Thanks so much all for the help! I'm gonna try to address this all logically and in order...

J M Fahey:
Check that it's fed +/-15V to pins 8/4 respectively.
Also that there's no DC (beyond a couple millivolts) on pins 1/7 .
You will *not* have audio on pin 6 but yes on pin 7 .

Pin 8 is getting 16.5V and pin 4 is getting -16.5
I could hear but the faintest of audio with the gain and volume cranked all the way on pin 7. I mean REALLY faint.

J M Fahey:
Worst case, carefully replace IC1.

The local Fry's Electronics store has the NTE equivalent in stock, so I'm going to do just that, after I install an IC socket... doing myself a favor for the future.

Roly:
First up, thanks for being your own best friend and posting the circuit/schem complete with your measurements - that's a huge flying start.

I've done enough lurking around forums to know better.  :)

Roly:
The readings at TP4 and TP5 say the +/-16V supplies are okay, despite the apparent reading at TP10, and I wouldn't expect any AC at TP4 or 5 (see table).

I'd misread TP10, it's good at 16V. I don't know what's going on at TP3, however... my new reading is .47VDC and -1.2mVAC. That's all sorts of not matching the table.

Roly:
TP13 should only be low and relay T90 pulled in if the output of the main amp ("half rail") is very close to ground, which it should be (the relay etc being there to protect the speakers from being burned up by DC).  So it's possible that R28 & 29 had only got very hot, and not actually failed, otherwise I would have expected to see some other damage (dead transistors?) or off voltages around the output stage.

It's been so long since I reopened this can of worms... I should've checked if those resistors were toasted or just toasty.

Roly:
If you could provide a voltage reading between point (E) - the main output - and ground, and across each of R31 through R38, the 3.3 and 0.33 ohm emitter resistors (~1V, and ~10-20mV), that would help a lot in getting a picture of the condition of the output stage.

I'm not getting any voltage at E... and I'm wondering about R39. I checked the Ohms in circuit, and it reads 0... I know that's not the proper way to check, but R40 is the same type and it reads 10 Ohm. I'll unsolder R39 tomorrow and double check. I'm not sure what you mean by read across the emitter resistors though... read each side of each resistor? Or from emitter to emitter?

Tomorrow, I'll celebrate Independence Day by shopping for some parts. Yes!

I found this to test an octocoupler...
http://www.antimath.info/electro/how-to-test-an-optocoupler/
And by my reckoning, I guess my octocoupler is working.

I was able to follow the audio up to R1 that leads back into IC1 on pin 6. Then it disappears. R1 tests fine, as does the cap and resistor between pins 6 and 7 on IC1.

My limited knowledge would assume I could have a problem with the IC and / or with the optocoupler OC1. I don't even know what an optocoupler does here, though, much less how to test it.

Enzo... you are king.

I did as you advised. The resistor got hot, and then I realized I'd hit the wrong part of the test point I'd marked on the PCB while tracing the circuit. Oops.

It is indeed getting 16V there.

Now I'm really stumped. I'm attaching the revised schematic with notes. Any idea of where to look next would be appreciated. The only real idea I have now is to do an audio probe.

First post after finding this forum...

I'm looking for some help. I've done just a tiny bit of tube amp repair for me and friends, but now I've got an old humdinger of a fix-it project back on the bench.

My old Ampeg SVT-200T was fried some time ago at a Bay Area club with bad power. Two other bass rigs were burned up that night (I guess the power wiring there is crap, or three people in a row don't know how to hook up their amps). It was fixed by a tech who just installed an entirely new power amp board from SLM. As I recall, it fried again in the same club and I just put the thing in storage until I could suss it out. Installing a new power amp board is no longer an option, Ampeg having been bought by LOUD.

On opening, the T-90 relay was burnt and fused and R28 and R29 were toast. I replaced all of these. Not one fuse was burnt, though, so I guess those didn't really work to save anything.

Now the amp will power on, but no sound. The pre-amp works when I put a lead from the pre-out to another amp. The contact from the pre to the power amp in the loop section seems good. I've removed and tested the power transistors and they all seemed fine.

I tested all the points listed on the schematic... being a bit of a n00b inside an solid state amp, I'm not sure entirely what's going on, but TP10 is definitely way off, reading .2mVDC when it should have 16 volts. I'm going to replace the diode and electrolytic there.

I've attached the schematic here with my test notes on it (hopefully it's big enough to view). Is there anything else I should be looking for? And what the eff could've happened?

I've searched for anyone else working on this amp, but didn't find much online. It seems like when a '90s Ampeg solid state breaks, it gets tossed. These PCBs are a pain, but I'm looking to this as an educational experience and maybe get a back-up amp for my tube monster. Any help is appreciated.