I had similar situation not too long ago of one of the 12V voltage lines missing in the ISA slots, which caused serial mouse to be undetectable, but otherwise 486 MB was fully functional. I'm not sure how -12V line is important for an XT motherboard to work, but if -12V is present at PSU, but not in ISA slots, it most likely means that AT power header had corroded.

Measure resistance between +5V and GND at power header.

Thank you very much for your reply and the tip! 😀

I' have checked the voltages on what looks like a testing pad..
All voltages are there, so I guess both the contacts and the power supply are fine at least.

You have several tantalum caps on the mainboard, those are notoriously unreliable as they age and don't show any symptoms of failing until they explode to your face. Many XT boards have them and most require replacement by now. Fortunately, they can be measured while still soldered in so it's a quick check and very easy to fix by just cutting the legs of the old one and soldering in a new replacement. Just make sure you orient them the right way around (+ and -).

I see.. Do I have to check/replace the yellow ones (10µF 15v unipolar ?) only or the blue ones (mainly 0.1µF aka 100nF ?), too ?
And if I have to replace them, can I use electrolytic types instead ?

Don't start replacing caps yet, you can find bad/shunting caps (if there are any) one by one visually or by measuring resistance between +5V and GND. Does -12V appear now at ISA slots?

Aw, I already cut one of each's yellow cap's leg. Now the other voltage is gone, too. 🙁

I would not trust POST card at all in this situation, better measure those voltages directly from ISA slots.

You are right. 😳 Just checked the ISA bus directly and according to the multimeter, all voltages are there.
Though, I do not know how much current flows through these pins. A real power-consuming component like an LED would be nice.
- The keyboard is also still dead (none of the lights flash during POST). I guess I will need to attach a speaker, as well.

Okay, so I attached an old PC speaker.. No beep tones. 🙁

You cannot make a device to test the power consumption between two spots that would fit in a ISA slot as far as I know. The only thing you can do is to put the multimeter between the two parts of the PSU connector. That's the only way I know (and probably that exist except induction) to measure the flow of current going through a circuit.

You might also want to measure the resistance between GND and +/- pins. If it's really low (I think 100ohm would be considered as very low, but don't quote me on that), there's probably some other bad caps.

You might also want to reseat the chips just in case.

Thanks. So I guess I have to continue to tink a bit of this issue for a while.
My main area of concern is the fact that the multimeter might be of too high-impedance.
The measurement shows voltage, were a real device would not get anything meaningful out of it.

Hi, even if I have not had direct experiences with the MB XT, I could suggest you a line check, in this way you could find if even some other voltage is not ok.
It is very simple, with the MB disconnected from the PSU (therefore P8 and P9 disconnected), put the black tip where in P8 there is a black wire (N ° 5 or N ° 6), and with the red tip measure (OhmX1 ) values.
With this system I found on the Asus MEZ-M an infinity value in the + 3.3V, and in other MB other problems in other lines.
It is a system that also works to repair PSUs, in which case I use a tester set on the diode test with an audible alarm.

Thanks, I will try this, too! 😀 - I will also re-install new caps (to replace the ones I did cut off).
Maybe this will bring back the "3.3v" LED to life (it checks 5v if installed in ISA slots, actually).

PS: Just out of curiosity, does anybody know which components of an XT system are prone to fail or are known to use -12v.
- The serial port uses +12v/-12v, I remember. But the XT board has no USART ob-board, so it does not matter.

Installing new capacitors could restart the MB.
In the past it worked on a VGA VLB GD5424, after checking them with my ESR70, I found values ​​slightly above those printed, at which point I changed them to new ones, and it restarted.
However, about the POST CARD ISA, I'm pretty sure (but I could be wrong ???) that in the MB there is no + 3.3V.
I remember the POST CARDS of the 90s, among these was the RACER II, perhaps they are not to be found because they probably cost a lot.
However if you know the exact model of the original MB, it is likely that you can find the service manual, so at least you might know what the -12V circuits are.

found a manual for an 8 MHz turbo board, located here:
http://minuszerodegrees.net/manuals/Turbo-XT% … ion%20Guide.pdf
In addition to the jumper settings, you will find connection diagrams.

Many old power supplies do not have overcurrent protection.
If a tantalum shorts, this can lead to burning the trace(s) on the board, as they are often thin and function as fuse.
What resistance is between the P8/P9 post and the ISA -12V line?

I would reccomend to solder a wire under the -12v pin of the ISA slot then connect it to the -12v on the power supply... maybe it will run... I have a pentium 3 system with some busted diodes I think because I hadn't +12v whatsoever on the mainboard.. but in the underside of the board I soldered a wire from the +12v connection on the ATX connector to the +12v connection on the fan header and the board sprang to life and always functioned correctly 🤣

Nothing on the board uses -12V so even if -12v is faulty the board should turn on.

So best way is to just check tantalums for connectivity?