Possible defective PSU \ VOGONS

Possible defective PSU

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First post, by Retro-o

Posted on 2025-12-20, 22:50

Retro-o Offline

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Evening folks,

System:
Socket 7: AMD K2 266MHz
128Mb RAM
Graphics: S3 Virge DX/GX 4Mb RAM
Sound: AWE 64
O/S: Win98SE

Looking to pick your brains guys, my old WIN98 PC was running fine and now when I start it up the monitor remains on standby (no signal) and the CPU fan won't spin. PSU fan is spinning. I thought it may be a RAM issue, but I've tried swapping around all three RAM boards in different slots, one RAM only, then two RAM etc... but no joy. I don't think it's the RAM causing the issue. PC does beep on and off when there's no RAM present. I've tried clearing the CMOS and I've put a new battery in the motherboard which was reading 3.3V so all good there.

I've just checked the PSU with a volt meter. The connecter for the motherboard has twelve wires, six split between two connectors P8 & P9. P8 yellow wire reads 11.7V - good , the red reads 5V - good, but the orange reads 5V instead of 3.3V - not so good. I've been told this is most likely the cause and I need a new PSU - Just looking to pick your brains too see if you think that this is indeed the issue or are there any other things I could check / try first. I've attached some pics of my PSU.

Regards

Colin.

Reply 1 of 8, by RetroLizard

Posted on 2025-12-20, 23:51

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I'm not as experienced with power supplies like others might be, but I don't even see 3.3V listed on the power supply as one of the rails available.

Reply 2 of 8, by paradigital

Posted on Yesterday, 07:44

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RetroLizard wrote on 2025-12-20, 23:51:

I'm not as experienced with power supplies like others might be, but I don't even see 3.3V listed on the power supply as one of the rails available.

Correct, AT doesn’t have a 3.3v typically. The orange wire is Power OK, which should be 5v.

Reply 3 of 8, by tehsiggi

Posted on Yesterday, 07:47

tehsiggi Offline

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Orange being 3.3V was not a thing until ATX came up. Your PSU is AT, meaning the orange wire is for power good (PG) which signals the mainboard that all voltage rails are within specification.

See: https://pinoutguide.com/Power/MotherboardPower_pinout.shtml

It will be 5V if PG is true, meaning all voltages are good.
From your measurements the PSU main rails appear to be fine. You can check the negative ones as well, but I don't feel like the PSU is at fault here.

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Reply 4 of 8, by Retro-o

Posted on Yesterday, 23:15

Retro-o Offline

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Thanks for all the help and information so far folks. Sounds then like the PSU is good. Does anyone have any ideas a s may be causing the non start up issues?

Reply 5 of 8, by RetroLizard

Posted on Yesterday, 23:17

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Can you show us pictures of the whole computer?

Reply 6 of 8, by paradigital

Posted on Today, 16:45

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First steps for me would be check the motherboard for shorted voltage rails and visibly dead/suspect caps.

Reply 7 of 8, by shevalier

Posted on Today, 17:10

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Retro-o wrote on 2025-12-20, 22:50:

System:
Socket 7:
and now when I start it up the monitor remains on standby (no signal) and the CPU fan won't spin. PSU fan is spinning.

As far as I know, Socket 7 motherboards did not have CPU fan control.
It seems very likely that the +12V is either disappearing somewhere on the board or decreasing significantly under load.
Check the PSU voltage with the board turned on.
The +12V should go to the CPU PWM controller. If it is not there, then the indirect sign will be the lack of CPU fan rotation.

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Reply 8 of 8, by kotel

Posted on Today, 17:39

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So the mainboard powers on after you turn on the AT PSU, but you get no video output. This seems like a dead GPU (which works barely enough to get detected but not enough to display). Although the CPU fan not spinning concerns me.
Are you sure the Vio and Vcore of the CPU was set correctly? Those tend to run at 3.3V Voltage I\O (although they can also work with 3.5Vio) and around 2.2V Vcore. Also check if the FSB and multiplier jumpers were set correctly.
Post photos of the PC as others said, alongside the mainboard model.
Also you can try turning on the mainboard with just the CPU and RAM. Check if it beeps about a missing GPU (usually 1 long and 3 short).

POST card is very helpful for these cases (see attachment). They're cheap on ebay, amazon or aliexpress.

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