT X9 16 NI POWER TRANSFORMER MOUNT WSHR FLAT 3 8X 614 NI J14 15 16 SPKR OUTPUT JACKS WSHR SHLDR FIBER 3 8X5 8 J15 XFMR 94 TWIN 120V T3 120V DOMESTIC ONLY XFMR PWR 94 TWIN EXPORT T3 100V 115V 230V 240V XFMR OUTPUT 100W 4 8 16 Q T2 XFMR REVERB VIBROVERB T1 FOOTSWITCH ASSEMBLY DESCRIPTION REFERENCE DESIGNATION CABLE ASSY FTSW RT ANG 12 FOOTSWITCH CABLE DIODE 1N4448 1N914B SIGNAL CR1 3 DIODE ZEN 1N5223B 2 7 V 5 CR2 4 FTSW ASSY 3BTN GAIN CH REV COMPLETE FOOTSWITCH JACK PHONE PCB MONO CA PREMIUM Ji LED BI COLOR 5MMX5MM LD1 2 3 NUT HEX 12MMX1MM NI J1 RES CF 1 4W 5 1000 R1 SCRW SMB 6X1 4 PHP END CAP MOUNT SPACER RND NYL 680X 250X 147 LD1 2 3 SWITCH PUSH SPDT S1 2 3 WSHR NYL 485K 775K 150TK QS1 2 3 MISCELLANEOUS DESCRIPTION REFERENCE DESIGNATION MANUAL OWNERS 94 TWIN AMP SCHEM REDU WI SRV 94 TWIN AMP 94 TWIN AMP BLOCK DIAGRAM Yolume Puil Bright Input Channel Select l Ch A o ARROW NT ah aa a Ch B d Volume Pull Gain Select Channel Select
3. Volume Pull Channel Select 1 1 1 L 1 U Effects Level Switch mw di SC na Sa ey eseu o E a P ne gt WWW Ka GH gt NSH impedance Reverb Reverb Reverb Reverb D E i Reverb Cathode 162 ries Drive Pan On Off Recovery canes a EM Spodka Pre ame Out Power Amp In 82 1 3dB p o Main Amp as Speaker Parallel Ext Speaker 94 TWIN AMP ELECTRONICS SERVICE MANUAL ADDENDUM This addendum contains important modification and revision information including parts and revision C schematics Please attach to the existing service manual for the 94 Twin Amp CONTENTS Notices Description of changes Parts list Schematics diagrams Fender Musical Instruments Corp 7975 North Hayden Road Scottsdale AZ 85258 94 TWIN AMP ADDENDUM This is the model name for warranty claims SERVICE MANUAL ADDENDUM MAY 1996 TYPE PR 266 IMPORTANT NOTICE The information contained herein is CONFIDENTIAL and PROPRIETARY to Fender Musical Instruments Corp It is disclosed solely for use by qualified technicians for purposes of equipment maintenance and service It is not to be disclosed to others without the expressed permission of Fender Musical Instruments Co All specifications subject to change without notice For warranty repair service only Fender specified part numbers are to be used It is recommended they also be used for post warranty maintenance and repair Parts marked with an asterisk
4. 024833 024835 024853 024854 027281 027286 024876 049028 047781 041510 037600 037597 047780 BAMA SAAN UONNYNNWNAABANANANNNNHAA ANS Aa PARTS LIST NOTE SHADED ITEMS ARE FOR REFERENCE ONLY PRINTED CIRCUIT BOARD ASSEMBLY QTY PART DESCRIPTION CABLE RIBBON 9 CKT 15 IN CABLE RIBBON 9 CKT 9 IN CAP AE AX 4 7 UF 50V CAP AE AX 22UF 25V 20 CAP AE AX 22UF 500V CAP AE RDL 22UF 63V 20 CAP AE AX 47UF 350V CAP AE RDL 47UF 50V 20 CAP AE RDL 100UF 100v 20 CAP AE AX 220UF 285V CAP AE RDL 220UF 50V 20 CAP AE RDL 1000UF 35V LOPROFIL CAP CD 68PF 1000v 10 CAPCD 250PF 1000V 10 CAP CD 470PF 1000v 10 CAP CD 680PF 1000v 10 CAP CD 1500PF 1000V 10 CAP CA 2200PF 100V CAP PFF 0022UF 600V CAP PFF 0047UF 400V CAP MPF 068 UF 100V CAP MPF RDL 022UF 400V 10 CAP MPF RDL 022UF 630V 10 CAP MPF RDL 1UF 250V 10 CAP MPF RDL 1UF 400V 10 CAP MPF 22UF 63V CAP MPF 47UF 63V CAP MPF RDL 68UF 250V 10 CONTROL POT 1M 2B W DPDT CONTROL SNAPIN 1K 15C CONTROL SNAPIN 25K B CONTROL SNAPIN 100K B CONTROL SNAPIN 250K 30A CONTROL SNAPIN 250 K 30A REFERENCE DESIGNATION P3A P2A C35 C1 5 11 24 28 53 61 C22 42 43 44 C36 37 59 62 C40 41 51 52 Cen C45 50 C38 39 C46 49 C47 48 CH C6 17 C25 C13 27 C31 C30 C10 14 C12 29 65 C57 58 C3 8 9 16 18 26 63 C32 33 C23 34 C2 7 15 19 21 C55 56 C54 C20 R39 CHANNEL 2 VOLUME R82 PRESENCE R19 37 CHANNELS 1 amp 2 MID R62 REVERB
