QSC PLX Series, Technical & Service Manual

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CLAMPING VOLTAGE DECREASES, BUT NO CURRENT CUTBACK
CHANNEL-POLARITY CH CH CH 2+ CH 2-
Measure voltage on: c21 c22 C56
Check clamp transistor Q18 Q23 Q63 Q68
If the measured voltage on the clamp capacitor is not decreasing to about 50% during the short, check the
circuitry which measures the current during short circuit,
CLAMPING VOLTAGE DOES NOT DECREASE TO 50% DURING SHORT.
CHANNEL-POLARITY CH
CH
CH 2+ CH 2-
Measure voltage on: c21
c22 C56 c57
Missing cutback transistor, resistor Q24, R67 Q25, R68 Q69, RI90 Q70, RI91
Missing voltage sense resistor, R 73 R 74
Shorted or low value shunt resistor R72 R75
Troubleshooting Thermal Tracking
MOUNTING PROBLEMS WITH SENSING NTC.
The thermal sensing for fan and bias tracking depends on a 1 OK NTC which is mounted in a hole in the
heat sink. The hole is filled with thermal grease to improve coupling. If the NTC is not straight while
mounting the heat sink, it may short out against the side of its hole. It is mounted on a standoff which
protrudes into the hole, so this should not occur if care is taken while installing the heatsink.
If shorted to the heat sink, the amplifier output voltage is coupled to the NTC. If the short is to the grounded
lead of the NTC, it may not damage anything. If to the other end, a large voltage is put across the NTC
which will probably damage it.
SHORT FROM “LIVE” END OF NTC TO HEAT SINK:
Replace affected NTC
BE AWARE! This can short (relatively) quietly and then appear to be operating normally. Poor bias
tracking can indicate this problem. Sometimes this short will not happen until the amplifier is driven past IV
input.
NTC BENT OVER AND SHORTED TO DRIVER TRANSISTORS.
May touch Q26, or Q64, Q71.
This causes severe overcurrent to the affected output cell, possibly damaging the parts in series with the
shorted transistor. It may also blow the power supply.
Replace affected NTC, drive transistors, check components in series with drive transistor
Ch 1: shorted, check, R381, Q20, R70, D14.
Q26 shorted, check ALL outputs and opposing driver transistor on this channel.
Ch 2: Q64 shorted, check R383, Q65, D35, D39.
Q71 shorted, check ALL outputs and opposing driver transistor on this channel.
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