(Warning, wall of text)

I have a Pentium 3 Mainboard with 512 megs of ram, an old ISA soundcard (yeah right. <.< and an older AGP graphicscard, the parts are from several PC´s.

Now it ran quite okay with the old (weak, 200-and-some-more watts) which I replaced with a new one (Bought on Ebay, but from a professional vendor who sold it as new).

Now all was good until yesterday, I did connect a 56k modem via USB and since all was working I did switch it off. When I returned a few hours later it would not power on. After messing with it a little I unplugged it and took it to another powersocket, it went on then. For the evening I had it shut off, and when I did want to switch it on, surprise: Same story.

After a few hours I found out several things:
1. It isn´t completely powerless, the red lamp of the optical mouse (bottom) is on and the keyboard lamps too.
2. When I disconnect it from power the powerled gets alight for a splitsecond as well as the fans turn a little bit.
3. When I push the switch on the backside AND disconnect the power on the socket, then plug it in again and switch power back on it works normal, after a few seconds of being shut off it won´t go on oncemore.
4. This isn´t always, sometimes I can switch it on a few times before the error occurs again.

I thought of the powercable maybe being faulty, or the powersupply/Motherboard even... but if it was the powersupply then it would not go on always, right? RIGHT? Sorry, just having high hopes...

If you did read until here, thanks for reading. Does anyone have a possible theory what this could be about?

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Check on the board for blown capacitors, a common fault as boards get older, will still get power round in most cases, but the machine won't boot.

Sometimes find reseting the BIOS using the jumper (normally found near the battery) might help, incase there's something that's causing a problem there, this means your BIOS will be set back to factory defaults, so you'll lose any tweaks you've done there.

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I had the thing open yesterday, nothing strange occured to me... Plus when the thing boots you don´t notice anything special. I thought if maybe the new powersupply gets too much power for the thing, but if that was the case it would not start at all I guess... and not just now and then again not.

Edit for everyone, I replaced the old (and I mean old) Powersupply through a brandnew one usable for P4 processors, does that maybe cause troubles?

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Double-check all your power connections and inspect for loose connections or even loose pins. A friend's Vaio I was working on last week had some problems after I did some re-organizing, it wasn't recognizing the hard drive sometimes. Turns out one of the actual molex pins had worked its way loose. Popped it back in, and no more problems.

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Dusted.

Hmm I guess when (If... *pray) it happens the next time I will disconnect everything and reassemble the thing.

Btw, knowing you are very good with this stuff, do you think that the powersupply could be too much for the old thing? I asked the vendor if a P4 supply could be used for P3 and got as an answer that it only has to be ATX, rest would not matter... but I somehow do not trust this anymore.

Also, would I need to reconfigure something in the bios for the new thing? I know this is maybe a pointless question, just a thought...

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Too much? That's not really possible, power supplies will only pull as much power as they need. You could have received a defective or damaged unit though.

And there are no serious power settings in your BIOS to mess with, although it wouldn't be a bad idea to clear your CMOS or possibly even flash your BIOS.

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Dusted.

May need to remove the heatsink to check the caps round the CPU.

No, it won't provide too much power, not possible, the bigger PSU just means you can add more cards and things. I've managed to overload one of my old PSU's by having 4 hard drives, a CD Writer, DVD Writer, and standard DVD drive, along with fans and stuff. So a new one really helped!

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Okay I will try that, thanks!

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Yeah well, was just a thought.

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