5. 10K SWITCH SLIDE 4P3T TERMINAL EYELET W LUG THERMISTOR 10 5A C60 11 VOLT REF LM4040DCZ 10V TO 92 XSTR N CH JFET J111 TO 92 CHASSIS ASSEMBLY DESCRIPTION SCRW M 8 32X1 2 PHP ZI SCRW SMB 6X3 8 PH PHS BLX SCRW SMB RX3 8 RH PHS BLK SCRW TF 6 32X5 8 PHPZI TAPTYT STANDOFF NYLON PCB SNAPIN 3 8 SWITCH SLIDE DPTT SWITCH TOGGLE DPST W NUTS SWITCH TOGGLE DPST W LEADS TUBE 12AT7 TUBE 7025 12AX7A SOVIET MADE TUBE SHIELD REFERENCE DESIGNATION R44 MIX R11 CHANNEL 1 CLEAN VOLUME R8 25 CHANNELS 1 2 GAIN R92 94 BIAS BALANCE ADJUST CR2 3 5 11 14 15 CR16 17 19 20 22 30 32 CR18 31 R50 R1 2 R96 R16 24 34 59 70 71 97 153 R51 54 55 93 95 142 147 R49 R22 26 29 32 58 64 141 R125 R63 76 77 129 R3 6 7 40 56 118 121 123 127 128 135 138 156 157 R65 R12 38 R108 109 R57 R134 R120 R146 R5 10 15 21 27 30 61 67 140 R99 102 110 111 R89 90 R80 R104 R42 R43 R79 R73 R72 R112 R103 R53 R91 154 R106 107 R105 S4 6 1 TH1 U1 Q1 REFERENCE DESIGNATION OUTPUT TRANSFORMER MOUNT J6 REVERB RCA JACKS J13 BALANCED LINE OUT PCB TO CHASSIS MOUNT PCB TO CHASSIS MOUNT S10 IMPEDANCE SELECT S7 8 POWER STANDBY S9 OUTPUT HIGH LOW VR4 5 8 V1 2 3 6 7 V1 2 3 4 5 6 7 8 QTY SAAB A WAWNN AOR e lt WWW0OmMAUWAUANN E 4 lt PART 023580 023606 039214 9904300100 026564 027520 022319 031153 026401 047776 049052 026478 037099
6. R17 18 35 36 CHANNELS 1 amp 2 TREBLE BASS R23 CHANNEL 1 DRIVE VOLUME QTY QTY ONWANADTGANAFA PART 041511 041512 037596 026133 026730 006260 031729 028990 024995 024996 024997 028955 025058 025059 028016 025065 025069 025077 025084 041741 025941 026549 026493 031065 025116 025117 033095 037354 041739 027349 041738 027350 027352 027353 041277 041740 041737 036924 029047 041742 038652 026001 028503 041261 014689 PART 016473 028684 026521 028937 025936 026472 036570 049976 023531 013341 023598 94 TWIN AMP PRINTED CIRCUIT BOARD ASSEMBLY CONT DESCRIPTION CONTROL SNAPIN 250K B DUAL CONTROL SNAPIN 1M 30A CONTROL SNAPIN 1 MEG J TAPER CONTROL TRIM 25K LIN DIODE 1N4006 800V DIODE 1N4448 SIGNAL DIODE ZEN 1N5231B 5W DIV 5 RES CF YAW 5 51K RES CF W 5 68K RES CF W 5 82K RES CF 4W 5 100K RES CF 4W 5 130K RES CF W 5 180K RES CF 4W 5 220K RES CF 4W 5 300K RES CF AM 5 470K RES CF W 5 1M RES CF 4W 5 3 3m RES CF 4W 5 10M RES MOX FP IO 5 470 RES CF ZW 5 5609 RES CF ZW 5 1 5K RES CF ZW 5 2 7K RES CF ZW 5 91K RES CF ZW 5 100K RES CF ZW 5 220K RES MOXFP 1W 5 10 RES FILM 1W 5 4 7K RES MOX FP 1W 5 6 8K RES FILM 1W 5 10K RES CF 1W 5 15K RES FILM 1W 5 22K RES FILM 1W 5 91K RES FILM 1W 5 100K RES CF 2W 5 8209 RES MOX FP 2W 5 4 7K RES CF 2W 5 33K RES WW BT 5W 10 1K RES WW BT 7W 10 2709 RES WW VT 7W 10
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8. 100 101 REVISION C S9 REFER TO TECH NOTE TN96 3 Q1 2 TECH NOTES Fender Musical Instruments Corp 7975 North Hayden Road Scottsdale Arizona 85258 TECH NOTE TN95 4 ISSUE DATE July 1995 ISSUED BY C Cola o PAGE 1of2 PRODUCT S AFFECTED 94 TWIN AMP All Serial numbers prior to LO 635399 SYMPTOMS When switching from channel two Red to channel one Drive Yellow there appears to be a loss in gain in the channel one Drive mode And or the channel one Drive Yellow and channel two Red are active at the same time The channel indicator LED s Yellow amp Red are not affected CONDITION Excessive voltage 3 5 4 Vdc across the relay coils K1 amp K4 when in the de energized state The modification will limit the voltage to an acceptable 2 Vdc REQUIRED ACTION 1 On the centermost PCB near IC U2 locate R119 and R124 Lift the PCB and change R124 from 680 ohms to 560 ohms 1 4W P N 024962 2 Install a 1N4448 small signal diode P N 006260 across R119 Orient the diode so the Cathode band connects to the junction of R119 and C60 Re install the PCB 3 Change R134 from 1K to 1 5K 1 2 watt P N 026549 This can be done without removing the preamp board R134 is located on the preamp board directly behind the yellow Led LD2 see Service Diagram R134 connects to the anode of LD2 De solder R134 and allow it to drop out of the PCB onto the chassis Install the 1 5K 1 2W resistor fro
