Inspect very carefully of each C7 3 legged capacitors, one of them might be cracked or blackened.

Use lens if you can borrow.

Cheers,

figure out which power rail is shorted.
+12v, +5v, -5v, -12v or PG

if it's the +-12v or -5v rail, it's going to be a capacitor.
if it's the 5v rail, it's likely a capacitor but it can also be one of the ICs.

If you have a bench power supply, you can hook it up to the shorted rail and figure out by touch what gets hot. limit it to half the max voltage the rail can do and limit the current to 1A.

fyi, the cap in the middle of the photo looks a bit sus to me.

pentiumspeed wrote on 2023-03-12, 22:04:

Inspect very carefully of each C7 3 legged capacitors, one of them might be cracked or blackened. These 3 legged capacitors are tantalum and they tend to fail at power on and short out with a bang.

Use lens if you can borrow.

Cheers,

Thanks for all the feedback. I'll look at the capacitors extra closely a second time.

When possible, could you post 4 pics of 4 quadrants of the board.
The pic isn't hq enough to see clearly.

Hope you solve as this is a very nice board. Really nice!

Nexxen wrote on 2023-03-12, 22:28:

Hope you solve as this is a very nice board. Really nice!

It is a nice board. I was really sad when the magic smoke came out.

Below D8255 chip, T02 is a rifa? (the green square, vertical standing 😀)
A corner facing the D82 looks chipped.

Tried removing all ram chips?

Edit: add 3rd pic, top right yellow tantalum with a black spot, unless it's dirt.

Is the oddness on the end of 8237 left over stuff from a sticker ? I cannot see any thing like burned cap or trace.
Does the relay have a funny smell ? What does the back of the board look like ? Just curious.

Horun wrote on 2023-03-13, 00:50:

Is the oddness on the end of 8237 left over stuff from a sticker ? I cannot see any thing like burned cap or trace.
Does the relay have a funny smell ? What does the back of the board look like ? Just curious.

The back of the board is fine, but the 8237 is strange. The discoloration didn't come off with water or naphtha.

It looks like the plastic might have cooked.

I'll try replacing it.

Have you tried to measure for shorts on each power input? It could lead you to a shorted powerrail wich can lead to a shorted capacitor.

DerBaum wrote on 2023-03-13, 02:23:

Have you tried to measure for shorts on each power input? It could lead you to a shorted powerrail wich can lead to a shorted capacitor.

Ok, that’s easy enough. I’ll try that tomorrow

Yes, the +12V is shorted to ground

And it is this capacitor right here:

This page looks like it's got the info I need for the capacitor replacement.

https://www.minuszerodegrees.net/failure/IBM% … 0capacitors.htm

Makes me want to make sure my old XT gets power every 6 months instead over every 2 years.