have you connected a fan to the cpu fan header?

Yes, I tried CPU Fan header #1 and then #2, both of which are very close to the ATX power socket. I also tried attaching fans to both #1 and #2 CPU fan headers. The fans always work, but the power always cuts out - I would say in 2 seconds, not 3.

Try removing the battery

I've just tried removing (and replacing) the battery. No joy I'm afraid. 🙁

Are you attaching 3-wire fans? It's possible that the motherboard requires the CPU to be cooled by a 3-wire fan so that it can get a reading on whether the fan is actually spinning.

If you can get into the BIOS you may be able to find some options to turn off fan monitoring.

Also, have you checked for POST beep codes?

Power switch means power button connected to the mainboard?

I've tried 2 CPU fans. I think they are both 3 wire. Certainly one of them is, as I can see all 3 wires on it.
Unfortunately I cannot get in to the BIOS.
I don't hear any beeps - the power cuts out so quickly, that I don't think there is any time to hear any sound!

Yes, power switch means that I am using a "mobo testing kit power button" which is plugged directly on to the 2 pins on the mobo's power pins header. I use this gadget all the time. I used it again, just a moment ago when I tested another board.

If you have the nerve, bridge green with black on the ATX plug, that will force the power supply on. If nothing happens, try the power button, too.

h-a-l-9000 wrote:

If you have the nerve, bridge green with black on the ATX plug, that will force the power supply on. If nothing happens, try the power button, too.

<the sound of chickens clucking> I think I'll give that a miss! 🤣

Progress! Well - kind of. I removed the CPU, and left the CPU socket empty. (I also removed the graphics card.) The only thing plugged in to the mobo is: keyboard, mouse, 1 stick of RAM, 1 CPU fan, and the power cable. Now, the power stays on when I switch on the power. The only problem with this scenario is that I might not be able to play any games on it. 😉

Edit: If I put any CPU (Duron, T-Bird or Palomino) back in to the mobo, the power cuts out again after about 2 seconds. I think the mobo is damaged. 🙁

where do you get all this, erm, "interesting" hardware from?

crap-bay. 😜

Edit: I have spent the best part of today testing crap which was advertised as "perfect working order". The only thing that has worked today is a heatsink! And here it is. It's quite heavy. Solid copper by the look of it. I like its "boat shape".

Looks like a thermalright slk-800-u, said to be brilliant heatsinks, though heavy. I ought to get one to replace my tiny copper qtec one that has a small high speed [noisy] fan on it.

I have two Thermalright copper Socket A coolers, a SLK700 and 800. That one does look like the SLK-800. It should fit a 60/70/80mm fan on the top "tiers" and have two springy metal wire clips to hold the fan. Those were the monster coolers of 2002/3.

I think those were about $40 w/o fan at the time.

Gasp! I guess on flea-bay you win some, you lose some. The heatsink's a SLK 900A, and came with a 92mm fan plus a good selection of different sized springy clips. The winning price was about a fiver, and the seller wrapped it carefully in big bubble wrap.

http://www.slcentral.com/SLK-900A/ (review of the heatsink)

Success! I plugged in my fastest fan (3300-3400 RPM) in to fan header #1, and also another fast fan (2500 RPM) in to fan header #2, and bingo, the mobo now POSTs! I will mess about with it now and see if I can get it to do anything useful...

Update: The mobo seems to be OK. I've gone in to the BIOS, and saved a few basic settings, and then reached the Windows 98 desktop. I powered the mobo down, replaced the fast and noisy fan with a slower quieter one, and the mobo boots up fine - no problems from the PSU.

Nice. SLK 900. Quite a monster heatsink for those days.