9. Plated CAP PFF Polyester Film Foil OHP Oval Head Phillips PB Particle Board RESISTOR CODES PHP Pan Head Phillips PHPS Pan Head Phillips Sems RES CC Carbon Comp SMA Sheet Metal A Point RES CF Carbon Film SMB Sheet Metal B Point RES FP Flame Proof SS _ Stainless Steel RESMF Metal Film TF Thread Forming RES WW Wire Wound Zl Zinc Plated Product Release No Part Number Power Requirements Input Impedance Effects Loop Effects Mix Control Reverb Preamp Out Power amp in Balanced Line Output Amplifier Load Impedance Power Output Speaker Complement Dimensions 94 TWIN AMP SPECIFICATIONS PR 266 This is not a model number 120V Version 21 4809 230V Version 21 4889 120V Version 120 volts AC 60 HZ 360 watts 230V Version 230 volts AC 50 HZ 360 watts Input 1 1M Q Input 2 136k Q Pre Reverb Nominal Level 2 5 5 13 dbv Output Impedance 4 2kQ maximum Input Impedance 130kQ minimum Continuously variable between the signal at the EFFECTS SEND and EFFECTS RETURN jacks Post effects loop Post reverb 1 3 dbv nominal level Recommended load 22 k Q minimum 1 3 dbv sensitivity 1130kQ input impedance Derived from the output transformer fully balanced 3 dbv nominal level into 600Q or greater Pin 1 floating pin 2 pin 3 Switch selectable for 4 8 or 16 Q OUTPUT HIGH setting 100 watts RMS 5 THD OUTPUT LOW
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12. please indicate the Tech Note Number TN96 3 in the Authorization Code Box Box 12 on the Warranty Reimbursement Form Labor reimbursement 1 Hour TECH NOTES Fender Musical Instruments Corp 7975 North Hayden Road Scottsdale Arizona 85258 TECH NOTE TN96 5 ISSUE DATE June 1996 ISSUED BY C Colago PAGE 1 of 2 PRODUCT S AFFECTED 94 TWIN AMP All units which contain an A B C or D revision PCB See REQUIRED ACTION SYMPTOMS Noisy Reverb At a low setting 2 4 on the reverb control the reverb appears to sound noisy The following modification will reduce but not eliminate the noise CONDITION The input to the reverb recovery tube V6A is susceptible to picking up noise The reverb noise is most noticeable when the A C line voltage is noisy The noise will transfer to the 6 3 volt Heater secondary of the power transformer where it can be induced into the reverb recovery circuit Changing the node at V6 pin 2 from high impedance to low impedance will reduce the circuit s susceptibility to picking up noise REQUIRED ACTION Determine which revision PCB is in the unit A B C 8 D revision PCBs do not contain the modification An E revision PCB will contain the factory installed mod The PCB part number and revision level is printed on the underside of the center most PCB near the Orange wire and filter capacitor C40 Another way to identify an E revision PCB is to 5 locate R117 1 8K a
