Look for fuses.
If none, multimeter time.
Check resistors for open circuit, and caps for short.

Unplugged!
Preferably earth it all before working on it.
Eg brush an earthed wire over the power pins and bigger components near caps.

It's a good quality PSU, made by Delta with Japanese capacitors. So I'd say it's worth to try and fix it.

Have you tested turning on the PSU without connecting the P8 & P9 to the motherboard? You can use a broken hard drive as a load. The reason I'm suggesting this is because the behavior (coil whine and fan not spinning) could be the PSU short circuit protection kicking in, in that case it could be a short on the motherboard for example.

If it still doesn't turn on, unplug it from the wall and try testing for shorts with the multimeter on the different rails of the P8/P9 or Molex connectors. If the fuse is OK then probably the primary side is fine and you should start by looking for failed components on the secondary (caps, rectifier diodes, resistors and so on).

Thanks for the suggestions. Made a little progress. Also have some additional questions.

Firstly I have to say that I found handeling the PSU board in general very difficult. It is very crowded and not possible to completely turn it over. The reason is that the main input is soldered to the power plug connector and cant be removed easily. So my first question is - if you fix PSU units - do you desolder the main power wires to get the board out of the housing completely simply for better handling ?

Anyway I checked as many components as possible and indeed found one larger capacitor on the secondary side that is definitively shorted. So far I have never recaped anything but I want to do so in the future. I have a mainboard which needs new capacitors so I want to learn to do this in the future but I will need sufficient time. So a second question is then - would I replace only this faulty capacitor or would you suggest recapping all electrolytic capacitors or even ALL capacitors incuding metal oxide ones (seems a bit of a dounting first recap job) ?

For the meantime, I came up with a solution. I have a working (also quite old) ATX PSU as well as an ATX to AT mainboard adapter. This adapter cable also has two additional wires that can be used for a power switch. Tried that and indeed the PC is booting! So now I know at least that the PC is not dead - nice!

Thanks again for the support

The plug can't be removed? Or at least poked out in a way that gives more access?

Someone assembled these somehow.
The reverse process is usually possible.
I'd just replace the dead one and see if it works.

With the size of those damn things, in low volumes, it'll probably cost you more in parts than buying a new one!

I wouldn't bother with the metal ones.

That said... I guess price it up and decide.

Aui wrote on 2025-01-05, 03:17:

So my first question is - if you fix PSU units - do you desolder the main power wires to get the board out of the housing completely simply for better handling ?

It depends, if the wires are so short that they make it very difficult to work on the PSU I would desolder them. If you can find a way of soldering comfortably without the metal casing getting on the way or the cables straining too much then don't bother with desoldering.

Aui wrote on 2025-01-05, 03:17:

Anyway I checked as many components as possible and indeed found one larger capacitor on the secondary side that is definitively shorted. So far I have never recaped anything but I want to do so in the future. I have a mainboard which needs new capacitors so I want to learn to do this in the future but I will need sufficient time. So a second question is then - would I replace only this faulty capacitor or would you suggest recapping all electrolytic capacitors or even ALL capacitors incuding metal oxide ones (seems a bit of a dounting first recap job) ?

I would replace all the caps on the secondary side, if one of them has failed some of the others may also be close to death. Also, you can replace the small caps near the transformers as some of them are critical. You can leave the 2 big ones on the primary side as those rarely fail and they're nowhere near under the same stress as the ones on the output (also, those big caps are the most expensive).

Replacing caps on an older power supply is usually much easier than on a motherboard as it's a one layer board with large through holes which makes desoldering and cleaning the holes way easier. Just make sure to note the series of the caps you remove so you can get proper replacements.

Aui wrote on 2025-01-04, 07:46:

Currently looking at an older 486 power supply. Despite looking almost as new, it is not working.

I have mixed feelings about this PSU. On one hand looks clean, has proper input filtering. On the other they used up more than 2/3 of the space for the primary side and had to cram the secondary into a corner, with all the wires and an extra vertical PCB pretty much walling off the caps in there. I'm not saying don't fix it but you want to put 105C capacitors of good quality (well known brand). In fact if you can get low-ESR caps better than 105C for the +5V and +12V rail filters, pick those if they fit.