Okay, so I thought of one last thing to try since it's so late...and figured it out!

I had plugged in the ATX power connector backwards! 🙁....Yes, it's keyed, but was compliant enough to let me plug it in backwards.

So...means I connected the +5 VSB to the -12 volt line.

So guessing I should look for what capacitors are on that line and replace? Or should I leave good enough alone?

Also, any recommendations for replacing tantalums on these old boards? Especially on the 12V lines? I feel like those are usually the problematic ones. Recommendations for what ratings to replace them with for an MLCC?

We are going to need to see pictures of the board.

If the power supply is a good one, it SHOULD detect overvoltage/overcurrent and turn off before TOO much damage occurs. We don't live in this world of unicorns.

most likely the power connector has suffered damage and caps on the 5v power rail are hoop-a-jooped.

Guld wrote on 2024-09-07, 01:29:

I had plugged in the ATX power connector backwards! 🙁....Yes, it's keyed, but was compliant enough to let me plug it in backwards.

HOW????? multiple pins are shaped differently, this is no https://www.youtube.com/watch?v=cUbIkNUFs-4
if the supply somehow actually started -12V rail is least of your problems

exactly the same question , i was thinking of the usual AT style pins in my post. now?

I have to see.

Guld wrote on 2024-09-07, 01:29:

So...means I connected the +5 VSB to the -12 volt line.

Not only that, but the PSU tried to start. Inserting the connector rotated 180 degrees* shorts out PS-ON to GND (among other things). Carefully check the board for burnt traces, swollen capacitors and anthing out of the ordinary, then try powering it with the connector in the proper orientation. If lucky it'll just work, else maybe follow the smoke/sparks/charred copper and try to repair it 😀

* - not hard to do, btw, especially when both connectors are made of the cheaper plastic, the one that's almost translucent (and very soft). I had to bodge a few cables for modular PSUs that were lacking them, and sometimes I had to improvise...

quicknick wrote on 2024-09-07, 22:29:

Guld wrote on 2024-09-07, 01:29:

So...means I connected the +5 VSB to the -12 volt line.

Not only that, but the PSU tried to start. Inserting the connector rotated 180 degrees* shorts out PS-ON to GND (among other things). Carefully check the board for burnt traces, swollen capacitors and anthing out of the ordinary, then try powering it with the connector in the proper orientation. If lucky it'll just work, else maybe follow the smoke/sparks/charred copper and try to repair it 😀

Yeah, I had been working with AT system lately and the ATX connector went right in without any difficult. It wasn't until later that I thought...hey.... and looked again and saw that I had plugged it in backwards...ugh.

So yes, +5 VSB would have grounded and so would several other lines (including PS_ON unfortunately).

Here is the connector on the motherboard...yes it's keyed, and has the clip connector. Kinda kicking myself for being tired and putting it in backwards. I was as shocked as anyone it provided so little resistance.

The good news is...the system does still power up (when connected correctly this time). But wondering if I should proactively look for any polarized caps on the -12 V line at a minimum.

Power supply is a Seasonic unit, and hopefully fairly good at protecting the motherboard! But the usual caveat of not all powersupplies from a brand are good 🙁.

nothing has obviously let the magic smoke out. I would leave it alone for new parts. It seems the power supply worked correctly and shut off before any damage occurred. I would count your blessings and just move forward at this point.

I've also never plugged in an atx power connector backwards before. At least not intentionally. I will pay more attention to this next time. 😀