Lots of experience here.
Just not with reversing the symptom.

Sorry. Boards tend to do that.

There are the BIOS bootup beeps, if you get that far, dignostic cards and of course, try another psu. Good luck!

Check the PSU indeed, check the caps and then try to find if there isn't a hot spot on the board (maybe one of the MOSFETs is shorted ? It happened to me recently, I had a Abit VH6T that was doing something like this and after I did it a few times something smelled like burned electronics. And that was a mostfet. I replaced it and the board worked, although the CPU got killed unfortunately. Rip tualeron 1300)

Check this https://www.vogons.org/viewtopic.php?f=46&t=86581

Deksor wrote on 2022-03-09, 16:58:

Check the PSU indeed, check the caps and then try to find if there isn't a hot spot on the board (maybe one of the MOSFETs is shorted ? It happened to me recently, I had a Abit VH6T that was doing something like this and after I did it a few times something smelled like burned electronics. And that was a mostfet. I replaced it and the board worked, although the CPU got killed unfortunately. Rip tualeron 1300)

I had a slocket in this thing last and was really torching up the MOSFETs. I put little heatsinks on them and left it at that. Maybe they got too stressed and something cracked inside as they cooled down.

I pulled out my multimeter to check this since it's an easy test and, sure enough, W1002C is shorted! It appears to be a PWM MOSFET, but I can't find the datasheet.

Now that I'm actually paying attention, both legs seem to be sitting on the same ground plane so they're supposed to be shorted? They are shorted to the back part of the MOSFET as well and that can't be right. I'll have to remove it to be sure.

Don't quote me on that, but I'm not sure this is a transistor, looking at the symbol on the bottom left, it seems like it's some sort of diode

This is a double diode from Hitachi.
If i find the datasheet, i post here...

Found it !
This is a HRW1002A, i don´t know the differences for the HRW1002C...
But your problem is not this component, you need to see the transistor next it...
Good luck

If it's a diode then I shouldn't be able to read resistance in both directions.

I've never seen a diode in a TO-220 package before.

It's a rectifier diode array, very common in that package. Basically Pin 1 -->|-- Pin 2 (Tab) --|<-- Pin 3.

Kahenraz wrote on 2022-03-10, 09:05:

I've never seen a diode in a TO-220 package before.

It's not a diode, it's two diodes.
You could probably buy it for 30 ct. If you're taking 10.000 of it.
https://www.transistorall.com/transistor-data … t?search=W1002C

TheMobRules wrote on 2022-03-10, 09:41:

It's a rectifier diode array, very common in that package. Basically Pin 1 -->|-- Pin 2 (Tab) --|<-- Pin 3.

I don't know much about electronics but if it's a diode then shouldn't voltage be blocked from the tab to the legs in one direction? The diode setting in my meter reads a signal in both directions.

I went ahead and lifted the two legs and measured the diodes to be fine. It's not a dead short but I'm reading very low resistance between the two pads that the diode connects to (anode to cathode). I wasn't able to find a dead short on any of the components I tested elsewhere. I don't know if this is normal, since I don't have another identical board to test against.

The capacitors are all genuine Rubycon and Sanyo capacitors and would be surprised if they were the problem. They all look to be in good condition.

Hi,
First thing to do : test the motherboard with a know good power supply.
If there is no boot, verify with a multimeter on resistance all this pins (3.3V, 5V, 12V, -5V, -12V) on connector to ground (ATX connector) see figure.
Important : Connector of the ATX power supply disconected from the motherboard
If you have a resistance very low, most the time near 0 ohm, you have found the short.
The idea is to have less components to verify, if you know where the problem is...
Sorry for my english...

fool wrote on 2022-03-09, 17:25:

Check this https://www.vogons.org/viewtopic.php?f=46&t=86581

Damn! Now I gotta go through a box full of P2B checking for that.

BitWrangler wrote on 2022-03-10, 15:54:

fool wrote on 2022-03-09, 17:25:

Check this https://www.vogons.org/viewtopic.php?f=46&t=86581

Damn! Now I gotta go through a box full of P2B checking for that.

My P2B is revison 1.10 and there are no large diodes like the one on the P3B-F. The next closest thing would be the TO-220 diode which tested fine once I lifted the legs.

It doesn't make sense for there to be continuity where the diode would connect. It's not a dead short but the diode setting on my meter beeps and the resistance measures very low.

Kahenraz, are you measuring with or without the cpu installed?
also
What resistance value are you measuring ?

My P2B-F has similar layout by the SIMMS. Those are not for the CPU but for the ram, for the CPU they are between Slot-1 and the parallel port and have dual coils....