Biasing issue

[MLAudio]

With SC99 in U1 and per biasing instructions, I can get the voltages the same (-.009 vdc at min and max gain). With 2nd SC99 in U2 (and U1 removed, of course), I am getting -12.75 vdc at out-2. I turned off power right away. I tried 3 more DOAs (1 SC99, 2 JH990Cs) under same scenario.

So, the out-2, should be around the same level (< 2V), right?

I have triple-checked every component, every solder joint, and cleaned the PCB. Any ideas?

[MLAudio]

FYI - I just checked All the DOAs. I put them in an API 500 series module and they checked out fine. I know this test was at +/-16 vdc, so it possible that if I lower the power from +/- 24 vdc, that it may solve the issue?

[MLAudio]

Lowered DOA voltages to +/- 18 vdc with 99 DOAs and still have the same issue. Also tried lowering to +/- 16 vdc and tried 2520 DOAs. Same issue.

Also put jumpers into production mode (non ADJ) and tried to pass signal. at lower gain settings got some "thumping" oscillation. At higher gain settings (> 3 clicks), oscillation stopped, but low frequencies are distorting. Higher frequencies (> 1kHz) sound normal.

U2 DOA tends to get hotter than U1.

[tpryan]

If the op-amps work normally when installed at U1 but give -12.75V when installed at U2, you have a short, an open, or a component installed incorrectly. There really isn't another explanation worth considering at this point. Go over your work very carefully, test continuity against the schematic, and re-flow any suspect solder joints. Don't even bother trying to pass a signal through the module until you get the DC voltages correct. You will eventually damage the op-amp in U2 if you do.

[MLAudio]

If the op-amps work normally when installed at U1 but give -12.75V when installed at U2, you have a short, an open, or a component installed incorrectly. There really isn't another explanation worth considering at this point. Go over your work very carefully, test continuity against the schematic, and re-flow any suspect solder joints. Don't even bother trying to pass a signal through the module until you get the DC voltages correct. You will eventually damage the op-amp in U2 if you do.

I have checked all components again. Re-soldered every joint that even looks marginally suspicious (including SW1) and buzzed the entire circuit.

OK. I have a known good SC99 that I will refer to here. Supply bipolar is +/- 16.

In U1, with no signal, I get 0 VDC (or very close to it) at +,-, and out. Basic op amp test here; tells me the omp ap is good. Just for grins, I run a sine wave into the U1 and get a nice, clean signal out at OUT (pin 6) on my scope. The op amp is good. I get the same signal at U2 pin 1 (+) empty Mil-Max socket. The signal is moving through the circuit.

I disconnect the output transformer from the OUT of U2 (no load now), by taking out L2 (don't want to put any more stress than I have to on U2). I put the op amp in U2 position. With no signal applied, I get 0,0, and -11.75 at +,-, and OUT respectively. My scope shows a uniform DC square wave. There is nothing on the inputs being amplified, so the DC must be coming from downstream of U2, pin 6 (OUT). This is regardless of the servos being bypassed or not (J6, J7).

I checked U3 (servo op amp), and got no voltages on the -,+, or OUT (with no signal) of either side there. Basic op amp test again. I have rarely (if ever) seen one of these go bad. I guess I could use the shotgun method and replace it.

I don't have the tools to test the ceramic caps.

I know it has to be a bad component or short, but honestly, I have spent 6 hours on this PCB and I am stumped. I am not a PhD-level electrical engineer, but I can get around an op amp fairly well, and know the basics. This is starting to look like something beyond that, and I can't figure it out.

Any help is appreciated.

[MLAudio]

I think I have it figured out. When I tried to put the module in the chassis (it's an older chassis) the hole for R24 trim pot was very tight. I ended up drilling them out 1/32" to get the bushings to fit. I hadn't originally epoxied it to the PCB. I did after I drilled the holes. Some time between trying to put it in the chassis the first time and epoxying the pot down one of the leads on R24 snapped and was making an intermittent connection. When I buzzed the board out it was fine. But putting voltage on it cause it to open. I ended up taking an Exacto and cutting out the epoxy to take a look underneath and and found it. Since I have 2 of these I decided to look at the second one and found that one of the leads there is also broken.

Now I just need to find 2 more Bourns trim pots.