Peavey 400BH Module

[jvm]

I have a XR600C mixer amp with low, distorted output. Pre amp is working fine into another power amp. Power supply voltages are right at schematic values. Output transistors all check ok. I am somewhat stumped by this one. Any ideas?

[Enzo]

Yeah. remove U2 and see if it wakes up.

Verify there is some + voltage on the gate of Q17 to turn it off.

Remove Q17 and see if it wakes up.

try a new TL074.

If the preamp works, then the one set of +/-15v is OK, but there is a separate set of +/-15v for the power amp itself.

If all those DC voltages scattered around the drawing are OK, then the power amp itself probably works, the input section is faulty.

[jvm]

Thanks for the suggestions Enzo. No change if removing U2. +14.7 V at gate of Q17. No change with new TLO74. All power supply voltages are present and accounted for.
I have a clean strong signal up to R36 then signal becomes low and distorted after R36.

[Enzo]

Q17 could still be bad, you are getting voltage at its gate, but it could still be shorted. You could remove Q17 and see, but the easy way is to measure resistance across it WITH POWER ON. I find it simpler to measure resistance from pin 12 of the TL074 to ground, than to get on hte transistor leags, but electrically that doesn't matter.

With power off or with no gate voltage, that JFET Q17 should show a low resistance. When powered up the gate voltage should turn it off and a higher resistance results. More like 100k in this circuit.

I'd leave U2 out until all this is fixed.

As for R36, as a rule one can;t make good readings at the input pins of op amps. However, a shorted Q17 would be consistent with your findings.

If you like, pop the TL074 from its socket, and again measure your signal on R36. if Q17 or whatever is bad, the low distorted signal will still be the case. If the signal is not strong there, then teh op amp was messing with you.

Make sure your 074 is in the socket the right direction.

And in general, I'd be more interested in the waveform at pin 14 than pin 12.

[jvm]

Thanks, Enzo. I have found the problem(s). Burnt circuit trace on power amp board and corroded aux input jack. Found the aux jack issue after getting the power amp working. It still seems to me that the signal is still compressed. Can I safely pull U2 and see if DDt is what I am hearing?

[jvm]

Sorry for not paying close enough attention. Obviously I can pull U2 since you already told me to do just that. It's been a long day and I am about ready for a nap! Thanks for being so generous with your knowledge and expertise. You have helped me several times in the past and I do appreciate it.