Fender 94 TWIN AMP Service Manual

’94 TWIN AMP

p/n 02-4809-000(120V)

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 REV A

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 (
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

RESISTOR CODES

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 ZI = Zinc Plated

*

) indicate the required use of that specific part. This is necessary for RELIABILITY

PB = Particle Board
PHP = Pan Head Phillips
PHPS = Pan Head Phillips Sems

’94 TWIN AMP

SPECIFICATIONS

Product Release No.: PR 266 (This is not a model number)

Part Number: 120V Version : 21-4809

230V Version : 21-4889

Power Requirements: 120V Version: 120 volts AC, 60 HZ, 360 watts.

230V Version: 230 volts AC, 50 HZ, 360 watts.

Input Impedance: Input 1: 1M Ω

Input 2: 136k Ω

Effects Loop: Pre Reverb.

Nominal Level: +2, -5.5, -13 dbv
Output Impedance: 4.2kΩ maximum
Input Impedance: 130kΩ minimum

Effects Mix Control: Continuously variable between the signal at the

EFFECTS SEND and EFFECTS RETURN jacks.

Reverb: Post effects loop.

Preamp Out: Post reverb, +1.3 dbv nominal level.

Recommended load: 22 k Ω minimum.

Power amp in: +1.3 dbv sensitivity, 1130kΩ input impedance.

Balanced Line Output: Derived from the output transformer, fully balanced.

+3 dbv nominal level into 600Ω or greater.
Pin 1 floating, pin 2 (+), pin 3 (-)

Amplifier Load Impedance: Switch selectable for 4, 8, or 16 Ω.

Power Output: OUTPUT HIGH setting: 100 watts RMS, 5% THD.

OUTPUT LOW setting: 25 watts RMS, 5% THD.

Speaker Complement: Two Fender P.N 026488 Special Design 12” 8 ohm

speakers wired in series.

Dimensions: Height:20 3/4” (52.7 cm)

Width: 26 3/8” (67 cm)
Depth: 11 1/2” (29.2 cm)
Weight: 77 lbs. (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 terminates 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 through 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 & K3B sum through R54&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. The

’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 Ω load. The Impedance Selector (S10) will switch between the respective taps of the
output transformer secondary. CR1&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.

BIAS

The Bias is set by measuring the voltage drop across the Flame Proof 1 Ω resistors R89 & R90. If the voltage drop
reads 80mV, then the current through the resistors will be 80mA (E/R=1) = (80mV divided by 1 Ω = 80mA). CR2 & 3
are protection diodes for R89 & R90. If an output tube shorts, the fault current will shunt through the diodes instead of
R89 & 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 & J12) labeled “Bias”. Adjust the Output Bias Adjust control (R94) to read .08 VDC
(80mVDC). This will Bias V9 &V10. Now connect the test probes across the “Balance” test points (J10 & J11). The
meter is now referenced to the 80mVDC instead of ground. Adjust the Output Balance Adjust control to obtain 0VDC.
This precisely matches V11 & V12 to V9 & 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 the 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 supplies 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 5881/6l6wgc
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 Ω load), the Impedance Selector switch must be set to 8 Ω. This
will produce 60 watts RMS in the Output High setting, and 15 watts RMS in the Output Low setting.

PARTS LIST

NOTE: SHADED ITEMS ARE FOR REFERENCE ONLY

PRINTED CIRCUIT BOARD ASSEMBLY

QTY PART # DESCRIPTION REFERENCE DESIGNATION

1 048973 CABLE RIBBON 9 CKT 15 ¾ IN P3A
1 048972 CABLE RIBBON 9 CKT 9 ¾ IN P2A
1 038691 CAP AE AX 4.7 UF 50V C35
7 009512 CAP AE AX 22 UF 25V 20% C1,5,11,24,28,53,61
4 024819 CAP AE AX 22UF 500V C22,42,43,44
4 036954 CAP AE RDL 22UF 63V 20% C36,37,59,62
4 024820 CAP AE AX 47UF 350V C40,41,51,52
1 028471 CAP AE RDL 47UF 50V 20% C60
2 031040 CAP AE RDL 100UF 100V 20% C45,50
2 013638 CAP AE AX 220UF 285V C38,39
2 028482 CAP AE RDL 220UF 50V 20% C46, 49
2 039362 CAP AE RDL 1000UF 35V LOPROFIL C47,48
1 025982 CAP CD 68PF 1000V 10% C4
2 020917 CAPCD 250 PF 1000V 10% C6, 17
1 025777 CAP CD 470PF 1000V 10% C25
2 011435 CAP CD 680PF 1000V 10% C13,27
1 017620 CAP CD 1500PF 1000V 10% C31
1 039265 CAP CA 2200PF 100V C30
2 026202 CAP PFF .0022UF 600V C10,14
3 025962 CAP PFF .0047UF 400V C12,29,65
2 027275 CAP MPF .068 UF 100V C57,58
7 024833 CAP MPF RDL .022UF 400V 10% C3,8,9,16,18,26,63
2 024835 CAP MPF RDL .022UF 630V 10% C32,33
2 024853 CAP MPF RDL .1UF 250V 10% C23,34
5 024854 CAP MPF RDL .1UF 400V 10% C2,7,15,19,21
2 027281 CAP MPF .22UF 63V C55,56
1 027286 CAP MPF .47UF 63V C54
1 024876 CAP MPF RDL .68UF 250V 10% C20
1 049028 CONTROL POT 1M 2B W/DPDT R39 (CHANNEL 2 VOLUME)
1 047781 CONTROL SNAPIN 1K 15C R82 (PRESENCE)
2 041510 CONTROL SNAPIN 25K B R19, 37 (CHANNELS 1&2 MID)
1 037600 CONTROL SNAPIN 100K B R62 (REVERB)
4 037597 CONTROL SNAPIN 250K 30A

