Fender KXR 200 Service Manual page 5

KXR 200
DELTACOMP (CONT)
current controls the output amplitude of U2. The inverted signal from U2 mixes with the input to 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 +15dvc.
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 white 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 of the Diff amp legs. 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. Therefore 27K + 1K ÷ 1K = 25K ÷ 1K = 28Av.
Q9 & Q10 make up a voltage amplification stage. Again, to operate properly, the same amount of current
must flow through these transistors. Thus the voltage drop across R20 & R21 must be the same. Diodes
CR16-CR19 (BYV26D) make up the Fixed Bias circuit for the output transistors. The body of the Diodes is
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.
Transistors Q11, Q12 and associated components comprise the Current Limiting or Short Proof protection
circuit. There are usually two conditions which demand excessive output current from the amplifier. A
shorted speaker cable, or a load impedance which is below the minimum rating of the amplifier. If these
condition occur, the voltage drop across the Emitter Resistors (.33Ω 5 Watt) will dramatically increase. On
the positive side of the amplifier, C11 charges through R29 & R34. On the negative side C12 charges
through R30 & R35. This will cause Q11 & Q12 to turn on. This limits the amount of signal which is
available to the Base of the Driver Transistors (Q13 & Q14). CR20-CR23 make up a latching circuit that
senses the signal through R22. This ensures that Q11 & Q12 turn on hard in conduction. When the fault
condition is removed from the output, the current limiting will remain latched up until the input signal is
removed.
Q12 & Q14 are the Driver Transistors for the output section. Q15-Q18 are the output transistors. Notice
on the schematic that the Base resistors carry the "Fp" designation. This indicates a Flame Proof/Fuse-