Try checking the other voltages +5V and +12V and the battery.
Try removing everything but the CPU - it should still make POST codes without any RAM or cards so it helps eliminate the chance those components are causing some kind of mayhem.

Have you confirmed the CPU is good? Jumpers? If the multiplier is wrong a P75 probably won't be happy.
If it's jumpered for BIOS recovery or CMOS reset I'm not sure if it would produce POST codes, seems like it would though.

If you are able to remove the BIOS chip and have a way to dump it's contents, you could check it against whatever BIOSes are available online, if there are any.
If you install a socket and get some extra chips, you could burn another BIOS for it and see if it starts producing any POST codes. If you can't find a BIOS for this exact board, maybe just a BIOS for a board very similar to this one would still be able to do something.

shamino wrote on 2023-07-15, 23:03:

Try checking the other voltages +5V and +12V and the battery.

They are fine, that's my standard AT PSU for testing. And new CR2032 for sure.

shamino wrote on 2023-07-15, 23:03:

Try removing everything but the CPU - it should still make POST codes without any RAM or cards so it helps eliminate the chance those components are causing some kind of mayhem.

Yep, just bare PSU/MB/CPU/POST card

shamino wrote on 2023-07-15, 23:03:

Have you confirmed the CPU is good? Jumpers? If the multiplier is wrong a P75 probably won't be happy.

I tried it on both known to work P100 and P75. All frequency combinations. Like, I have S7 ASUS motherboard but I want S5 for such a low build and put MMX CPU on the second one with Necroware's VRM help.

shamino wrote on 2023-07-15, 23:03:

If it's jumpered for BIOS recovery or CMOS reset I'm not sure if it would produce POST codes, seems like it would though.

Tried all variants, first resetting, then enabling.

shamino wrote on 2023-07-15, 23:03:

If you are able to remove the BIOS chip and have a way to dump it's contents, you could check it against whatever BIOSes are available online, if there are any.
If you install a socket and get some extra chips, you could burn another BIOS for it and see if it starts producing any POST codes. If you can't find a BIOS for this exact board, maybe just a BIOS for a board very similar to this one would still be able to do something.

Seems like it's the only remaining option for me. I found there was Mr. BIOS for such boards, that could be interesting. So I will try socketing and will find a use for this BIOS chips at last.

Thank you!

Also look very closely for any physical damage, including to the pins on the surface mounted chips. Do any of the chips get hot?

Some board problems could drag down the voltages even with a good power supply.

shamino wrote on 2023-07-16, 11:19:

Also look very closely for any physical damage, including to the pins on the surface mounted chips. Do any of the chips get hot?

Some board problems could drag down the voltages even with a good power supply.

Voltages are ok, capacitors also looks ok. Nothing gets hot except for CPU for sure. Chipsets are barely warm.

There is BIOS recovery feature that is supposed to flash the BIOS from RAM using the small bootloader that doesn't even support video.

As I understand it requires memory to be installed, I have to connect floppy drive and set the jumper so it could read the BIOS but I don't see any FDD activity while I set this jumper to REC position.

Also it has to perform at least some POST tests, isn't it?

What else can I check before removing the BIOS chip? Should I see some activity on its pins with scope even if it's dead? Some bus activity?

rkurbatov wrote on 2023-07-17, 14:26:

Voltages are ok, capacitors also looks ok. Nothing gets hot except for CPU for sure. Chipsets are barely warm. […]
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Voltages are ok, capacitors also looks ok. Nothing gets hot except for CPU for sure. Chipsets are barely warm.

There is BIOS recovery feature that is supposed to flash the BIOS from RAM using the small bootloader that doesn't even support video.

As I understand it requires memory to be installed, I have to connect floppy drive and set the jumper so it could read the BIOS but I don't see any FDD activity while I set this jumper to REC position.

Also it has to perform at least some POST tests, isn't it?

Yes, I would think there should be some POST codes even in the recovery mode. If the boot block is damaged then recovery wont work though.

What else can I check before removing the BIOS chip? Should I see some activity on its pins with scope even if it's dead? Some bus activity?

I'm not sure how to confirm if it's attempting to access the BIOS chip, but it's an interesting idea. Maybe by scoping the CS pin?