1 047780 CONTROL SNAPIN 250 K 30A R23 (CHANNEL 1 DRIVE VOLUME)

R17,18,35,36 (CHANNELS 1&2
TREBLE,BASS)

’94 TWIN AMP

PRINTED CIRCUIT BOARD ASSEMBLY (CONT)

QTY PART # DESCRIPTION REFERENCE DESIGNATION

1 041511 CONTROL SNAPIN 250K B DUAL R44 (MIX)
1 041512 CONTROL SNAPIN 1M 30A R11 (CHANNEL 1 CLEAN VOLUME)
2 037596 CONTROL SNAPIN 1 MEG J TAPER R8,25 (CHANNELS 1,2 GAIN)

2 026133 CONTROL TRIM 25K LIN R92,94 (BIAS/BALANCE ADJUST)
11 026730 DIODE 1N4006 800V CR2,3,5-11, 14, 15
14 006260 DIODE 1N4448 SIGNAL CR16,17,19,20,22-30,32

2 031729 DIODE ZEN 1N5231B .5W 5.1V 5% CR18, 31

1 028990 RES CF ¼W 5% 51K R50

2 024995 RES CF ¼W 5% 68K R1,2

1 024996 RES CF ¼W 5% 82K R96

8 024997 RES CF ¼W 5% 100K R16,24,34,59,70,71,97,153

7 028955 RES CF ¼W 5% 130K R51,54,55,93,95,142,147

1 025058 RES CF ¼W 5% 180K R49

7 025059 RES CF ¼W 5% 220K R22,26,29,32,58,64,141

1 028016 RES CF ¼W 5% 300K R125

4 025065 RES CF ¼W 5% 470K R63,76,77,129
14 025069 RES CF ¼W 5% 1M

1 025077 RES CF ¼W 5% 3.3m R65

2 025084 RES CF ¼W 5% 10M R12,38

2 041741

1 025941

1 026549 RES CF ½W 5% 1.5K R134

1 026493 RES CF ½W 5% 2.7K R120

1 031065 RES CF ½W 5% 91K R146

9 025116 RES CF ½W 5% 100K R5,10,15,21,27,30,61,67,140

6 025117 RES CF ½W 5% 220K R99-102,110,111

2 033095

1 037354 RES FILM 1W 5% 4.7K R80

1 041739 RES MOX FP 1W 5% 6.8K R104

1 027349 RES FILM 1W 5% 10K R42

1 041738 RES CF 1W 5% 15K R43

1 027350 RES FILM 1W 5% 22K R79

1 027352 RES FILM 1W 5% 91K R73

1 027353 RES FILM 1W 5% 100K R72

1 041277

1 041740 RES MOX FP 2W 5% 4.7K R103

1 041737 RES CF 2W 5% 33K R53

2 036924 RES WW BT 5W 10% 1K R91,154

2 029047

1 041742 RES WW VT 7W 10% 10K R105

3 038652 SWITCH SLIDE 4P3T S4-6

9 026001 TERMINAL EYELET W/ LUG 1

1 028503

1 041261 VOLT REF LM4040DCZ-10V TO-92 U1

1 014689 XSTR N-CH JFET J111 TO-92 Q1

RES MOX FP ½W 5% 47Ω
RES CF ½W 5% 560Ω

RES MOX FP 1W 5% 1Ω

RES CF 2W 5% 820Ω

RES WW BT 7W 10% 270Ω

THERMISTOR 10 Ω 5A C60-11

R3,6,7,40,56,118,121,123,127,128,135,138,156,157

R108,109
R57

R89,90

R112

R106,107

TH1

CHASSIS ASSEMBLY

QTY PART # DESCRIPTION REFERENCE DESIGNATION

4 016473 SCRW M 8-32X1/2 PHP ZI (OUTPUT TRANSFORMER MOUNT)

1 028684 SCRW SMB 6X3/8 PH PHS BLX (@J6, REVERB RCA JACKS)

2 026521 SCRW SMB RX3/8 RH PHS BLK (@ J13, BALANCED LINE OUT)
15 028937 SCRW TF 6-32X5/8 PHPZI TAPTYT (PCB TO CHASSIS MOUNT)
15 025936 STANDOFF NYLON PCB SNAPIN 3/8” (PCB TO CHASSIS MOUNT)

1 026472 SWITCH SLIDE DPTT S10 (IMPEDANCE SELECT)

2 036570 SWITCH TOGGLE DPST W/ NUTS S7,8 (POWER, STANDBY)

1 049976 SWITCH TOGGLE DPST W/ LEADS S9 (OUTPUT HIGH,LOW)

3 023531 TUBE 12AT7 VR4,5,8

5 013341 TUBE 7025/12AX7A SOVIET MADE V1,2,3,6,7

8 023598 TUBE SHIELD (@V1,2,3,4,5,6,7,8)