So, I was putting together a SS7 system, and stuff went south. It seems like my board is fried, and I would like to at least try to diagnose it to learn something. I have a PCI POST Card on the way.

However, I am a bit confused as to what happened and where to start. I bought an old AT-case, with an old Seasonic PSU that the seller told me worked. He is legit, so I don't feel cheated, but maybe the PSU was faulty. This is what happened:

Note: the board is a PC-Chips M577 baby AT, with support for both AT and ATX PSUs.

I inserted the new CPU (switched from 6x86 to a K6-3 450mhz), set the jumpers to the correct clock speed, multiplier and of course voltage. I turned the system on. It booted fine, and I had it running in the case for a couple of hours. At one point it felt like static in the chassis of the case, but the mainboard was not touching anything it shouldn't touch, however I turned the computer off at the power switch. When I turned it on again 15-30 min later, nothing. Except for the CPU-fan spinning. No beeps from the speaker (which worked as I tested it).

What I have tried:
- Using a different ATX psu
- The video card works in another system.
- Have tried inserting only one stick of memory in slot 1, 2 or 3
- Have tried the 6x86 again (with correct jumpers), it just gets hot as does the K6-3.
- Booting without RAM or videocard to provoke a beep.
- AT keyboard only lights up when system is switched off
- Reset the CMOS.
- Checked for blown caps or chips, but nothing to see

Nothing works

What the hell happened? It worked, I turned it off, then turned it on again, literally the only thing I did. I didn't touch anything.

Second question: With a multimeter, is there anything I can do to diagnose the system? Or will I actually harm it even more?

Third question: Is there any chance of repairing such a system?

Bonus question: I don't like to throw out old tech, so is there anyway to safely test, if the PSU is faulty or is that a fools errand or outright dangerous?

Hope you guys can help, I am a bit depressed, really wished to have it running and enjoying my old games.