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Repairing an IBM XT motherboard

mR_Slug

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Hi, I recently came across a 64-256 XT system. There is one slight problem with the motherboard. Capacitor C56 is blown, its located near slot 5 at the back of the computer. If you power on the computer the cap catches fire:-D

The cap is charred and I cant read the value of it. Please could someone give me the correct value? Also does anyone know what may have caused the cap to blow in the first place?

I tried to get the tech reference manual from here:
ftp://ftp.oldskool.org/pub/misc/IBM5160TechRef.pdf

But the link seems to be broken. Not sure if it contains troubleshooting info on what to do in case of fire:)

Any help appreciated
 

modem7

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C56 is a tantalum identical to C68, C63, C46, C28 (and others).

Labeled "10-16". Motherboard schematic indicates that C56 is 10uF.
Therefore, C56 is a 10uF tantalum rated at 16V.

Schematic indicates that together with the adjacent C55, it is filtering the +12V line.

Cap can blow due to overvoltage (the 16V rating being exceeded by ?? %), so check the voltage on the +12V line.
Cap might have blown due to old age (e.g. deteriation). I had a tantalum in a Tandon 360K drive convert to smoke a few months ago.
 

nige the hippy

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Was just about to say the same, I've had tants go short on their own. you can probably just snip it off for now, it's just a de-coupler, the PC should still work ok.

Thanks for the "dead" link to the xt manual, it's a bit alive sometimes.
 

mR_Slug

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Wow thanks for the info. I appreciate the effort to look that up Modem7.

I took your advice nige the hippy, worked like a treat. I got some beeps! After connecting a MDA card/monitor and setting the correct dip switches…. It Posts!!!

I bought this motherboard + case just for the RAM, that turned out to be unsuitable for my AT.

I really am astonished this works at all, I had removed all the RAM, so I had to put all this back in. The power supply is an XT clone 65W model with a power connector that’s not quite compatible (its one single connector instead of two, with a clip I had to shave off,). Not to mention the cigarette lighting Cap:)

Now as for replacement, is it worth trying to solder in a new cap?

Oh and I got the tech ref manual too, great bedtime reading!!
 

modem7

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Now as for replacement, is it worth trying to solder in a new cap?
On a PC/XT motherboard, the +12V is sent to the I/O expansion connectors for possible use by expansion cards. The +12V is not actually used by the motherboard itself.
Any I/O card that uses the +12V is going to have it's own filter caps anyway. If it were my motherboard, I wouldn't be fussed about C56 being missing.
 

per

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I had an XT motherboard do that too. It's quite impressive, and rather pretty :rolleyes:

How do you detect when it happens? does it make a loud noise? (louder than the PSU?) or do you just have to wait until smoke comes out of the floppy drives?
 
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