Parts List - Fender PRINCETON 112 PLUS Service Manual

THEORY OF OPERATION CONT.
CHANNEL AND REVERB SWITCHING
The A.C. voltage from the power transformer secondary is tapped off through dropping resistor
R73, and presented to the footswitch jack (Approx 20 V R.M.S.). By rectifying the positive or
negative side of the waveform, we create a D.C. control voltage that is sensed by comparators
U6A and U6B. Diode's CR18 and CR21 set up the reference voltages for the comparators.
Therefore the control voltage must toggle above and below the reference voltage to cause the
output of the comparator to toggle between +15Vdc and -15Vdc. The comparator output
controls the state of the CMOS switches and the indicator LEDs. Without the footswitch
connected to the footswitch jack, CR15 rectifies the negative half of the waveform. U6B senses
the DC control voltage, and provides -15vdc to U7A, allowing the reverb output to function.
POWER AMPLIFIER
U5 is a high voltage op-amp that provides voltage gain for the power amplifier. CR3 and CR4
act as differential protection diodes that prevent the positive and negative inputs of U5 from
becoming more than 0.6 volts apart. Bootstrap Capacitors C38 and C39 sense the output
through R60. This provides more voltage swing for the opamp with respect to ground.
Diodes' CR5 - CR8 (BYV26D) make up the Fixed Bias circuit for the output transistors. The Bias
Diodes mount through a hole in the transistor heatsink. This insures the Diodes will properly
track the temperature of the output transistors. These Diodes were selected because they
exhibit a 2mV decrease in Knee Voltage for every 1 degree (Celsius) increase in temperature.
Pin 6 of U5 is at 0 Vdc. Therefore the Bias Diodes provide 2 voltage drops (+/-1.2volts) to the
Base of Darlington Transistors Q1 and Q2.
TROUBLESHOOTING TIP: With the circuit board removed, the heatsinking for the output
transistors is greatly reduced. With no signal applied, the Bias/Idle current will cause the output
transistors to quickly overheat. Use a jumper wire to short across the bias diodes. The easiest
way is to jumper from the bases of opposing output transistors. This will turn off the transistors.
Diodes' CR9 - CR12 make up a voltage clamp protection circuit. If the bases of the output
transistors reach 4.5 volts with respect to the output, the diodes will turn on, clamping the bases
of the output transistors.
The output signal feeds through the Headphone jack circuit, before driving the speaker. With
headphones connected to the Headphone Jack, the speaker is muted and a new feedback path
is created through R78.
PRINTED CIRCUIT BOARD ASSEMBLY
QTY PART # DESCRIPTION
1 048523001 PCB ASSY PRINCETON 112 PLUS
1 028104 BUTTON PUSH SWITCH BLACK
1 028577 CABLE ASSY REVERB 30
4 038689 CAP AE AX
6 025960 CAP AE AX
1 038690 CAP AE AX
5 026517 CAP AE AX
1 038691 CAP AE AX
4 028471 CAP AE RDL
2 041048 CAP AE RDL 2200µF
2 038703 CAP CA
1 038873 CAP CA
1 038698 CAP CA
3 038699 CAP CA
1 038701 CAP CA
1 039262 CAP CA
1 039263 CAP CA 1000pF
1 039264 CAP CA 1500pF
2 039265 CAP CA 2200pF
1 039266 CAP CA 3300pF
2 039268 CAP CA 4700pF
1 027281 CAP MPF
2 027286 CAP MPF
1 036234 CAP PFF RDL .015µF
1 033590 CAP PFF RDL .022µF
6 033591 CAP PFF RDL .033µF
3 033592 CAP PFF RDL .047µF
2 037324 CONTROL 100K 30C SNAP IN
2 037323 CONTROL 50K 15A SNAP IN
2 034719 CONTROL 50KB SNAP IN
2 038658 CONTROL RTRY 50K 30C
1 040552 CONTROL 250K 30C
2 064089 DIODE 1N4003 200PRV
13 006260 DIODE 1N4448 75PRV
4 020534 DIODE 1N5402 RECTIFIER 200PRV C&F
*
4 028776 DIODE BIAS BYV26D LEAD FORMED
2 027329 DIODE ZEN 1N5228B 3.9V 5%
2 028119 DIODE ZEN 1N5353B 16V 5W 5%
2 026000 FSTN TAB MALE PCB MT .187x.032
10 025802 FSTN TAB MALE .250x.032 PCB MT
2 025996 FUSE CLIP PCB .250 & 5MM FUSE
1 048827 FUSE QA 1-1/4x1/4 250V 2AMP
3

PARTS LIST

REFERENCE
(@ S1 DRIVE SELECT)
(@ P1-4)
.47µF 100V C15,16,28,34
22µF 16V C9,10,25,32,40,41
1.0µF 100V C53
2.2µF 50V C7,35,37,42,43
4.7µF 50V C36
47µF 50V 20% C38,39,51,52
50V 20% C47,48
.1µF 50V 5% C49,50
22pF 100V 5% C31
47pF 100V 5% C33
100pF 100V 5% C6,24,27
470pF 50V 5% C14
680pF 100V 5% C17
100V 5% C3
100V 5% C22
100V 5% C8,26
100V 5% C21
50V 5% C1,2
.22µF 63V 10% C5
.47µF 63V 10% C4,44
100V 10% C29
100V 10% C20
100V 10% C11-13,30,45,46
100V 10% C18,19,23
R23,32 (GAIN,DRIVE CH-TREBLE)
R8,9 (CLEAN CH TREBLE,BASS)
R30,44 (CONTOUR,REVERB)
R16,31 (CLEAN/DRIVE VOLUME)
R37 (DRIVE CHANNEL BASS)
CR13,14
CR1-4,9,12,15-21
CR22-25
CR5-8
CR10,11
CR26,27
CP11,12
CP1-10
(@ XF1)
F1(100V/120V ONLY)
DESIGNATION