52 QSC Audio Products, LLC
may also damage the power supply.
Replace the affected thermistor and driver transistors. Check these
components in series with the drive transistors:
• Channel 1
Q19 shorted: Check, R381, Q20, R70, D10, and D14.
Q26 shorted: Check
all
the output transistors on this channel, as
well as the opposing driver transistor, Q19.
• Channel 2
Q64 shorted: Check R383, Q65, R193, D35, and D39.
Q71 shorted: Check
all
the output transistors on this channel, as
well as the opposing driver transistor, Q64.
Troubleshooting Fan Speed
The customer complains of fan runs full speed or fan doesn’t run.
Symptoms covered:
• The fan is stuck at full speed.
• The fan doesn’t run.
Fault: The fan is stuck on high speed
• C85 may be leaking
• If the amplifier is in thermal protection, C81 may be leaking
• Q88, Q89, or Q91 failed
• Thermistor R30 or R155 shorted to heatsink
• R266 or R271 missing
Fault: The fan doesn’t run
Check the fan voltage; it should be 11 V when the amplifier is cold,
and up to 29 V when hot.
• If the fan voltage is okay, replace the fan.
• No voltage: check Q89–Q91, R264, and 265 for open or missing
components. Confirm that ±15 V supply voltages are present.
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
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