13. BIAS The Bias is set by measuring the voltage drop across the Flame Proof 1 Q resistors R89 amp R90 If the voltage drop reads 80mV then the current through the resistors will be BOMA E R 1 80mV divided by 1 Q 80mA CR2 amp 3 are protection diodes for R89 amp R90 If an output tube shorts the fault current will shunt through the diodes instead of R89 amp R90 BIAS ADJUSTMENT The Bias Adjust and Balance Adjust controls along with the Bias and Balance test points are located on the rear panel for easy access Allow the amplifier to warm up for at least 2 3 minutes with the Output switch set to High and the Standby switch in the On position With a digital voltmeter set to its most sensitive DC voltage scale connect the test probes to the test points J11 amp J12 labeled Bias Adjust the Output Bias Adjust control R94 to read 08 VDC 80mVDC This will Bias V9 amp V10 Now connect the test probes across the Balance test points J10 amp J11 The meter is now referenced to the 80mVDC instead of ground Adjust the Output Balance Adjust control to obtain OVDC This precisely matches V11 amp V12 to V9 amp V10 For optimum sonic performances set Bias to 80mVDC then set Balance For optimum sonic performance with increased tube life set Bias to 60mVDC then set Balance For optimum tube life set Bias to 40mVDC then set Balance Note If a bias measurement of 40mVDC cannot be obtained it may be time to replace t
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15. LAIM INSTRUCTIONS The above modification should be performed on any 94 Twin Amp requiring service in or out of warranty For proper labor reimbursement please indicate the Tech Note Number TN96 5 in the Authorization Code Box Box 12 on the Warranty Reimbursement Form Labor reimbursement 3 4 Hour NIAL 15 WUYDUIO JI LUWIHOS YSN 82416 43 QUOJOJ WATO v uggkean 1292 S1NAWNALSNI TWOITSAN SLIM Ape Om ABEZ 104 Jw SISHU HI geen WY WITHIN Bh 3P ONOS Z HIG OM T MLA 43 1GJALJA NIU G7 WRIT Xe AUBE eise Aug SI S SVIS Zwee HMH TO ONIYTM I UJWNOJSNUJI LYOdX3 TAD Tana ug eng AMI NI op UOA AN COT dd ant Act 1 HUTA 1531 Aug WI Budi 1V OUO JAI1SISJE VE NY QINI geg Et A LINO BE SJONODUA HILHA D i Lu ran 274 D OFZ wan w i nl HLIA Gar SJOYLDA WSIS W TW Y i TARET OL 238 GAS 400 8193200 INDILULON Soe HS OL LIS STOWINDJ ag TW RD TINS EI NIYI 2 UNO ADD THs Eu NEI OW XIN BNA SES JABI wm WIT 1SNOILIONO2 1531 KOR AWWA AWU SANDA 290 Leet 20 VUIT ONO IOMETA It Ov VWI LSEJI JW A KA b SNISN gue QL 133453 HIJA OISU OU TW O S SAU Lwa Os JW gen LIBIO XIS Ch HILI Wee ZS AM R Sat TW UE LYTA1 TTL WEES KO IA EN THA EL BIS weng TO Te 201 IVT TT bA STF 4 TEND LIAO ESO ISZOWIMII ISWI 71 031419345 3S1AX3H10 SSZIN 3 KA ese ER EES Soe 18 S9VLIOA BNID OJLU8 Hin ZHAI OI XG oan 3411515334 UG NY OLNI SHAE 82 W 31831 inaano VIAE zx Ea epi bts Abh te Ya UAB81 vexv
16. e 94 TWIN AMP THEORY OF OPERATION CONT Reverb return circuit uses JFET Q1 Without a footswitch connected to the footswitch jack diode CR17 enables the reverb to function V6A amplifies the reverb return signal Note that V6A should provide a minimum gain of 49 The output from V6A drives the Reverb control R62 then sums with the dry signal through R63 V6B feeds the Cathode Follower V7B which drives the Preamp Out jack The signal is normalled to the Power Amp in jack and then feeds V7A V7A contains the Presence control accepts feed back from the output and feeds the Phase Inverter V8 OUTPUT SECTION The output tubes V9 V10 V11 V12 5881 6L6WGC are arranged in a push pull configuration that will produce 100watts into a 4 8 or 16 Q load The Impedance Selector S10 will switch between the respective taps of the output transformer secondary CR1 amp CR4 are flyback protection diodes that prevent the output transformer from an over voltage condition This can occur when the output transformer experiences an open load A separate secondary winding drives an XLR jack J13 which provides a balanced line out with a floating ground pin pin1 This can be used to drive a slave amp and additional speaker cabinets It can also be used as a send to a mixing console However to get a useful sound the signal should be pre equalized before the console input A low pass filter set a 5 to 6 kHz with a minimum 18 db per octave slope works well
