Do you have a multimeter? If something on the motherboard was damaged, you'd need to start performing a systematic diagnosis of the power circuit on the motherboard and determine which component(s) have failed.

A POST diagnostics card can also help in indicating if voltages are being distributed and if the motherboard is even trying to POST.

The other thing to test is swap out all the components for different ones (e.g. RAM, CPU) to verify that it is indeed the motherboard that is damaged, and not something like the RAM or CPU.

Somehow I doubt these are 470uF primary side caps. Look too small for my taste, and why you'd even need 2x470u for 300W PSU. But counterfeit capacitors rarely cause such problems, in fact good quality but rebranded 220u or 330u would work fine - and that's how this scam existed since the '90.
The diodes look fine. Caps don't have blown tops. Can't quite see but the switching transistors look intact too. They don't always break apart due to a short but it's pretty common. In other words can't really tell what's wrong just from the photo.

I concur, get yourself a POST card, try the mobo with a different PSU. Use the reset button in case the PWR_GOOD line got somehow damaged and the mobo is just not starting properly. After that swap RAM sticks and CPU. With a POST card you can try running with no cards (including video card), it should at least start and output some codes before giving up.

Shponglefan wrote on 2024-01-22, 17:36:

Do you have a multimeter? If something on the motherboard was damaged, you'd need to start performing a systematic diagnosis of the power circuit on the motherboard and determine which component(s) have failed.

A POST diagnostics card can also help in indicating if voltages are being distributed and if the motherboard is even trying to POST.

The other thing to test is swap out all the components for different ones (e.g. RAM, CPU) to verify that it is indeed the motherboard that is damaged, and not something like the RAM or CPU.

I have a multimeter, but not enough experience to perform a systematic diagnosis of the power ciruit. But I could try a Post diagnostics card. I don't have much experience in using those either. Will this work on my Soyo 025N-card? (it has ISA slots).

https://www.ebay.com/itm/315088282363?hash=it … ABk9SR7zMrKumYw

Deunan wrote on 2024-01-22, 19:11:

I concur, get yourself a POST card, try the mobo with a different PSU. Use the reset button in case the PWR_GOOD line got somehow damaged and the mobo is just not starting properly. After that swap RAM sticks and CPU. With a POST card you can try running with no cards (including video card), it should at least start and output some codes before giving up.

Yes, I have a different PSU. I'll try to get a POST card, and do some more testing. But in the end I guess I have to buy a new motherboard. Anyway, thanks for the help.

Hey mate ,

I see there is switch between 110V and 200V AC .Maybe it set in wrong position ?

ciornyi wrote on 2024-01-22, 20:33:

Hey mate ,

I see there is switch between 110V and 200V AC .Maybe it set in wrong position ?

No, that was checked and double checked before connecting 😀 And checked again afterwards to make sure that wasn't the reason.

Anyway we use 220V, så 110V probably wouldn't have damaged anything.

Definitely not an Antec. I have an actual PP-300V and it's packed full of components, the transformer is much larger and more appropriate for 300W. Quite heavy.

ciornyi wrote on 2024-01-22, 20:33:

Hey mate ,

I see there is switch between 110V and 200V AC .Maybe it set in wrong position ?

Funny I just had this problem . My new powersupply was 220VAC and caused the exact problem you have. Power supply heated up and nothing else really happened.

I vote again for the post-code and checking the voltages. I found since I was feeding the PSU 120VAC the whole thing was under-voltage.

Runar77 wrote on 2024-01-22, 21:10:

Anyway we use 220V, så 110V probably wouldn't have damaged anything.

110V setting + 220...230V input is exactly what causes damage, when the PSU is set to 110V it engages internal voltage doubling to basically convert it to 230V operation. Doing this conversion again will exceed all breakdown voltages and causes the primary side parts of the PSU catastrophically fail.

..you'd have to wonder why the head of this screw looks the way it does...

...junk 85C caps from looks like at least 6 different suppliers.... geez, cheap and nasty....even the TL494 is a knock-off...

