Gsquared wrote on 2025-11-02, 15:43:

Initial power-on did nothing, the power supply fan turned on but nothing else.

One thing you should be aware of: On the old XT supply, the fan is an AC fan, connect directly to the 120V AC input (unsure about the European version). So for XT supplies, a running fan in the power supply only means AC is present, and does not imply that any part of the power supply is working. This is different in newer power supplies: Those newer supplies generally use 12V DC fans, and fan spin means that 12V DC is up and running. You observed it yourself:

Gsquared wrote on 2025-11-02, 15:43:

I checked the voltages on that first, and the 12v line output 2v.

12V was not up and running. The reason for the 12V to not work will be obvious in a moment. In your case, it was most likely not due to a broken power supply.

Gsquared wrote on 2025-11-02, 15:43:

So I bought another power supply which according to the seller was pulled from a working system. [...] but still nothing happened when I powered it on.

Your other power supply had similar issues.

Gsquared wrote on 2025-11-02, 15:43:

I took everything out except the video card, but when I powered it on a capacitor blew next to the AT connector on the motherboard labelled C58, followed shortly by the three-pin on the top right of the video card.

And that's actually due to the original fault of the system: These capacitors were broken all along, and kept the +12V line down by drawing excess current. After you removed the extra components, the power supply had enough power to blow the shorts on the +12V away.

Gsquared wrote on 2025-11-02, 15:43:

1. Did the power supply cause this? If so, how? I checked all of the voltages and they lined up well.

The power supply is not to blame. If the original XT supply is strong enough, it would have blown the caps as well. They were already bad when you got the system.

Gsquared wrote on 2025-11-02, 15:43:

2. How did the capacitors blow with the minimal setup but not the full setup? I imagine this has something to do with the first question.

There are two theories: One theory is that the power supply entered overload/short cirucuit protection mode with the full setup before heating the capacitors enough to catch fire. The other theory is that the caps degraded further on each power-up attempt, and it is pure coincidence that they blew after you removed the components.

Gsquared wrote on 2025-11-02, 15:43:

3. Stupidly, I didn't keep track of the position of the capacitor on the video card and I'm not sure if I put it in the right way. There seems to be only one trace on the board going to the middle pin, so does it even matter? I've attached some images for reference.

The 3-pin tantalum capacitors have the two outer pins connected to each other, so the orientation doesn't matter. If you replace them by more modern 2-pin tantalums, you need to connect the positive end to the center pin, and the negative pin to one of the outer pins. You can find further information about this type of capacitor and how to replace them on "-0°", one of the most comprehensive information sites about the IBM PC, XT and AT: https://minuszerodegrees.net/failure/IBM%2051 … 0capacitors.htm

It is very common for this type of capacitor to blow up when you power up an XT that has been in storage for a long time, so I don't think you did anything bad or wrong. If you were more experienced , you might have concluded that most likely one or multiple capacitors on the +12V are shorted before they caught fire, and have them removed without them exploding. But as this kind of failure (while slightly spectacular) does not typically cause any collateral damage, I wouldn't worry about it.

The +12V line on the CGA card is not used for anything except for the connector next to that capacitor which is meant to connect a 12V-powered monitor (like the internal monitor in the IBM 5155 Portable Personal Computer), so the CGA card would work fine even if you did not install a new capacitor. After replacing the capacitors, you should be able to run the system with either of your power supplies without further problems.

mkarcher wrote on 2025-11-02, 16:29:

12V was not up and running. The reason for the 12V to not work will be obvious in a moment. In your case, it was most likely not due to a broken power supply.
...
These capacitors were broken all along, and kept the +12V line down by drawing excess current.

I tested the old power supply with nothing connected to it. Nothing outside of the supply was pulling the +12v line down to +2v. That being said, it would make sense that these caps are going bad, I just figured they would be more robust than the electrolytic capacitors I'm used to working with and started worrying if the new power supply was bad in a different way.

You gave me the courage to power the motherboard again to see what would happen, and lo and behold another capacitor blew, C56 this time. At this point I just bought a new working motherboard instead of going through each capacitor until none of them blew anymore, or just replacing all of them outright. I'll test it again when it arrives.

