Standard AT PSU doesn't have 15V rail. 12V is max. https://en.wikipedia.org/wiki/Power_sup ... g_diagrams
Is this some special/ custom computer ? Or maybe you multimeter has bad battery 😀

It's very high values and definitely have to be replaced.

I measured on an IDE connector. You are right, it should be 12 V, not 15 V. Maybe my multimeter is also bad? The PC runs fine..

Any hints on how to remove the AT power connector from the socket?

Did you have any load on it?

peter_shaw wrote on 2023-08-24, 09:34:

Any hints on how to remove the AT power connector from the socket?

Try gently rocking each connector front to back whilst pulling up on them - they can be quite stiff, especially if they've never been removed since installation (may also be some mild oxidation in play)

then just tilt to clear any locking lugs

Any pics you have may make things clearer

peter_shaw wrote on 2023-08-24, 09:34:

I measured on an IDE connector. You are right, it should be 12 V, not 15 V. Maybe my multimeter is also bad? The PC runs fine..

Measure 12V line in another good known PC PSU. You will know for sure !

any pointers on atx to at adaptors? I just read there might be Problems with some soundcards because they need -5 V, which ATX doesn't provide?

You can check if your cards need -5v if you look at the card and determinate if B5 of the ISA connector is connected on the card.
https://upload.wikimedia.org/wikipedia/common … SA_Bus_pins.png
Most of them dont use it...