I'd say it's related to the video card.

Is there any difference if you have RAM installed vs no RAM?

Can you try a different BIOS/EEPROM?

Do you have a diagnostic POST card?

8 beeps – Display (Graphic card) memory read/write test failure.

Cards getting power?

Hmm I see 14 big caps total but you replaced 13 (from picture at TRW). If any where bulging all the rest are suspect...including the 6 small ones.
Have you reset the bios after replacing the coin cell ?
Added: Suggest you forget about the AGP for now. The PCI slots should be getting 5v so try again with a known older PCI card....

tauro wrote on 2024-12-23, 22:09:

I'd say it's related to the video card. […]
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I'd say it's related to the video card.

Is there any difference if you have RAM installed vs no RAM?

Can you try a different BIOS/EEPROM?

Do you have a diagnostic POST card?

With no RAM I get 4x rapid beeps repeating
I have a POST card somewhere which I’ll test
I’ll burn another bios & try that also

myne wrote on 2024-12-24, 01:20:

8 beeps – Display (Graphic card) memory read/write test failure.

Cards getting power?

I tested with an AGP card that a heat sink/fan, Fan spins as it should

So that's 5v working.
There's more

Some gigabyte atx mobo have 3 lags linear regulator from +5v to 3.3v for agp
Don't know why
Check the +3.3v agp voltage

shevalier wrote on 2024-12-24, 09:58:

Some gigabyte atx mobo have 3 lags linear regulator from +5v to 3.3v for agp
Don't know why
Check the +3.3v agp voltage

Just checked that voltage regulator, 4.9v in & only 1.25v out, Looks like we’ve found the culprit

I don’t have any spare LX8384 on hand but do have an LMS1587 on hand, Is that a suitable equivalent?

https://www.alldatasheet.com/html-pdf/76461/M … 5/1/LX8384.html
https://www.alldatasheet.com/datasheet-pdf/pd … I1/LMS1587.html

You could just solder wire directly from ATX connector 3.3v pin to AGP power plane, even if not as a permanent mod then just for testing.

ERX400 wrote on 2024-12-24, 14:40:

only 1.25v out,

Looks like Vtt voltage
Are you sure this is a regulator for an AGP slot?

If it's definitely an AGP slot, then it's better to replace it with a jumper from the power connector.
The regulator is redundant in that location, a leftover from the AT motherboard era.

The LX8384 above the north bridge generates vtt, 1.25v for Tualatin, 1.5v for coppermine off 5v rail , do not short it to 3.3v rail.
the NEC mosfet under the pci slot 1 sends 3.3v from psu to the agp slot.

I read the voltages of the 3x NEC mosfets on the board while board powered on. My readings are as follows: MOSFET closest to CPU fan connector - Gate: 3.71v Source 1.75v. MOSFET closest to CPU socket - Gate: 5.51v Source: 0v. MOSFET closest to PCI slot 1 - Gate: 5.28 Source 3.28v