PL3 Series Service Manual 45
TD-000274-00 Rev. A
Table 3.2. Troubleshooting clamp malfunctions
Fault: Premature clipping at about 60% of maximum
peak voltage, at any impedance
This indicates a malfunctioning step in the channel’s class H
circuitry. See
Troubleshooting a Step Problem
.
Fault: Constant premature clipping, worse at low load
impedances
If the amplifier can produce a full output voltage sine wave signal
into a load of 8 ohms (PL325: greater than 60 V rms or 85 V peak;
PL340: greater than 75 V rms or >110 V peak) but clips prematurely
into 4 or 2 ohms, it is safe to assume that the class H step circuitry
is all right but the output current capability is too low.
First, check the clamping voltages across C21 (Channel 1 +), C22
(Channel 1 -), C56 (Channel 2 +), C57 (Channel 2 -), as shown in
Table 3.1. At idle, all four voltages in the amplifier should be similar.
If one is significantly different from the others, check parts accord-
ing to the following table
The exact voltages will vary with temperature, but the one that does
not match the others will indicate the weak cell. A voltage that is
too low will cause premature clamping on that polarity of the
output section.
If the voltage is correct and current is still low, also check for
missing or unsoldered output transistors or emitter resistors.
Audio Power Stage: Troubleshooting current
limiting
Fault: Output collapses prematurely
Power supply cutback normally will cause the output signal to
collapse after several seconds at continuous level at or near full
power into low load impedances, such as 2 ohms per channel or a
4-ohm load in bridged mono. This is done to protect the power
supply circuitry from excessive long-term current or overheating
while still allowing full power for audio peaks and transients.
If the output collapses immediately instead of after several seconds,
it may indicate a premature triggering of the power supply cutback.
Cutback in one or both output sections at high temperature and
nearing maximum power into two ohms is also normal. But cutback
that occurs when driving loads of 4 ohms or higher per channel (or 8
ohms or higher in bridged mono), or when the amplifier is cold, into
2 ohms per channel or 4 ohms in bridged mono, indicates a problem
with the transistor power measuring circuitry (see Table 3.2).
Audio Power Stage: Troubleshooting power
supply cutback
Overview
As noted in the power supply description, the amplifier’s current
limit cuts back when necessary to protect the power supply circuitry.
Because the observed effect is a reduced maximum output voltage,
in response to prolonged operation above the long-term current
Channel and polarity ☛ Channel 1 + Channel 1 - Channel 2 + Channel 2 -
Measure clamping voltage across: C21 C22 C56 C57
Clamping
voltages collapse
too soon
Cutback
occurs
prematurely:
Check for low resistance R67, R73 R68, R74 R190, R196 R191, R197
Check for high resistance R51, R72 R53, R75 R174, R195 R176, R198
Check for open or missing
diode
D9 D8 D34 D33
Clamping
voltages are
correct but
current is still
weak
Check for shorted diode D10, D14 D11, D12 D35, D39 D36, D37
Check for missing, open, or unsoldered output
transistors or emitter resistors
Q28, Q34, Q36,
Q39; R88, R91,
R94, R97, R102,
R107, R111, R113
Q29, Q35, Q37,
Q40; R89, R92,
R95, R99, R103,
R108, R112, R114
Q73, Q79, Q81,
Q84; R211, R214,
R217, R220, R225,
R230, R234, R236
Q74, Q80, Q82,
Q85; R212, R215,
R218, R222, R226,
R231, R235, R327
Check for stressed emitter resistors that may be
out of tolerance
R88, R91, R94, R97,
R102, R107, R111,
R113
R89, R92, R95, R99,
R103, R108, R112,
R114
R211, R214, R217,
R220, R225, R230,
R234, R236
R212, R215, R218,
R222, R226, R231,
R235, R327
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
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