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19. he output tubes CHANNEL REVERB SWITCHING A 27 5VAC signal is tapped off from the Brown secondary of the power transformer This signal is presented to the Footswitch jack By rectifying the positive or negative half of the waveform a DC control voltage is created This voltage is used to control several opamp comparator circuits TROUBLESHOOTING TIP Most channel switching problems will probably be related to intermittent or sticky relays However troubleshooting the switching circuitry is easy All test point voltages are on the schematic First verify the reference voltage at the comparator The reference voltage is applied to the non inverting input of the opamp Then measure the output of the comparator If it doesn t toggle properly check the control voltage If the control voltage is not correct hang a scope probe on the footswitch jack Look at the AC waveform activate the channel select switches and look for the rectified waveform The Reverb is switched by rectifying the negative side of the waveform The Gain Select and Channel Select are switched by rectifying the positive side of the waveform at two different voltage levels Diode CR16 and Zener CR18 set the two voltage levels POWER LEVEL SWITCHING Via the output switch S9 the output power of the amplifier can be switched between 25 and 100 watts S9A selects between the center tap and full winding of the high voltage secondary of the power transformer This causes the B and Z su
20. indicate the required use of that specific part This is necessary for RELIABILITY and SAFETY requirements DO NOT USE A SUBSTITUTE A coded naming convention is used in the description of certain parts The codes and what they mean are as follows CAPACITOR CODES HARDWARE CODES CAP AE Aluminum Electrolytic BLX Black Oxide CAP CA Ceramic Axial CR Chrome Plated CAP CD Ceramic Disk HWH Hex Washer Head CAP MPF Metalized Polyester Film M Machine Screw CAP MY Mylar NI Nickel Plated CAP PFF Polyester Film Foil OHP Oval Head Phillips PB Particle Board RESISTOR CODES PHP Pan Head Phillips PHPS Pan Head Phillips Sems RES CC Carbon Comp SMA Sheet Metal A Point RES CF Carbon Film SMB Sheet Metal B Point RES FP Flame Proof Ss Stainless Steel RES MF Metal Film TF Thread Forming RES WW Wire Wound Zl Zinc Plated 94 TWIN AMP ADDENDUM DESCRIPTION OF CHANGES All of the following modifications are present on revision E printed circuit boards Some of the other modifications appear on earlier revision PCBs The PCB revision level is located on the solder side of the centermost PCB near the orange wire and filter capacitor C40 Reason For Changes Compliance with Safety Regulatory Agencies Added Crowbar protection circuit CR33 38 to the screen Z supply capacitors C40 41 The crowbar circuit is present on Revision D amp E printed circuit boards Do not at
21. m the solder side of the PCB Don t forget to remove the old resistor from inside the chassis ED ED ED TECH NOTE TN95 4 PAGE 20F2 WARRANTY CLAIM INSTRUCTIONS The above modification should be performed on any 94 Twin Amp requiring service in or out of warranty For proper labor reimbursement please indicate the Tech Note Number TN95 4 in the Authorization Code Box Box 12 on the Warranty Reimbursement Form Labor reimbursement 1 Hour Fender Musical Instruments Corp 7975 North Hayden Road Scottsdale Arizona 85258 del TECH NOTES TECH NOTE