Some capacitors are not necessarily critical to the operation of the motherboard and tantalum capacitors are known for popping and their black smoke.
However, I would in any event look into replacing those capacitors as soon as possible as it might not right now present any problems but, could in future lead to further issues.
I suggest you refer to this website, which contains a vast amount of documentation and information on the various IBM models: http://minuszerodegrees.net/

duddingston wrote:

I located and removed some problematic capacitors (C56 and C58) and sure enough the thing kicked in.
...

Does anyone know what the C56 and C58 capacitors do (i.e. whether it might be linked to the poppng 'incident'), and whether I should replace them? Once I have the keyboard I'll be able to work out what exactly the 'pop' may have killed.

Ugh.. Maybe your not the best qualified person to be identifying and randomly removing capacitors so dont do that? 😒

duddingston wrote:

and whether I should replace them?

yeah. useless caps. remove them all, they are not needed. 🤣

Thanks jesolo - I've read up on the site you shared and see that both capacitors filter 12V lines going to the expansion slots and "aren't critical to the operation of the motherboard". I'm going to replace them in due course, though. Thanks for your help.

BloodyCactus - If I only did the things I was the most qualified person to do, I'd not get very much done.

I had the same thing on my 5160 but I had them replaced,rather than left out.

I'd just go and replace ALL the voltage line capacitors anyway since they cost a penny and it's really an easy job to do. If one goes you can bet the rest of them are going to start popping quite soon.

Murugan wrote:

I had the same thing on my 5160 but I had them replaced,rather than left out.

Thanks - love your profile pic btw.

sf78 wrote:

I'd just go and replace ALL the voltage line capacitors anyway since they cost a penny and it's really an easy job to do. If one goes you can bet the rest of them are going to start popping quite soon.

Great idea. Will do.

Filter capacitors are, in theory, completely useless. In practice, they're only somewhat useless.

Good electrical engineering practice dictates that you should place a supply rail decoupling capacitor as close as possible to every IC. That's why you see lots of old through-hole boards with rows of ICs and an accompanying ceramic or tantalum capacitor right next to them. Digital circuits, by their nature, switch between V+ and 0V as quickly as possible. This causes tiny surge currents that, combined with the inherent resistance in all conductors, causes voltage droops on the V+ rail, and ground bounce on the 0V rail. Decoupling caps help to mitigate some of these effects by providing a low-impedance voltage source to satisfy that sudden surge. The circuit will continue to work without them, but it gets noisier the fewer caps you have, until the point where it gets so chaotic that logic circuits have trouble differentiating the high and low levels properly.

Tant caps tend to fail as a short circuit, so they might cause the PSU to go into protect mode, or (if the PSU is brawny enough) will just catastrophically remove that short circuit. It's somewhat unpredictable whether the bad cap will fail first, or if it will burn up a PCB trace first. It depends which has the highest current-carrying capacity -- which is not a spec that you would ordinarily design to.

This is all "teach a man to fish" style, and essentially to say... the error you're getting COULD be related, if for example, the failed cap took long enough to pop that it damaged part of the circuit which supplies power to itself and the keyboard controller. Or it might be totally coincidental and you just need to use some contact cleaner on the keyboard port.

You got yourself a piece of history there, so treat it like one. Now you are not just a computer geek. Now you are a custodian of a technical monument.
While sometimes caps or resistors are intentionally omitted by factory even when there is a place for them on the motherboard (for cost reduction purposes or maybe, they are not required in later revisions, while the PCB stays the same) it's counter logical to remove electronic componenst that were installed (and are not intended to be removed or reconfigured like FPUs or RAM), wishing that everything will work fine. Believe me - corporation like IBM wouldn't install unnecessary parts because it means a loss of money for them. If they were installed - they are required. Caps serve a pretty important role for voltage stabilization, buffering, filtering unwanted noise. Remove them and you will go into all sorts of problems.