weirdah wrote on 2020-05-09, 09:47:

The problem with this unit seems to be the power supply. The PC components seem to work for a few seconds after turning on the PSU, after 3-4 seconds power goes down. If I try it for some more times, PSU powers for some more seconds, so I was able to get the boot screen once.

Brush it some more, especially near the output wires, there's some black dust still there. In general PSUs are difficult and dangerous to work with and I don't know how much experience you have so my suggestion would be to replace the electrolytic caps. Even though they don't look vented or leaked, some might just have slowly dried over time. Not just the big ones, also the smaller ones like 10uF in auxilary/feedback circuits - it's sometimes these that end up causing all the problems.

Does the mobo or any of the expansion cards on it have tantalum capacitors?

Thanks for the reply, motherboard does not have any of that old tantalum capacitors, power goes to a ISA riser card that has some resistors and capacitors, only a few of them, but PSU turns off after a few seconds with any kind of load, being hard drives and floppy or mobo, so I discarded a mobo problem at this first stage.

I cleaned up more the output section without any result, so I will change small capacitors to see the result.

Thanks a lot.

weirdah wrote on 2020-05-09, 13:49:

I cleaned up more the output section without any result, so I will change small capacitors to see the result.

Replace all the electrolytics in there. I'm not a "recap everything" beliver but in this case it's probably the easiest and fastest way to be sure, and not expensive either. While doing that inspect all the solder joints on the PSU. Especially all the coils and transformers and the power dissipating stuff (primary transistor, secondary diodes). This is more of a CRT failure mode but sometimes cracks can develop due to vibrations and/or thermal cycling. This too can cause random behaviour.