QSC PLX-1202 Service Manual download pdf (Page 23)

CLIP LIMITING DOESN’T WORK:

BOTH CHANNELS:

Check U3 missing,

Check U3 supply voltages, +I

3VCL,

-13VCL,

C73*

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

RI8

(141) 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

complete its transition.

Step Switching Too Close

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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QSC PLX-1202 Service Manual Page 23

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