QSC PLX Series Technical & Service Manual page 21

CLAMPING VOLTAGE DECREASES, BUT NO CURRENT CUTBACK
CHANNEL-POLARITY
Measure voltage on:
Check clamp transistor
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
Measure voltage on:
Missing cutback transistor, resistor Q24, R67
Missing voltage sense resistor,
Shorted or low value shunt resistor R72
Troubleshooting Thermal Tracking
MOUNTING PROBLEMS WITH IOK 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 Ql9, 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:
Q26 shorted, check ALL outputs and opposing driver transistor on this channel.
Ch 2: Q64 shorted, check R383, Q65, Rl93, D35, D39.
Q71 shorted, check ALL outputs and opposing driver transistor on this channel.
CH I+ CH l-
c21
c22
Q18
Q23
CH I+ CH l-
c21 c22
Q25, R68
R 73 R 74
R75
CH 2+ CH 2-
C56
Q63 Q68
CH 2+ CH 2-
C56 c57
Q69, RI90 Q70, RI91
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