Desktop PC powers on, then off

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Desktop PC powers on, then off

Postby dubsola » Wed Apr 04, 2012 7:50 am UTC

When I hit the power button, the fan spins, the power light comes on, but for <1 second, then it all turns off again.

I thought it was the power supply, but lacking a tester, I took it to my local PC repair shop. Straight away the guy says 'It's the motherboard, the capacitors are bulging, there's a short somewhere, you'll need a new motherboard.' I asked him to check the power supply, sure enough, it's not putting out -5V. He says he will replace the power supply for me for £35, which is a pretty sweet deal - I know how to do it, but that's pretty cheap and it saves me the effort.

When I go to pick it up, he says 'There is a problem with the motherboard, I've blown two new powersupplies. It might be a short or a bad capacitor. They both blew on the -5V. I can order and fit a new motherboard for you, same make and model, £110.' I tell him I will think about it. I noticed that the old power supply was back in the case - either he took it out, put new ones in and then took them out and put the old ones back, or he tested the new power supplies without fitting them.

Questions:
1. Is it possible for a short in the motherboard, or a bad capacitor, to blow a power supply?
2. Is it bad when capacitors bulge at the top?
3. Is it possible to test a new power supply without fitting it? I'm pretty sure this is possible, but if it isn't, then it makes me think the computer repair guy is being a bit dodgy.

Thanks in advance.
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Re: Desktop PC powers on, then off

Postby ElWanderer » Wed Apr 04, 2012 11:13 am UTC

dubsola wrote:Questions:
1. Is it possible for a short in the motherboard, or a bad capacitor, to blow a power supply?

I don't know for definite, but I'd have thought so. Ideally, all that would "blow" would be something replaceable like a fuse, or it'd shut itself down before it could be damaged. Probably depends on the quality of the power supply.
dubsola wrote:2. Is it bad when capacitors bulge at the top?

Yes, it's the magic smoke trying to escape.

Or more seriously, it's a sign the electrolyte inside has tried to burst out, which is bad. They're usually designed to vent out the top as a safety valve. On a related note:
http://en.wikipedia.org/wiki/Capacitor_ ... l_symptoms
dubsola wrote:3. Is it possible to test a new power supply without fitting it?

Yes. All you're doing by "fitting" one is screwing it into a case. Unless you mean: is it possible to test a power supply without powering any PC components with it, in which case:
"Typically the power supply has a protection circuit that prevents it from operating under no-load conditions"
"It is possible to troubleshoot a power supply by placing a dummy load across its power connectors to see if it turns on. If you don’t mind spending a couple dollars, there are pre-built troubleshooting devices you can get down at your local pc repair shop."
http://www.pcmemoirs.com/2011/04/18/how ... therboard/

For testing PC components, you'd typically assemble and test on a bench/table, rather than screwing everything into a case where it's harder to access and replace things (if multiple components may have failed, there may be a lot of combinations to try before you work out what's working and what isn't). It also eliminates shorting problems caused by pins on the motherboard touching the case (which can happen if you screw the board in too tightly or without the right screws).

The bit that doesn't make sense is that if the motherboard is visually damaged, why plug a brand new power supply into it... and when that fails, why plug in a second?
Now I am become Geoff, the destroyer of worlds
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Re: Desktop PC powers on, then off

Postby dubsola » Wed Apr 04, 2012 1:08 pm UTC

It's an Antec power supply. It's possible that a fuse has blown in the supply, I'll take a look to see if it looks like there are any that need replacing / can be replaced. Still, seems like the motherboard is definitely damaged what with the bulging capacitors. I can't afford to replace it now so I'll make do without the PC.

Thanks for your help.
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Re: Desktop PC powers on, then off

Postby cphite » Wed Apr 04, 2012 5:55 pm UTC

dubsola wrote:Questions:
1. Is it possible for a short in the motherboard, or a bad capacitor, to blow a power supply?
2. Is it bad when capacitors bulge at the top?
3. Is it possible to test a new power supply without fitting it? I'm pretty sure this is possible, but if it isn't, then it makes me think the computer repair guy is being a bit dodgy.

Thanks in advance.


1. Possible? Yes. Not likely, but not impossible.
2. Yes, definitely. If it's one, you might get away with replacing it; more than one I wouldn't even bother trying. You need a new motherboard.
3. Yes, you can (and should) test a power supply without putting it into the case.

The "dodgy" bit is why this guy even bothered to test a new power supply, let alone another one after the first one was blown, against a bad motherboard.
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