44 QSC Audio Products, LLC
Table 3.1. Clamping voltage troubleshooting
Troubleshooting hint: Identifying shorted transistors in-
circuit
Although the output transistors are arranged in two banks (one NPN
and the other, PNP) of four devices, you can identify a shorted
output transistor among them without first removing them from the
circuit. This requires an ohmmeter that can resolve to tenths of an
ohm reliably.
Measure the resistance across the collector and emitter of each
transistor in the bank. The ohmmeter’s polarity is not important. A
damaged transistor will measure about 0.4 Ω, while each good
transistor will measure about 0.7 Ω (see Figure 3.11). This test can
be done on both NPN and PNP transistors.
Remove and replace damaged transistors with genuine ones of the
same type. Do not substitute.
Troubleshooting hint: Checking emitter and base
resistors while transistors are removed
When you remove a failed output or driver transistor for replace-
ment, check also for damage in the adjacent resistors. Replace any
that are physically damaged or do not measure within 10% of their
correct value.
• On each output transistor’s emitter is a pair of 0.47-ohm resistors
in parallel.
• Each bank of output transistors has a 15-ohm resistor from the
base to the supply rail (R85, R86, R208, or R209).
• Each driver transistor has a 200 ohm resistor (R81, R82, R204, or
R205) from its base to the supply rail.
Fault: Supply clamping diode shorted
This would draw excessive current from the power supply and
trigger DC protection. Check D19 and D20 on channel 1, or D44 and
D45 on channel 2.
Fault: Baker clamping circuit problem
A shorted driver transistor may cause damage to one of the Baker
clamp diodes.
• Channel 1: check D10–D14 and D16.
• Channel 2: check D35–D39 and D41.
Audio Output: Troubleshooting current limiting
Overview
The usual symptom of weak output current is premature clipping on
peaks of the audio signal. This could be caused by a malfunctioning
class H step, weak current limiting, or a dead output section.
Fault: No output on one polarity
If the positive or negative half of the output signal is missing, there
is an open in the part of the current splitter circuit that leads to the
dead output polarity. Check for missing or open components in these
locations:
• Channel 1, positive: Q19, Q20, R70, R381, D10, and D14;
negative: Q21, Q22, R71, R382, D11, and D12.
• Channel 2, positive: Q64, Q65, R193, R383, D35, and D39;
negative: Q66, Q67, R194, R384, D36, and D37
Channel and polarity ☛ Channel 1 + Channel 1 - Channel 2 + Channel 2 -
Measure across: C21 C22 C56 C57
normal voltage on PL325 6.0 V 6.0 V 6.0 V 6.0 V
normal voltage on PL340 6.5 V 6.5 V 6.5 V 6.5 V
If voltage is too high
check for missing resistor R60 R61 R183 R184
or missing transistor Q14 Q15 Q59 Q60
If voltage is about 0–0.3 V
check for shorted transistor Q14, Q18, Q24 Q15, Q23, Q25 Q59, Q69, Q71 Q60, Q68, Q70
or missing resistor R51, R72 R53, R75 R174, R195 R176, R198
If voltage is approx. 0.7 V check for missing resistor: R59 R62 R182 R185
If voltage is wrong check for wrong value R59, R60 R61, R62 R182, R183 R184, R185
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
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