TN96 3 ISSUE DATE March 1996 ISSUED BY C Colago PAGE 1 of 2 PRODUCT S AFFECTED 94 TWIN AMP All Serial numbers prior to LO 673648 should require this modification SYMPTOMS Overheating of power tubes when in the LOW OUTPUT power mode Unit appears to be locked into the HIGH OUTPUT power mode CONDITION Arcing between switch contacts in the high voltage leg of the HIGH LOW OUTPUT power switch S9 The arcing occurs only when the HIGH LOW OUTPUT switch is operated while the amplifier is in the fully on mode NOT IN STANDBY This may cause burning pitting and eventual welding together of the switch contacts Therefore when the amplifier is switched to the LOW power mode the high voltages at the output tubes remain high but the Bias voltage drops The result is a severely under Biased output stage that quickly overheats REQUIRED ACTION A new style s
22. minates at pin 6 of Relay K2A K2A selects between the output of the Clean or Vintage Drive circuit VINTAGE DRIVE The signal from Pin 9 of Relay K2B feeds through the Gain control R8 to the first stage V2A of the Vintage Drive circuitry V2A provides a gain of about 20 then drives the second gain stage V2B V2B provides a gain of about 48 then couples the signal to the vintage drive Volume control R23 which also contains the select switch for the Clean Drive feature When in the Channel 2 mode Relay K1A connects the wiper of the Gain control R8 to ground muting the input to the Vintage Drive stage The Vintage Drive circuit terminates at pin 8 of Relay K2A CHANNEL TWO Channel Two is like owning a second modern hot rodded amp with more soaring gain than previously found in a Fender tube amp The signal from pin 9 of Relay K1B feeds through the Channel Two Gain control R25 to the first V3A and then second V3B stage of the overdrive circuitry From V3B the signal is fed to through the Treble R35 Bass R36 and Mid R37 tone controls Finally the signal travels through the channel two Volume control R39 which also contains the Channel Select switch Channel two terminates at pin 9 of Relay K4B K4B selects between channels one and two EFFECTS LOOP The selected Ch 1 or Ch 2 signal from Relay K4B is fed to a Split Load Phase Inverter V4A The output from Pin 1 of V4A follows two paths to Relay K3A One path travels thr
23. ough one half of the Mix control The second path connects directly to K3A The signal from Pin 3 of V4A drives the Effects Send Jack J3 and is normalled to the Effects Return jack J4 The Effects Level switch provides three signal levels for matching a wide variety of external signal processing devices The switch also maintains unity gain between the Send and Return jacks The signal from the Effects Return jack feeds V4B then travels through the other half of the Mix control R44B and to Relay K3B The dry and wet signals sum together through R54 and R55 Relay K3 is controlled by the Effects Select switch in the Both position K3 will be in the Normally Closed position Any signal available at the Effects Return jack will pass whether in Ch 1 or Ch 2 The Mix control will be active With the Effects Select switch in the Ch 1 position the Effects Return will be active while the amp is in the Ch 1 mode When the amp is switched to the Ch 2 mode the control voltage from U2B will energize relay K3 K3A will toggle bypassing one half of the Mix control R44A and K3B will break the connection from the Effects Return Therefore the Effects Return will be active only for the channle that is selected by the Effects Select switch REVERB The signals from Relays K3A amp K3B sum through R54 amp R55 then split to feed the Reverb Drive circuit and the summing amp for the Wet and Dry signal V5 12AT7 and T1 make up the standard Reverb Drive circuit Th
