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Topics - sewage666

#1
Amplifier Discussion / Fried Ampeg SVT-200T
July 03, 2012, 01:26:40 PM
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.