Yeah, it's probably a short circuit. If you can spot any blown caps it may be easy to fix, or not.

If it was just a dead BIOS or a problem with the Power Good signal detection the PSU would still turn on. If it doesn't, it's almost certain the PSU's short circuit protection is kicking in.

edit: nevermind, I only read the thread title without reading your post 🤣

If you have an ohmmeter, could try checking for shorts between ground and the other pins on the power connector. Here's a pinout: http://pinouts.ru/Power/MotherboardPower_pinout.shtml

I wish I'd me more knowledgable in electronics... so, I would use ohmmeter or continuity test to check between the ground and every pin on the power connector on the motherboard? If there is a connection then this is a problematic line, right?

Ok. I measured it. On my multimeter 1 is no connection and 0 is good connection. I get the following readings:

Orange, White, Blue lines = 1 (no connection)
Black are ground, so there is 0 (good connection)
On All Reds I get around 0.435 (so there is some connection)

What now?? 😕

I'll contact the seller as this problem is beyond my abilities.

kixs wrote:

Ok. I measured it. On my multimeter 1 is no connection and 0 is good connection. I get the following readings: […]
Show full quote

Ok. I measured it. On my multimeter 1 is no connection and 0 is good connection. I get the following readings:

Orange, White, Blue lines = 1 (no connection)
Black are ground, so there is 0 (good connection)
On All Reds I get around 0.435 (so there is some connection)

What now?? 😕

Is that 0.435 Ohm or kiloOhm (or MegaOhm)? .435 Ohm is almost a short circuit. Does the resistance start low and increase quickly? (It should, if the caps still do something)

And did you measure with the black or red lead on ground?

kixs wrote:

I'll contact the seller as this problem is beyond my abilities.

If it was advertised as working, that's the best thing to do 😉

It was advertised as Used. Not tested, but should work.

That 0.430 is in the continuity mode. I should recheck in resistance mode.

Exchange it or get a refund and buy a different one.

kixs wrote:

It was advertised as Used. Not tested, but should work.

That 0.430 is in the continuity mode. I should recheck in resistance mode.

If it's some super rare board it might be worthwile to try and fix it. Soldering iron and multimeter can get you pretty far.

If you checked on continuity/diode test mode, that would mean there's a 0.430V voltage drop across the pins. Looks normal to me.

I measured the resistance between ground and other lines.

Orange = 2.7K Ohm
Red = 1.7K Ohm
Yellow = 1
Blue = 40 Ohm
White = 1

In the mean time I got reply from the seller, telling me to replace the battery - which is in a bad shape, but I don't think it will solve the problem.

kixs wrote:

Yellow = 1
White = 1

1 what? Ohm? Is that with the PSU connected to the motherboard?

kixs wrote:

In the mean time I got reply from the seller, telling me to replace the battery - which is in a bad shape

And by "bad shape" you mean it leaked all over the motherboard and caused corrosion and oxidation on a few tracks and components?

40 ohm on blue (-12V) seems suspicious. I don't think the mainboard even uses that, only expansion cards do. Try cleaning the ISA slots and see if you can find out where the -12V trace leads to.. however that might be difficult with a multi-layer board.
You could also try removing the blue wire from the PSU plug and see if it'll boot without it.

devius wrote:

1 what? Ohm? Is that with the PSU connected to the motherboard? […]
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kixs wrote:

Yellow = 1
White = 1

1 what? Ohm? Is that with the PSU connected to the motherboard?

kixs wrote:

In the mean time I got reply from the seller, telling me to replace the battery - which is in a bad shape

And by "bad shape" you mean it leaked all over the motherboard and caused corrosion and oxidation on a few tracks and components?

Yellow = 1
White = 1

This is what the multimeter shows as default number and measuring resistance doesn't change it. Like there is no connection between Yellow/White with ground.

Yes, battery has leaked, but I've seen worse. I'll upload the picture later.

jwt27:
I'll do that.

This is an area around leaking battery:

Visually there is nothing suspicious with -12V line.

Removing the battery won't fix this, so I'm not removing it. I'll wait some more if someone can help me fix it. Otherwise I see no other option except to return it.

Leaked batteries usually do a lot of damage that isn't apparent at first glance. You would have to remove the battery and clean the area very well. Then, even if you could fix the short circuit it's extremely likely that the keyboard won't work, since that is the first thing to go when batteries leak. If you're really lucky, no traces would be cut and the keyboard would still work, but that's a risk.

Either return it or ask for a refund. This board is in bad shape and will require a lot of work to recover, if it is recoverable at all.

This board looks like worth saving with simm slots.
The guy sold it cant help the age of the parts, it is good they still sell to be able for us to fix it rather than throwing them in the bin -> then theyre ever lost. so return it, and it turns scrap probably.

The -12v should be broken, tantalum caps short out like this at age measure what cap is connected to the -12v pin and remove to see it starts.
the psu is overcurrented at the -12v line and cannot start up properly.

remove the bad cap(s) and clean the oxide with vinegar and wash with water / ipa afterwards.

Also make sure to load the 12v line with a cd drive, or some psu's dont start properly either or start overvolting some power rails.
It kills hw to underload a psu. 5v load and 12v minimal 500ma to 1A current load.
Modern psu's don't bother to much about it, old at style are not so much technique wise 😒

Thanks for the tips.

I also found this page about restoring a broken XT with similar problems:
http://www.classic-computers.org.nz/blog/2010 … g-an-IBM-xt.htm

I managed to get a partial refund on the motherboard and now I can start fixing it.

There are many Tantalum caps on the (-12V) line... I just hope the broken one(s) is closest to the power connector.