24. pplies to 94 TWIN AMP THEORY OF OPERATION CONT switch from 454Vdc to 230Vdc This voltage change only affects the four output tubes The high voltage supplies for the preamp tubes are tapped off prior to the output switch S9 When switching the high voltages at the power tubes the negative Bias voltage must also be switched The Bias supply is derived through a voltage doubler from the Brown secondary of the power transformer S9B determines the path of the Bias current The low power path is through R97 100K and the high power path is through R98 18K and R97 100K in parallel R96 82K sets up the voltage divider that feeds the Bias and Balance controls 52Vdc High 21 Vdc Low Note It is recommended that the amplifier be in the Stand by mode before switching the High Low Output switch LOW POWER OPTION The 94 Twin Amp can be run with only two output tubes instead of four This is done by removing the inner two 5881 6l wgc tubes The Impedance Selector switch must be set to one half the total speaker load The rule is half the tubes half the impedance Therefore when using the internal two speakers 16 load the Impedance Selector switch must be set to 8 Q This will produce 60 watts RMS in the Output High setting and 15 watts RMS in the Output Low setting 048973 048972 038691 009512 024819 036954 024820 028471 031040 013638 028482 039362 025982 020917 025777 011435 017620 039265 026202 025962 027275
25. setting 25 watts RMS 5 THD Two Fender P N 026488 Special Design 12 8 ohm speakers wired in series Height 20 3 4 52 7 cm Width 26 3 8 67 cm Depth 11 1 2 29 2 cm Weight 77 Ibs 35 kg Product specifications are subject to change without notice 94 TWIN AMP THEORY OF OPERATION INPUTS J1 and j2 are high and low sensitivity inputs that feed the first preamp stage V1A V1A provides a gain of about 38 for channel 1 and about 49 for channel 2 The gain difference is due to different loading of the plate of V1A The signal then couples to relay K1B which routes it to the channel one or channel two circuitry CHANNEL ONE The 94 Twin Amp is a direct descendent of classic tube amps like the Twin Reverb and the 410 Bassman Channel one is the traditional Fender design which offers the vintage sound as well as contemporary sounds with the use of switch selectable Gain and Master Volume controls It s like two channels in one The first stage of Channel One contains the Treble R17 Bass R18 and Mid R19 tone controls The signal from the Treble control feeds through Relay K2B which routes the signal to either the Clean or Vintage Overdrive section of channel one CLEAN The signal from pin 11 of Relay K2B feeds through the Clean Volume control R11 to V1B R12 and C4 provide the Bright boost feature via a pull switch on the Clean Volume control V1B provides a gain of about 45 The Clean channel circuitry ter
26. t the far left side of the center most PCB near IC U2 On an E revision PCB R117 will be a small 1 6 Watt resistor about half the size of a 1 4 watt resistor Earlier revisions used a 1 4 watt resistor The following modification can be performed on the solder side of the preamp PCB without removing it from the chassis Refer to the service diagram revision A or B to ocate Q1 J111 and R59 100K They are located behind relay K3 which is positioned between the Volume R39 and Mix R44 controls SEE FIGURE 1 TECH NOTES TN96 5 PAGE 2of2 Replace R59 100k with two 2 1 8K 1 6 watt P N 047234001 or 1 4 watt P N 024970001 resistors in series