I just bought an Atlas Sound CP700 amp for $60. I'm quite excited considering these are close to $1,000 new. I bought it knowing that both channels just sat in protect mode and the power switch was stuck on. I brought it home, jiggled the power switch until it started working, then removed a bunch of random wires that were attached to the back. It powered on as it should and the protect lights went out!
Thinking I was successful, I hooked up a speaker, my phone as an audio source, and turned up the input level. As soon as I touched the input level knob, a squealing began coming from inside the amplifier and the signal and limit lights came on. Even with nothing connected, moving either channel's input level above zero would make the disturbing squealing noise and the signal and limit lights come on. The whole amp was rather filthy, so I decided the pots were dirty and it needed a good cleaning. I took the cover off, blew out all the dust, and turned it on. The strange input behavior was still there, then I heard a sizzling noise and shut it off. Nothing was visibly wrong, so I turned it on again. That's when a cap on the power supply board exploded.
After that incident, I disassembled the entire amp and plugged pieces in one by one. The power supply board appeared to survive, I think the violently terminated cap was one of three mains filters. I reconnected the power supply's toroidal transformer, and still nothing violent. Then I connected the 25v line to the main board. The fan and indicator lights came on, followed by a sizzling pop and a puff of smoke. Not seeing where it came from, I turned it on again and received a second puff of smoke. Upon closer inspection, an ML7815A voltage regulator and a JRC4580D chip were the source of the smoke. It still powers on with fan and indicators, but those two chips are clearly cooking along with who knows what else.
I have removed the main board, but have no idea where to go from here. There are over half a dozen fuses on top of the electronic protection, and not a single one has blown. As far as I can tell the problem is in the input circuitry and not the high power amplifier section. I'd really like to fix this amplifier as it is high end equipment and appears to all be simple analog components. However, I have no idea what the original problem was or why so many components are being violently fried. Hopefully helpful pictures of internals: imgur.com/a/cyyxAzU