46 QSC Audio Products, LLC
Figure 3.12. The overcurrent detection circuit for power supply cutback.
Table 3.3. Troubleshooting clamp transistors
limit, we commonly refer to this behavior as “power supply
cutback”, but we must remember that it is actually
amplifier current
limiting
in response to an overload signal sent from the power
supply. Full-power operation into 2-ohm loads (on both channels)
should produce a 50% cutback of current after several seconds.
If the amplifier fails to cut back both channels after about 3 seconds
when driving a 2-ohm load on each channel with a continuous test
tone at maximum output, the cutback signal is probably missing.
CAUTION: Prolonged operation under these conditions with no
cutback could blow the IGBTs or burn out capacitor C144. Conduct
testing for no longer than 6–10 seconds.
Check the output (secondary side) pins of the MOC8101 opto-
coupler U17 (see Figure 3.12). Confirm the presence of +6 V
(reference to amplifier’s ground) on pin 5. The voltage on pin 4
should normally be at about 0 V, and go high (+1 to +5 V) after about
3 seconds at full power.
If pin 4 on U17 does not go high, examine the IC itself. If it appears
undamaged, trace the circuitry driving U17 (CAUTION: this circuitry
is on the primary side, which uses PRI_LO as its reference). Check
for continuity through L6:2 to PRI_LO (from pin 3 to pin 4); also check
for missing or open R343, D61, Q95, or R347, all of which drive the
input of opto-coupler U17. A short in R346 or C131 will also prevent
voltage reaching U17.
If pin 4 of U17 goes high as it should yet both channels fail to cut
back, trace the voltage on the PS_OL bus to R273, which connects
to the MUTE+ bus. Continue tracing voltage on MUTE+ to Q16 and
Q61. If only one channel fails to cut back, look for missing Q16, R65,
or Q17 or Q61, R188, Q62.
Capacitor C131 controls how quickly the power supply cutback
reacts. If it is missing, the amplifier current limits will enter cutback
almost immediately at or above full power into 4 ohms per channel.
Channel and polarity ☛ Channel 1 + Channel 1 - Channel 2 + Channel 2 -
Clamping voltage
decreases, but no
current cutback
Measure voltage on C21 C22 C56 C57
Check clamp transistor Q18 Q23 Q63 Q68
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
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