Power supplies usually have a protection circuit that can cut the voltage on short, but short on 12V rail can cause damage to both mobo and the ISA cards. First of all check for broken traces, also check the mosfets.

Blown tantalum on the card means high current went thru motherboard, take it out and look carefully at 12V track between furthest ISA slot (or the one where you put bad card during incident) all the way to power connector. Most likely that track is fried, it might have a break or it might be slightly charred raising its resistance thus supplying lower voltage and power.

Fix is easy, solder bodge cable on back of the board between AT power connector and all ISA 12V pins.

Alright, on some level, I didn't really need to make this thread, turns out the problem was pretty obvious haha. But it was right at the top edge of the board, and I was more inclined to check more centrally on the board, and nearer the expansion slots. So I suppose a more confident reiteration of what I presumed I needed to look for was helpful to get me looking again with more perseverance heh.

And it was a simple fix too.
And fortunately, it seems nothing else was damaged!

Thanks!

teh_Foxx0rz wrote on 2024-03-01, 08:44:

And fortunately, it seems nothing else was damaged!

That's what fuses are designed to achieve, and that trace just behaved like a fuse. Although I seriously doubt that this trace was intentionally designed as fuse. Possibly that trace was already damaged before the incidence and thus was the weakest point of the short circuit.

I see some original and probably not-so-great capacitor brands on the board, so I can only assume it's the same affair in the PSU if it's an old unit. If that's the case, have a look inside. Excessive ripple can sometimes damage tantalum caps, which is why it could have burned on that card. Or it could have burned due to old age. In any case, it's always worth checking out the power supply from time to time. I check mine every few years if I know they are not units with quality Japanese capacitors.

Also, now that you've identified the "weakest" trace on the 12V rail of the motherboard, I would strongly suggest placing a cut on that trace and an SMD fuse. 3-4 Amps should be more that adequate.

Moreover, it should go out as a warning to anyone using modern ATX PSUs with a single 12V rail capable of more than 18 Amps that such motherboard trace damage becomes much more likely to occur when a short-circuit happens. For this reason, I prefer to use older PSUs with more reasonable output ratings relative to the machine they are meant to power... and preferably from known OEMs that have good protections. OEM 200-300W PSUs from Dell and HP are pretty good candidates, especially after being recapped. I have many of these still in service 2 decades later. They shut off quickly upon detecting a short-circuit.

mkarcher wrote on 2024-03-01, 14:57:

That's what fuses are designed to achieve, and that trace just behaved like a fuse. Although I seriously doubt that this trace was intentionally designed as fuse. Possibly that trace was already damaged before the incidence and thus was the weakest point of the short circuit.

Electronics background here...

That trace couldn't have been damaged from before, otherwise it would not have worked at all.

The probable reason it got burned there rather than anywhere else on the board is likely that the trace is the thinnest there or has the least amount of cooling / coupling to the PCB, thus allowing it to overheat and burn the quickest there. It also comes down to manufacturing tolerances. The traces might look uniform to the naked eye, but some places it might be microns thinner than at others.

momaka wrote on 2024-03-01, 17:29:

Electronics background here... […]
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mkarcher wrote on 2024-03-01, 14:57:

That's what fuses are designed to achieve, and that trace just behaved like a fuse. Although I seriously doubt that this trace was intentionally designed as fuse. Possibly that trace was already damaged before the incidence and thus was the weakest point of the short circuit.

Electronics background here...

That trace couldn't have been damaged from before, otherwise it would not have worked at all.

The probable reason it got burned there rather than anywhere else on the board is likely that the trace is the thinnest there or has the least amount of cooling / coupling to the PCB, thus allowing it to overheat and burn the quickest there. It also comes down to manufacturing tolerances. The traces might look uniform to the naked eye, but some places it might be microns thinner than at others.

When I said the trace was already "damaged" before, I did not mean that the trace was completely broken (open circuit), but degraded due to corrosion or scratches, making it higher resistance at that point, which basically boils down the the same idea as "being some microns thinner", but due to external influences instead of due to manufacturing tolerances.

mkarcher wrote on 2024-03-01, 18:15:

When I said the trace was already "damaged" before, I did not mean that the trace was completely broken (open circuit), but degraded due to corrosion or scratches, making it higher resistance at that point, which basically boils down the the same idea as "being some microns thinner", but due to external influences instead of due to manufacturing tolerances.

Good point.
Corrosion could indeed become a factor.
I have some old motherboards from the PII era that are starting to get a lot of corrosion ingress on some of the traces. They still POST and work... so fingers crossed that continues to be the case for another decade. Not that an external trace can't be fixed. But if an internal one (inner PCB layers) opens, that'll make repairs exponentially more difficult if not impossible.

momaka wrote on 2024-03-01, 17:29:

Moreover, it should go out as a warning to anyone using modern ATX PSUs with a single 12V rail capable of more than 18 Amps that such motherboard trace damage becomes much more likely to occur when a short-circuit happens. For this reason, I prefer to use older PSUs with more reasonable output ratings relative to the machine they are meant to power... and preferably from known OEMs that have good protections. OEM 200-300W PSUs from Dell and HP are pretty good candidates, especially after being recapped. I have many of these still in service 2 decades later. They shut off quickly upon detecting a short-circuit.

