QSC PowerLight3 Series Technical & Service Manual page 51

3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
• Q7 (channel 1) or Q52 (channel 2), may be missing or open.
• R28 (channel 1) or Q151 (channel 2), may be missing or open.
• Q6 (channel 1) or Q51 (channel 2), may be missing or open.
• R18 (channel 1), or R141 (channel 2), may be missing or open.
• R19 (channel 1), or R142 (channel 2), may be missing or open.
• On the rear panel DIP switches, SW1:1 (channel 1), or SW1:10
(channel 2), may not be making contact.
• Check each pin on U3.
Fault: Clip limiter oscillates
• C13 (channel 1), or C48 (channel 2), may be missing or open.
• R21 or R27 (channel 1), or R144 or R150 (channel 2), may be
missing or open.
Audio Power Stage: Troubleshooting Step
Problems
Fault: Step won't turn on (premature clipping)
• First, inspect the step FETs—Q38 and Q41 on channel 1, or Q83
and Q86 on channel 2—as well as diodes D21 and D22 on
channel 1 or D46 and D47 on channel 2 for broken leads.
Replace any damaged components.
• Check the DC supply voltages on the LM311 step comparators,
U4 and U5 on channel 1 or U8 and U9 of channel 2. Measure
across pins 8 (V+) and 4 (V-).
The supply voltages should measure about 14 V on the positive
step driver comparators U4 and U8, and about 12 V on U5 and
U9, the negative step driver comparators.
If any step circuit will not switch to the high rail, the channel will
clip prematurely on that polarity, with any load or even with no load.
Make sure the clipping is not actually current cutback, which is
usually evident only when driving signal into 2-ohm loads.
With your oscilloscope, probe the output voltage and intermediate
rail voltages to confirm the clip point and the lack of step action.
Trace the circuit back from the step FET via the gate drive to the
drive circuit, to locate cracks, missing parts, etc.
Check the voltage of the nodes POSREF (it should be about 20 V
below the +MID-RAIL node) and NEGREF (it should be about 17.5 V
above the -MID-RAIL node). Look also for severe mismatches among
the comparator resistor ladders: R52 and R54–R56; R57, R58, R63,
and R64; R175 and R177–R179; R180, R181, R186, and R187.
PL3 Series Service Manual
TD-000274-00 Rev. A
Fault: FET does not stay fully turned on
This generally causes clipping and possible distortion problems at
low frequencies when the amp drives 2-ohm loads.
Use a 20 Hz sine wave signal and set it to just the onset of clipping
on the waveform peaks. Confirm that the step FETs remain fully on
for the entire time that the signal exceeds the step threshold and
shut off slightly after the signal descends below the threshold. If
they shut off prematurely, check for weak gate drive and determine
the cause.
• Weak positive step gate drive: check for about 14 V across C32
(channel 1) or C51 (channel 2).
Check capacitor C31 (channel 1) or C66 (channel 2).
Resistor R104 (channel 1) or R227 (channel 2) may be low in
value, and D18 (channel 1) or D148 (channel 2) may be missing or
open.
Also check R78, D15, and Q30 (channel 1) or R201, D40, and Q75
(channel 2).
• Weak negative step gate drive: check for about 12 V across C67.
Check R83, D17, and Q32 (channel 1) or R206, D42, and Q77
(channel 2).
Fault: FET turns on or off very slowly
This generally causes problems at high frequencies when driving 2-
ohm loads.
If the turn-on and turn-off slopes are both equally slow, check
capacitor C30 (channel 1) or C65 (channel 2) on the positive steps
and C29 (channel 1) or C64 (channel 2) on the negative.
If only one slope is slow, check these components and buffer
transistors:
• Positive step: R78, R79, D15, Q30, and Q31 (channel 1); R201,
R202, D40, Q75, and Q76 (channel 2).
• Negative step: R83, R84, D17, Q32, and Q33 (channel 1); R206,
R207, D42, Q77, and Q78 (channel 2).
Fault: Step stuck on (stays on high-voltage rail)
• Check for shorted FETs Q38 and Q41 (channel 1) or Q83 and Q86
(channel 2) and replace damaged FET if necessary.
If the positive step is stuck on, (as evidenced by a permanent high
voltage on the switched rail) the FET is certainly bad, since the
positive gate drive circuitry cannot indefinitely stay turned on due to
its bootstrapped supply.
If the negative step is stuck on, the cause could be a bad FET, or the
gate drive circuit could be holding the FET on, which can be
confirmed easily by measuring the gate voltage with no signal
present; the gate voltage should be very close to the negative high
rail voltage. If it is several volts positive with respect to the negative
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