Did you use the same PSU in both cases?

That time feels like if the ATX power switch is stuck shorted.. It would power on then force a power off after a few secs.. I'd try a different PSU or momentarilly shorting the power header with a jumper and releasing it quickly.

Also, does the same happen without any CPU installed?

Shponglefan wrote on 2026-02-28, 14:35:

Did you use the same PSU in both cases?

Hi, I tried 3 different PSU's. All 3 are confirmed working.

Cheers,

wbahnassi wrote on 2026-02-28, 14:58:

That time feels like if the ATX power switch is stuck shorted.. It would power on then force a power off after a few secs.. I'd try a different PSU or momentarilly shorting the power header with a jumper and releasing it quickly.

Also, does the same happen without any CPU installed?

Hiya, I tried 3 different PSU's. All confirmed working units.

Same symptoms without the CPU installed.

Cheers,

Does it turn on the moment the ATX PSU's hard switch is turned on? Is there continuity between the two pins of the power header on the front panel connectors block?

wbahnassi wrote on Yesterday, 05:26:

Does it turn on the moment the ATX PSU's hard switch is turned on? Is there continuity between the two pins of the power header on the front panel connectors block?

No, I have to push the power button on the case to power the system on. No continuity between the two pins of the power switch header on the motherboard.

Cheers,

In general I find it unlikely that a CPU can cause MB failure, especially when it is a correct CPU for the platform. Your case just sounds a coincidence.

Your problem sounds like a power delivery issue, like perhaps a regulator is failed. It also just may be that the new CPU has higher power consumpition and it pushed the already failing component over the edge.

Did you test it before as a bare motherboard? Do you now have it installed in a case?

CharlieFoxtrot wrote on Yesterday, 06:44:

In general I find it unlikely that a CPU can cause MB failure, especially when it is a correct CPU for the platform. Your case just sounds a coincidence.

Your problem sounds like a power delivery issue, like perhaps a regulator is failed. It also just may be that the new CPU has higher power consumpition and it pushed the already failing component over the edge.

This.

It would be good to test the CPU on another board but you gotta work with what you have.
Even if the K6-III/333 isn't faulty it is still one of the more power hungry socket 7 CPUs.
I'd be checking the power delivery on the board, particularly the 5 big caps next to the CPU socket. They're from the era of limited lifetime caps.

Take the motherboard out, check for shorts on the bottom, check all metal standoffs are in the right place...

1 Congrats on a CPU that officially doesnt exist according to AMD 😀 its the year 2000 Fry's Electronics Black Friday special. Afaik available only there and then for cool $29.
2 diagram for GIGABYTE GA-5AA rev3.2 Re: 430VX or 580VPX schematics and post describing 2.2 vs 3.2 differences Gigabyte GA-5AA revision 1.1, 2.2 and 3.2 differences

on Ver2.2 CPU Vcore is generated by non stnchronous DC/DC converter. U11 controller + q8 mosfer + D10 diode + L10 coil.
You can check if everything is working by measuring voltage on L10 coil and on pin2 of U11.
Personally I would start by recapping this board. Dont fret too much about special brands, any cheapest lowesr caps with more or less similar parameters will do. Even the crappiest caps wont see hard load here and will deliver thousands of hours of continued use, more than you and your children would ever use this board 😀
TC23 _non_ lowesr 330uF 6.3V, pretty much any random cap
TC24 TC39 TC40 C114 and 5 ones along CPU socket all lowesr 1000-1500uF 6.3V

but shutting down after 3 seconds I suspect might be due to shorting to something inside the case, maybe one of the motherboard metal studs didnt line up with hole? try with mobo out of the case