QSC PowerLight3 Series Technical & Service Manual page 53

3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Audio Power Stage and Power Supply:
Troubleshooting DC Fault Shutdown
Normal circuit behavior
Any amplifier fault that causes a severely asymmetrical output, such
as premature clipping of one polarity, a missing step, etc., may
trigger DC fault shutdown. This is normal behavior of the DC fault
shutdown circuit.
Tracing the cause of false triggers
If amplifier is shutting down for no apparent cause, you must locate
the source of the false DC fault signal. Check the amplifier outputs
with a DC coupled oscilloscope to confirm the actual absence of a
DC offset. The circuit will trip on DC offsets exceeding about 4 V,
positive or negative.
The input of opto-coupler U15 can be safely disabled by shorting
pins 1 and 2. This will indicate if the false DC signal's source is
before or after U15:
• If the amp comes out of shutdown when you disable U15's input,
then look for faults in the DC sense circuitry, including the
comparator U10:1. Check the output of U10:1, pin 1. If it goes
low during DC shutdown, it is sending the false signal.
• If the amplifier stays in DC shutdown, then the fault lies in U15
or the power supply control circuitry, which is referenced to
PRI_LO.
Fault: Shutdown occurs as soon as power supply
switching starts
NOTE: Connect a 50-watt resistor in series with AC line to limit
fault current, in case the output transistors are shorted. For
120 volts the resistor should be 50 ohms, and 100 ohms for 230
volts.
Connect the DC-coupled oscilloscope to an amplifier channel's
ouput. Briefly disable U15 as described under Tracing the cause of
false triggers to determine if there is actually a DC fault. Repeat for
the other channel.
If the amplifier outputs look okay, check the comparator output at
pin 1 of U10:1. If it is low, check the voltages on pins 2 and 3:
• Pin 2: should be zero (no signal)
• Pin 3: should be about 2 V, set by R243, R244, and R245.
• Check R348 at U15.
PL3 Series Service Manual
TD-000274-00 Rev. A
Fault: Shutdown occurs when output signal exceeds
4 V
• Check Q87, C7, R240, and D48 for open or missing components.
• Confirm that D48 is pulled low (-13 V), holding transistor Q87 on.
If not, check R117, R118, and Q42.
NOTE: This control voltage responds to the Bridge Mono mode
switch.
• Check R348 at U15.
• Check for open or bad connections at step diodes D21, D22, D46,
and D47.
Troubleshooting Thermal Tracking
Fault: Mounting problems with 10K NTC sensing
thermistor
Each channel's thermal sensing for fan and bias tracking depends on
a 10K NTC thermistor, R30 (channel 1) or R155 (channel 2), that is
mounted in a hole in its heat sink. The hole is filled with thermal
grease to improve coupling. If the thermistor is not straight while
the heat sink is mounted, it may short out against the side of the
hole. The thermistor is mounted on a standoff that protrudes into
the hole, so it should not short-circuit if care is taken while installing
the heat sink.
The heat sinks are live, as they carry the output voltage of their
respective channels. If a thermistor short-circuits to its heat sink,
this output voltage is coupled to the it. If the short is to the
thermistor's grounded lead, the thermistor might not be damaged,
although the channel output will then be shorted to ground. If the
other lead—the "live" end—shorts to the heat sink, a large signal
voltage could be put across the thermistor, which could damage it.
Fault: Short from the "live" end of the thermistor to the
heat sink
NOTE: This type of short circuit may occur unnoticed and the
amplifier will appear to operate normally. However, the amplifier
will exhibit poor bias tracking, and the fan speed may fluctuates
between low and high.
Inspect and replace affected NTC.
Fault: Thermistor is bent over and shorted to the driver
transistors
The thermistor may touch Q19 or Q26 (channel 1) or Q64, or Q71
(channel 2).
This will cause severe overcurrent to the affected output cell,
possibly damaging the parts in series with the shorted transistor. It
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