Fender BXR 200 Service Manual - page 5
BXR 200
DELTACOMP
(Cont)
The attack and release circuit for the DeltaComp is made up by the Diode, Resistor, and Capacitor network
which drives the Base of Q19. Comparator U1A senses the voltage from the Collectors of Q20 & Q21.
When the power amplifier approaches clipping on a positive swing, the collector of Q5 pulls up R56, which
turns on Q20, transferring –16Vdc to the collector, pulling down R60. This causes the output (pin 1) of U1A
to switch from +15Vdc to –15Vdc, lighting the DeltaComp/Clip Led.
Note: when the DeltaComp is disabled,
the Led indicates clipping of the output stage
. WIth the DeltaComp enabled, the Led indicates an active
DeltaComp circuit. The negative control voltage from R60 also feeds through P1A to P1B on the preamp
PCB. The voltage is routed to the DeltaComp switch (S5A), located on the front panel. With the switch
engaged, the control voltage is sent back through the ribbon cable (via P1 pin 6) to the attack/release
circuit.
The negative voltage drives through Diodes CR31-CR34, and charges Capacitors C16-C19 all at once in
parallel as a one pole filter through a single time constant of R60 X (C6+17+18+19) or 2.2k X 8.8 uF = 19.4
mSec. As they charge, Q19 turns on and provides current to pin 5 of the OTA (U2). The current controls the
output amplitude of U2. The inverted signal from U2 mixes with the input to U1B providing cancellation
which reduces amplitude of U2. The inverted signal from U2 mixes with the input ot U1B providing
cancellation which reduces the input to the power amplifier. This prevents the amplifier from clipping.
When the output of the power amplifier is reduced, the output of U1A toggles back to +15vdc. Due to the
blocking action of Diodes CR31-CR34, Capacitors C16-C19 are forced to discharge as a 4-pole filter with
different time constants through R55. R51 prevents Parasitic oscillation while Zener CR30 provides 2
slopes which results in smoother limiting.
When the power amplifier approaches clipping on a negative swing, R62 is pulled low, turning on Q22 which
pulls up the Base of Q21, transferring –16vdc to the collector.
POWER AMPLIFIER
JFET Q1 and associated components provide a 4-5 second turn-on delay for the audio input to the power
amplifier. When the power is switched on, Capacitor C5 charges through Resistor R7. The negative Gate
voltage pinches off the JFET, removing the ground from the input of the amplifier. When the power is
switched off, C5 immediately discharges through Diode CR1, grounding the input of the amplifier.
TROUBLESHOOTING TIP:
Check for proper operation of this circuit when experiencing excessive turn-on
or turn-off “Pops”, or no output when signal applied to the input. Many times the JFET itself can be the
culprit.
NOTE:
Excessive turn-off “Pops” can also be caused by uneven discharge of the +/- power
supplies. Usually a mismatch in the Filter Capacitors will cause this problem. Its easy to look at both
supplies on an oscilloscope. Invert one scope input and check for even discharge to zero volts.
Q4 is the current source for the Differential Amplifier (Q2,Q3). For the Diff amp to work properly, one half of
the current from the current source must flow through each leg of the Diff amp. If the same amount of
current flows through each leg of the Diff amp, the voltage drop across resistors R14 &R15 must be the
same. If not, there will be a DC offset at the output of the amplifier. The overall gain of the amplifier is set
up around the Diff amp. R16 (27k) is the feedback resistor and R17 (1k) is the pull-down resistor. Rfb
+Rpd
÷
Rpd = Av. Therfore 27k + 1k
÷
1k = 28k
÷
1k = 28Av.
Q9 & Q10 make up a voltage amplification stage. Again, to operate properly, the same amount of current
must flow trhough these transistors. Thus the voltage drop across R20 & R21 must be the same. Diodes
CR16-CR9 (BYV26D) make up the Fixed Bias circuit for the output transistors. The bodies of the Diodes
are mounted through the heatsink to properly track the temperature of the transistors. These Diodes were
selected because they exhibit a 2mV decrease in Knee Voltage for every 1 degree (Celsius) increase in
temperature. The Collectors of Q9 & Q10 are at 0 volts. Therefore the Bias Diodes provide 2 voltage drops
(1.2 volts) to the Base of Driver Transistors Q13 & Q14. One Diode drop (0.6 volts) Biases on the Driver
Transistors. The remaining 0.6 volts drops across the output transistors (Q15-Q18) in parallel, Biasing them
on.