PL3 Series Service Manual 47
TD-000274-00 Rev. A
Table 3.4. Troubleshooting short circuit cutback clamping
Audio Power Stage: Troubleshooting short
circuit cutback
Fault: Short circuit current doesn’t cut back
WARNING: Do not maintain a shorted load if cutback fails to occur
within 1 second.
It will be necessary to measure the output current with a DC current
probe, or by noting the voltage across a low value resistance with a
DC scope, in order to determine which output cell—the positive or
the negative one—is failing to cut back.
Failure to cut back could indicate either lack of clamping, or lack of
voltage cutback. Measure the voltage on the respective clamp
capacitor. If the voltage decreases, but current limiting does not cut
back, check the clamping transistor (see Table 3.3).
If the measured voltage on the clamp capacitor does not decrease
to about 50% during the short, check the circuitry that measures the
current during short circuit (see Table 3.4).
Audio Output Stage: Troubleshooting Stability
and Feedback
Fault: High frequency oscillations
Symptoms are severe oscillations accompanied by significant
current draw and gross distortion.
• Channel 1: C27 or R367 missing, open, or wrong value;
Channel 2: C62 or R368 missing, open, or wrong value.
• Channel 1: C25 or C26 missing, open, or wrong value;
Channel 2: C60 or C61 missing, open, or wrong value.
• Secondary reservoir capacitors missing or open (it would be very
unlikely that all are defective).
Symptoms are severe oscillation but no large current draw, with low
voltage gain.
• Channel 1: R22 open or missing;
Channel 2: R146 open or missing.
Fault: Marginal instability—may appear only as
excessive distortion
• Channel 1: C14 missing or open;
Channel 2: C49 missing or open.
• Channel 1: C16 missing or too large;
Channel 2: C50 missing or too large.
• Channel 1: C28 missing or open;
Channel 2: C63 missing or open.
• Channel 1: C25 or C26 missing or too large;
Channel 2: C60 or C61 missing or too large.
• C195 or C196 missing on the input board.
Fault: Excessive oscillation just below clipping into 2–
4 ohms per channel
NOTE: Into 2-ohm loads, about 0.1% oscillation at just below
clipping is normal.
• Channel 1: C17 missing or open; Channel 2: C52 missing or open.
Fault: Excessive switching noise
Switching noise may look like instability on an oscilloscope, but its
frequency will be much lower (110 kHz) than an oscillation typically
will be. It will be more visible on low-frequency signals (about
200 Hz and lower) and with lower load impedances.
• C129 or C134 missing on output board.
• Bad chassis-to-ground connections; check mounting screws on
the output and main boards.
Channel and polarity ☛ Channel 1 + Channel 1 - Channel 2 + Channel 2 -
Measure clamping voltage across: C21 C22 C56 C57
Clamping
voltage does
not decrease
50%
Check cutback transistor and
resistor
Q24, R67 Q25, R68 Q69, R190 Q70, R191
Check voltage sense resistor R73 R74 R196 R197
Check for shorted or low value
resistor
R72 R75 R195 R198
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
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