Allow access to the connection between the two resistors as another component will connect there Install a 2nd JFET Q2 P N 014689 in parallel with Q1 Face the flat side of the JFET towards the rear of the chassis and solder pins 2 amp 3 directly to the solder pads of JFET Q1 Pin 1 of Q2 must connect between the two 1 8K resistors This may require a short piece of light gauge wire Use shrink tubing if necessary Reference schematic 047767 047768 rev C zone 4C Using the footswitch verify the reverb on off function Figure 1 does not show all of the solder pads and traces on the PCB What is shown will clarify placement of the modification FIGURE 1 GREEN WIRE TO V6 PIN2 R59 1 8K R155 1 8K BLUE WIRE TO V6 PIN 1 OOO VOLUME MIX REVERB PRESENCE WARRANTY C
27. tempt the above modification to earlier revision PCBs Added RF emissions suppression capacitor C64 P N 025995 across the high voltage transformer secondary prior to the full wave bridge rectifier circuit Performance modifications Added CR32 and changed R124 and R134 to ensure that Relays K1 K2 and K3 drop out when de energized See zone 7B on Rev C schematic Refer to Tech Note TN95 4 Upgraded the HIGH LOW OUTPUT power switch Refer to Tech Note TN96 3 Changed the Reverb muting circuit to eliminate noise in the reverb recovery circuit caused by poor power line conditions Change R59 from 100k to 1 8k add Q2 JFET and R155 1 8k See zone 4C on rev C schematic Refer to Tech Note TN96 5 Attached is a Revision C schematic service diagram QTY PART N gt gt v RUNER ANA 024819 024820 025995 026730 006260 041811 047234 024962 026549 025117 047768 049976 014689 94 TWIN AMP ADDENDUM PARTS LIST REVISIONS PRINTED CIRCUIT BOARD ASSEMBLY DESCRIPTION CAP AE AX 22uF 500V CAP AE AX 47uF 350V CAP CD 8200PF 1000V 20 DIODE 1N4006 800V DIODE 1N4448 SIGNAL DIODE ZEN 1N5368B 47V 5W 5 RES CF 1 6W 5 1 8k RES CF 1 4W 5 5600 RES CF 1 2W 5 1 5k RES CF 1 2W 5 220k SCHEM REDU W SERV 94 TWIN AMP SWITCH TOGGLE DPST DC W NUT XSTR N CH JFET J111 TO 92 REFERENCE DESIGNATION C22 42 43 44 51 C40 41 C64 CR2 3 5 11 14 15 34 37 CR16 17 19 20 22 30 32 CR33 38 R59 117 155 R41 52 124 137 R134 R99
28. witch has been specified for this application Determine if the old style switch is installed in the unit The old style switch is identical to the Power and Standby switches If the old style is present order P N 049976 SWITCH TOGGLE DPST DC W NUT The new style switch will have a pair of White amp Black leads as opposed to Faston lugs The leads must be soldered directly to the power supply PCB This is to comply with safety regulatory agencies U L C S A DO NOT SPLICE THE SWITCH LEADS WITH THE EXISTING JUMPER WIRES TECH NOTE TN96 3 PAGE 2of2 To gain access to the solder side of the PCB disconnect the Black wires from CP1 amp CP2 Disconnect the wires at the Stand by switch This will allow the PCB to be tilted up from the rear Remove the old style HIGH LOW OUTPUT switch amp wires Note the location where the Blue and Brown wires solder to the PCB Early revision A D PCB s are silk screened TO OUTPUT SWITCH BLU 8 DN Solder the pair of White switch leads to the two points labeled BLU Solder the pair of Black switch leads to the two points labeled BN Revision E PCB s are silk screened TO OUTPUT SWITCH WHT amp BLK White leads to WHT Black leads to BLK Re install the PCB Verify proper switching of the B and negative Bias voltages WARRANTY CLAIM INSTRUCTIONS The above modification should be performed on any 94 Twin Amp requiring service in or out of warranty For proper labor reimbursement
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