QSC PLX Series Technical & Service Manual page 23

CLIP LIMITING DOESN'T WORK:
BOTH CHANNELS:
Check U3 missing,
Check U3 supply voltages, +I 3VCL, -13VCL, on C73* 74
ONLY ONE CHANNEL BAD:
Probe output of U2 (7), pin 7 while clipping. If output exceeds 4V during clipping, check
R38 (161) missing.
R38-39 (161-162) have wrong values.
Q9, 10 (54, 55) missing.
R34 (157) or R35 (158) missing
Q8 (53) missing
If output at pin 7 clamps at 3.5.4V as expected, check parts surrounding U3:
R32 (154) missing
Q7 (52) missing
R28 (151) missing
Q6 (51) missing
RI9 (142) missing
SW I:1 (1:lO) not making contact
Check each pin on U3
CLIP LIMITING OSCILLATES:
Cl 3 (48) missing.
R21, 27 (144, 150) missing
Troubleshooting Step Problems
EXCESSIVE STEP DISTORTION (STEP GLITCH)
Close scrutiny of the distortion trace, and scope probing of the switched waveform, will help determine the
cause of excess step distortion, The step should switch when the output voltage is within IO-12 volts of its
respective rail, This switching margin should be fairly constant from 20- 20kHz. The switching event itself
should be a fairly uniform up and down ramp, moving at about 25 volts/us, therefore taking about 2us to
complete its transition.
Step Switching Too Close to the Rail:
This will cause increased step glitch, especially at low impedances. If present at all frequencies,
check the reference voltages:
Negref: 17.5 volts above its intermediate rail: D88, R276-7-8.
PosRef: 20V below its intermediate rails: D87, R256, 257, D53.
Confirm correct values in output voltage divider: R48 loaded by R49-50 (Rl71, and Rl72-173). If present
only at high frequencies, check the value of the speed up capacitor C20 (C55) in the output voltage divider,
or look for slow switching (see below).
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