Yes, I've been saying this for a while now, but for some reason people seem to think that using these modern power supplies on retro boards is inherently "safer". An 800W monster capable to deliver more than 60A on the 12V line paired with an old board full of dodgy tantalums is a fire waiting to happen.

The power supply is an interesting aspect of this in fact:

When I first plugged the culprit card into the board, the power supply I normally have in this PC just wouldn't start. It would nudge the fans but nothing else.

Naturally, because it's old, I thought the power supply was dead.
So I tried a different (also old, but seemingly cheaper) power supply I had spare.

And then that was when the damage happened.

I'm no expert but I feel pretty confident in saying that the tantalum capacitor was already shorted, hence the original power supply presumably having its overvoltage protection kick in. I'd also presume the trace blew where it did because either it just happened to be the weakest there, or perhaps it's somewhat of a bottleneck trace where other traces branch off from that, and for a split second were all trying to power up other parts of this system when the shorted capacitor added to that current draw through it.

I'll obviously be sure to check for shorts on new cards in future!

Of course for the record the power supplies don't have bulging capacitors and the voltages measure okay. I realise the hypothetical being mentioned was the ripple rather than the voltage but it doesn't feel like any problem with the power supplies.

The card with this presumably failed tantalum capacitor on was a ReelMagic CD MPEG-1 decoder; was eager to check it out. Even has the OPL3 addon board. A hunch tells me that it should still work despite everything, once the capacitor is replaced. Obviously, I'll check continuity is correct thoroughly after fixing it, haha.

Those tantalums decouple the power rails. They're not going to "pull" power through other components and damage anything as some have implied. What's going to get damaged is traces and possibly the power supply.

teh_Foxx0rz wrote on 2024-03-01, 23:35:

The power supply is an interesting aspect of this in fact: […]
Show full quote

The power supply is an interesting aspect of this in fact:

When I first plugged the culprit card into the board, the power supply I normally have in this PC just wouldn't start. It would nudge the fans but nothing else.

Naturally, because it's old, I thought the power supply was dead.
So I tried a different (also old, but seemingly cheaper) power supply I had spare.

And then that was when the damage happened.

I'm no expert but I feel pretty confident in saying that the tantalum capacitor was already shorted, hence the original power supply presumably having its overvoltage protection kick in.

Yes, that's indeed what happened most likely. Old Tantalum caps are known to bring new excitement to any system from time to time. 🤣

kingcake wrote on 2024-03-02, 01:56:

Those tantalums decouple the power rails. They're not going to "pull" power through other components and damage anything as some have implied. What's going to get damaged is traces and possibly the power supply.

Precisely.
O/P should just replace that Tant (and maybe the others if feeling nervous about them) and all should be well again.

teh_Foxx0rz wrote on 2024-03-01, 23:35:

When I first plugged the culprit card into the board, the power supply I normally have in this PC just wouldn't start. It would nudge the fans but nothing else.

...

I'm no expert but I feel pretty confident in saying that the tantalum capacitor was already shorted, hence the original power supply presumably having its overvoltage protection kick in.

Yeah, that's likely what happened, except you are using the wrong term. It's not overvoltage protection, but overcurrent protection on the original supply that prevented it from starting up. With the shorted tantalum on the 12V rail, the old supply was unable to get that rail even close to 12V, triggering an error indicator that shuts down the supply. The other supply tried harder to push more current into the 12V rail, desparately trying to get it up to 12V, with the consequence of burning the trace.

teh_Foxx0rz wrote on 2024-03-01, 23:35:

The card with this presumably failed tantalum capacitor on was a ReelMagic CD MPEG-1 decoder; was eager to check it out. Even has the OPL3 addon board. A hunch tells me that it should still work despite everything, once the capacitor is replaced. Obviously, I'll check continuity is correct thoroughly after fixing it, haha.

Exactly. Blown tantalums are just annoying and cause extra work, but seldomly damage something beyond hobbyist repair capabilities.

Alright, I've finally got around to replacing the capacitor.

Unfortunately though...
I just seem to get a white screen from the card. I have the VESA feature connector cable connected correctly, and then I plug my display into the ReelMagic card, but no.
I also get beeps from the BIOS as though there's no video card installed, if the VGA cable is connected to the ReelMagic card.

I'm not too familiar with the VESA feature connector though, so maybe there's something I'm missing. Or maybe this card did get damaged somehow after all...!

There is the complication that my BIOS doesn't seem to be writing the ESCD at the moment (just says "unknown flash type"), though I don't know what effect that would have on anything.