Niborius wrote on 2022-06-07, 15:00:

(...)
Could it be that the PSU is producing voltages that are too high/low?

That could be. You only know when you measure.
Some thing I do to measure when plugged in, is turn the system on and gently press the pin heads from the multimeter into the connector from above, next to the cable. This way the contact is shielded and less chance for shorting out the PSU. If the pins are thin enough, it should work for any PSU connector, at and atx.

While measuring on the connector is obvious,
I once ran into the problem where the contacts of the connector were corroded and measuring on the connector coming from the psu the voltages were always correct,
yet measuring on the mainboard itself, it was clear that there was a voltage drop high enough to cause instability of the system.

So measuring on the board... the ISA/PCI/PCIe bus offer some voltages.
the USB connector only offers 5V
main voltages of interest are +12v +5v +3v3
3v3 might be found on some ICs, though with all the balls these days fairly unavailable to measure.
measuring on the ceramic capacitors can help, though don't press your probes into them. Breaking them, fracturing 'm will cause all sorts of nastiness .

Thanks for the replies!

@Waterbeesje, your tip worked great to measure the connector directly. All voltages appear fine.

Meanwhile I learned that the IDE to CF Card reader got fried for more people if I read the reviews (It was a StarTech IDE2CF), so this may have been the troublemaker all along. I ordered a different one now.

If that doesn't resolve it I will do as weedeewee says and measure on the mainboard directly.

Niborius wrote on 2022-06-07, 15:00:

Forgive me if this is a stupid question, but if possible I would like to test the voltages of my PSU while it is connected to th […]
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Forgive me if this is a stupid question, but if possible I would like to test the voltages of my PSU while it is connected to the motherboard, using the metal ends of the pins on the back of the board.
The reason I'd like to do it this way is because the cable (it's an AT PSU) was very hard to get onto the motherboard and if possible I'd just prefer to leave it untouched now.

I should also add that I haven't used a multimeter before, let alone test a PSU. Is there anything in particular I should look out for?

The reason I'd like to test the PSU is because I'm having numerous issues with my retro PC and I can't find the source of it. Just last week a IDE CF Card reader seemed to cause a short and is now fried, although I'm not 100% if perhaps a metal bit of the back of the card reader touched the case.
But other than that I also have issues with I/O Controllers not working properly and/or dying.

Could it be that the PSU is producing voltages that are too high/low?

Push the test leads into the top of the atx connector.