Weird PC hardware problem is driving me crazy

[Jules]

I'm not a hardware engineer but, up until now, I've never had a hardware problem that I couldn't solve by swapping some components  out. Today I  am absolutely baffled. Please help!!!

Its a Athlon XP-2000 circa 2002 that I  use as a web and testing server at home. It stopped powering up so I  guessed the PSU had blown so I  went and bought a cheap replacement  (Trust 370 watt). This didn't fix the problem either so I  guessed that either the motherboard or CPU was dead so I've just bought another, this time an Athlon XP board with CPU and memory off Ebay. It was guaranteed working and the vendor is 100% so I have no reason to suppose I've bought a dud. I  swapped back to the original PSU, plumbed it in and fired it up. The processor and case fans did a quarter turn and then stopped. Cycling the power did not make this repeat however pulling the power lead from the PSU and replacing it and then cycling the power allowed the fans to make another quarter turn and then stop.  I guess (I don't know much about Switch mode PSU's) that the board was trying to draw more power than the PSU wanted to give, the PSU was shutting down to protect itself and removing the power lead from the PSU reset this condition.

So I  swapped in the new PSU and... Exactly the same thing happens, quarter turn on the fans and then nothing until the power is removed from the PSU. Back to the "broken" board and guess what, exactly the same behaviour.

I'm now confused and would welcome any ideas. All I  can come up with is...

The first PSU is broken and the new one is too? The only other thing I  have round here only has a 160 watt PSU. I tried that with no joy and no fan movements. BTW I  should say that I'm testing this just with a bare motherboard so there should not be a problem with a drive trying to take too much power.

The first PSU went wrong, took out the original motherboard and, as I  tried that first, thinking I  had a motherboard problem, it's taken out the second one too?

The first motherboard went wrong, took out the original PSU and took out the second too when I tried that?

Are either of these scenarios likely??

BTW I think the case switch is OK as I  can get the same behaviour shorting the pins of the power switch connector on the board.


Thanks


Jules

edit - Sorry RZ - just saw your announcement bu don't think I  can move this?

[Zipperhead]

I can't make any suggestions as to what it might be, but if it helps I've got a 450w PSU sitting on the shelf that I know works, and I'm not very far from you (I'm in Wandsworth).

You're welcome to try it if you want.

[Jules]

I can't make any suggestions as to what it might be, but if it helps I've got a 450w PSU sitting on the shelf that I know works, and I'm not very far from you (I'm in Wandsworth).

You're welcome to try it if you want.

Thanks. I'll let you know.

[Maladict]

Have you tried removing all the expansion cards (graphics etc), drives, and the RAM?  Take 'em all out just in case one of those components is duff, or not properly seated.  If the BIOS is line mine on the M2N-E motherboard then it should still be able to emit angry beeps (meaning things like "where is my RAM?").

[Polar Bear]

I'm guessing that you've swapped out all the cards etc when you swapped the mainboards.  Might be an idea to have the minimum of peripherals attached, i.e. if you have onboard graphics use that and only the one boot HD.   If tha works then add one component at a time and retry.

Is there a short between the switch and the chassis maybe?
   

[Jules]

Thanks both. The motherboards, which have onboard graphics, are being run without any option cards fitted or drives attached and I've powered up  them out of of the cases to make sure there are no shorts and with the memory removed.  I'm going to wire in a new power switch to see if that helps now.

[Jules]

Sitting somewhere quiet can help Reinstalled the original PSU, shorted the power switch buttons on the card, i.e switching the thing on while leaving the case switch out of the equation, the fans do their quarter of a turn and then there is a tiny click (sounds like a relay opening) in the PSU. As noted in my first post removing and replacing the power lead from the PSU allows this procedure to be repeated. Tried the second PSU and no click but no movement of the fans either and its the same with both boards

Latest wild theory. PSU1 is broken in that it opens some form of protective relay whenever asked to produce power.  I'd imagine this could also happen if there is some form of short circuit on the board - but I've got the same issue with both boards.

PSU2 (that doesn't click) either doesn't have any form of overload protection, switches it so quickly and noiselessly that there's no sign of it ever working  - or is DOA.

or... The original board has a short which has taken out both PSUs.


Questions)

1)  Does anyone know whether the latter is  possible. Can you destroy a PSU by shorting its outputs or is it protected against that?

2) Can I measure voltages on the pins of a 20 pin ATX connector to check if it is working and what do I  need to do to get the PSU to power on. As I understand this the PSU is not powered up until the case button is pressed so I'm assuming some connections on the ATX connector are shorted by the case button - or is it not that easy

Thanks


Jules

[токамак]

It might possibly be a dud capacitor on the mainboard - we repair PC's for schools where I work and this is a fairly common fault (symptoms as you describe). You could try opening up the box and wiggling every capacitor to make sure it has a good connection - some may be quite loose, I have actually managed to resuscitate a PC this way.  Our engineers solder new capacitors onto failed boards, but if soldering isn't your thing you might require a whole new motherboard.