majestyk wrote on 2022-12-13, 08:36:
The southbridge contains the real time clock, ATX start and shutdown circuits ect. so it´s hooked to the 5V standby rail of the PSU.
There are several PSU models that don´t provide sufficient protection against a too high standby voltage in case of a PSU defect. It´s quite possible the faulty standby voltage damaged the southbridge.
The "ground issue" could be due to some burned ground traces on the mainboard. You should inspect both sides very carefully with a good magnifying glass.
I think you are right about the +5VSB causing this, because I smelled (and heard) that something was not right as soon as I plugged it in.
At first I heard the faint buzzing sounds, and I asked my relative if that was normal for that PSU. Seconds later I smelled the bad news.
The relative quickly concluded that I broke it by installing a new CMOS battery (old one was dead after years of faithful service, so I replaced it before hooking the system up).
Quick inspection of the PSU revealed the blown up caps, and I was reluctantly absolved of the fault 😀.
Based on the advice here, I will go over the traces.
I also checked some pins from the ATX connectors against ground on the board, only pins 17-19 show proper ground, all others were open circuit or very high resistance, including pins 3,5,7. Now it's possible that not all are supposed to be used, but I thought it was strange.
What are the chances that the "meltdown" of the south bridge is the main problem here, and that a replacement would fix it? If the problem occurs outside of it (causing the extreme heating up of the chip), it might be harder to pinpoint.