The PSU has blown it's fuse ~ either rectifier diodes (or more likely), main chopper(s) have failed.... and I agree, doesn't look like a 300watt supply to me, maybe 200w...

What mainboard is it? A possible scenario is a tantalum cap has failed on the 5volt rail, which took the PSU beyond it's rating and kaboom....it failed before the tant. exploded....and that capacitor is still shorted on the mainboard (hence no boot and squeaky beep) ~ you would need remove cards/cpu/ram and check the resistance of the 5volt rails.

Tiido wrote on 2024-01-23, 00:26:

110V setting + 220...230V input is exactly what causes damage

Yeah, I was thinking the "wrong way" there. You are right. But it was set to the correct voltage.

Runar77 wrote on 2024-01-22, 17:16:

Yesterday I installed a brand new AT PSU in my 486 computer (Labeled "Antec PP-300V", but it's not Antec apparently. Stay clear of that cr*p). When I turned on the computer, there was one beep from the PC-speaker, it checked the RAM and correctly told me that there was no keyboard detected. Then the room went dark as the PSU blew my fuse.

Turned the fuse back on, and tried to turn the PC on again, but nothing happened. The PSU was dead.

I have seen AT and ATX psu, using tl494 pwm / lm393 comparator badly configured, with bad caps and leaky diodes on the secondary kill pc's. Even in a group regulated topology the 12V rail went nuts only for 300- 600ms sending some noisy 16- 18V DC and kill motherboards. Looking at the way the fuse was blown in your case it looks like something failed catastrophically on the high voltage primary side. Troubleshooting and fixing up a crap unit like this does not worth your time and money. Diagnose ad save the mainboard.

demiurge wrote on 2024-01-23, 00:09:

Funny I just had this problem . My new powersupply was 220VAC and caused the exact problem you have. Power supply heated up and nothing else really happened.

I don't know if the PSU heated up. Didn't seem to do anything really. Wrong voltage was not the case anyway, and I was thinking wrong. Have I used the wrong setting, I would have ended up feeding it 220 volts when it was set to 110 which obviously would have gone wrong.

giantclam wrote on 2024-01-23, 00:39:

What mainboard is it? A possible scenario is a tantalum cap has failed on the 5volt rail, which took the PSU beyond it's rating and kaboom....it failed before the tant. exploded....and that capacitor is still shorted on the mainboard (hence no boot and squeaky beep) ~ you would need remove cards/cpu/ram and check the resistance of the 5volt rails.

It's a Soyo 025N mainboard. I have been offered some assistance from a guy I know. I can ask him to check this. Thanks.

DAVE86 wrote on 2024-01-23, 08:35:

Troubleshooting and fixing up a crap unit like this does not worth your time and money. Diagnose ad save the mainboard.

Thanks.
I have no interest in trying to save the PSU. But if it's possible to save the main board, I would like to try that.

I honestly don't know what is happening. The mainboard has gradually improved. From being totally dead (only the CPU warming up), it now posts to the point where it displays the "no keyboard present" message. Sadly I found out I don't have a keyboard with the right plug, so I don't know if it works beyond that point.

But between being totally dead, and now seems to completely post, it was very unstable. Sometimes nothing happened, sometimes it displayed part of the "Award Modular Bios"-message, sometimes displaying the CPU, sometimes gradually checking the RAM (could stop in the middle of a count), sometimes seems to complete the post. But I have tried 20 times now, and it seems to work.

Can a mainboard just start to work like that after being dead? What has happened? Can I count on it being stable? (I don't know yet if it will boot into an operating system, I have to get hold of a keyboard or an adapter for PS/2 to do that).

There are tantalum caps on the board, maybe a shorted cap gradually burned out to the point 5V is high enough.

rasz_pl wrote on 2024-01-23, 19:38:

There are tantalum caps on the board, maybe a shorted cap gradually burned out to the point 5V is high enough.

If that's the case, do you think the mainboard will be stable? Or should I just get a new one?

measure resistance on 5V rail on the power connector with supply unplugged