Gsquared wrote on 2025-11-02, 18:36:

I tested the old power supply with nothing connected to it. Nothing outside of the supply was pulling the +12v line down to +2v.

OK, so I misunderstood that part. In that case, the +12V did not work, because you did not have sufficient load on +5V. The old PC supplies typically require a load of at least 1A at +5V to operate properly.

Gsquared wrote on 2025-11-02, 18:36:

You gave me the courage to power the motherboard again to see what would happen, and lo and behold another capacitor blew, C56 this time. At this point I just bought a new working motherboard instead of going through each capacitor until none of them blew anymore, or just replacing all of them outright. I'll test it again when it arrives.

If your new board is an original IBM 5160 again, it might have bad caps as well. If the condition is "worked before it was put to storage, so most likely still working", you might face the same issue with the new board. The tantalums on IBM hardware of that age are well-known for going bad. Recapping at least all +12V and -12V tantalums on a 5150 or 5160 mainboard is somehow a rite of passage for retro computing enthusiasts.

Recapping early IBMs in almost always needed/recommanded.

Also, Are you absolutely sure the connector is wired correctly and voltages are in the right place.
I noticed a few times you referred to the "AT" connector - but the that connector didn't come into use until the PC-AT
The XT had a different connector - I don't recall how compatible the power supplies were otherwise.

DaveDDS wrote on 2025-11-02, 23:09:

Recapping early IBMs in almost always needed/recommanded. […]
Show full quote

Recapping early IBMs in almost always needed/recommanded.

Also, Are you absolutely sure the connector is wired correctly and voltages are in the right place.
I noticed a few times you referred to the "AT" connector - but the that connector didn't come into use until the PC-AT
The XT had a different connector - I don't recall how compatible the power supplies were otherwise.

Huh, I didn't know that. Still, the other power supply I got was made for an XT clone. I made sure the voltage of each pin matched the old power supply, it's just the connector itself didn't fit on the motherboard.

mkarcher wrote on 2025-11-02, 21:48:

If your new board is an original IBM 5160 again, it might have bad caps as well. If the condition is "worked before it was put to storage, so most likely still working", you might face the same issue with the new board. The tantalums on IBM hardware of that age are well-known for going bad. Recapping at least all +12V and -12V tantalums on a 5150 or 5160 mainboard is somehow a rite of passage for retro computing enthusiasts.

Well then, good to know lmao. I should have figured since the same thing applies for old Macintoshes, which I have worked on before. The listing on eBay showed it working in a screenshot, so it should be fine for a bit at least. Silver lining, the board the computer came with is the 64-256k board while the one I bought is the 256-640k, so I won't need the RAM expansion boards the computer came with.

DaveDDS wrote on 2025-11-02, 23:09:

Also, Are you absolutely sure the connector is wired correctly and voltages are in the right place.
I noticed a few times you referred to the "AT" connector - but the that connector didn't come into use until the PC-AT
The XT had a different connector - I don't recall how compatible the power supplies were otherwise.

As far as I know, the only difference between the XT and the AT power supply pinout is that pin 2 is not connected/key on the XT and +5V on the AT.

I just rechecked the IBM PC technical reference manual (16KB-64KB), and it seems to confirm my claim. It has this pinout for the power supply:

11 - PWR GOOD
22 - KEY
33 - +12V
44 - -12V
55,6 - GND
6-----
77,8 - GND
89 - -5V
910,11,12 - +5V

I continued counting pins over both 6-pin connectors.

The usual references for AT power supplies indicate

11 - PWR GOOD (typically orange)
22 - +5 (typially red)
33 - +12 (typically yellow)
44 - -12 (typically blue)
55,6 - GND (typically black)
6---
77,8 - GND (typically black)
89 - -5 (typically white)
910,11,12 - +5 (typically red)

The connector with pins 1-6 is called "P8", the connector with pins 7-12 is called P9. The colors may deviate, especially in early AT supplies by big brands (HP, Compaq). I've repeatedly seen orange for +12V, and thus some other color (green?) for PWR GOOD.