Fender Ultimate Chorus Service Manual - Theory of Operation (Cont)
ULTIMATE CHORUS
THEORY OF OPERATION (CONT)
1 second) set up by R139 and C73. This allows time for the circuitry to stabilize, which prevents turn-on
pops and snaps.
The inhibit switches are located within the input jacks. The control voltage through CR55 to U10A locks the
unit in the DRIVE channel mode, muting the NORMAL channel and NORMAL REVERB. At the same time,
the inhibit voltage mutes the DRIVE channel and DRIVE REVERB. AT JFET Q6, the Chorus output has
it’s own turn-on delay circuit. If the Chorus button is engaged before power-up, R88 and C49 will provide a
delayed turn-on for the Chorus. Again to prevent pops and snaps.
POWER AMPLIFIERS (The following reference designators apply to the Left amplifier)
U8 (MC1436) is a high-voltage op-amp that provides voltage gain for the power amplifier. CR28 and CR29
act as differential protection diodes that prevent the positive and negative inputs of U8 from becoming more
than 0.6 volts apart. Bootstrap Capacitors C59 and C60 sense the output through R107. This provides
more voltage swing for the opamp with respect to ground.
Diodes’ CR30-CR33 (BYV26D) make up the Fixed-Bias circuit for the output transistors. The Bias Diodes
mount through a hole in the transistor heatsink. This ensures 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 U8 is at 0Vdc. Therefore
the Bias Diodes provide 2 voltage drops (+/-1.2volts) to the Base of Darlington Transistors Q8 and Q9.
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’ CR34-CR37 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 power amplifiers each provide 63 watts at 5% THD into a single 8 ohm speaker.
PARTS LIST
PRINTED CIRCUIT BOARD ASSEMBLY
QTY PART # DESCRIPTION REFERENCE DESIGNATION
1
039246
PCB ASY “ULTIMATE CHORUS” ALL VOLTS
(STUFFED)
1
031910
BREAKER THERMAL 15A 155/195 DEG F
TS1
3
028104
BUTTON PUSH SWITCH BLACK
(@ S1,2,3)
1
028577
CABLE ASSY REVERB 30”
(REVERB CABLE)
6
038689
CAP AE AX 100V .47uF 20%
C14,16,29,32,38,48
4
038690
CAP AE AX 100V 1uF 20%
C49,73,80,83
7
026517
CAP AE AX 50V 2.2uF 20%
C37,55,56,58,64,65,67
2
038691
CAP AE AX 50V 4.7uF 20%
C57,66
2
038692
CAP AE AX 35V 10uF 20%
C3,4
10
025960
CAP AE AX 16V 22uF 20%
C5,30,36,42,51,53,61,62,70,71
1
038693
CAP AE AX 50V 47uF 20%
C78
4
028471
CAP AE RDL 50V 47uF 20%
C59,60,68,69
2
028474
CAP AE RDL 25V 100uF 20%
C88,89
1
028479
CAP AE RDL 25V 220iF 20%
C41
2
031756
CAP AE RDL 50V 4700uF
C84,85
2
038873
CAP CA 100V 22pF 5%
C28,82
3
038698
CAP CA 100V 47pF 5%
C35,54,63
4
038699
CAP CA 100V 100pF 5%
C33,40,50,52
1
039259
CAP CA 100V 220pF 5%
C46
3
038701
CAP CA 50V 470pF 5%
C7,15,25