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My computer has developed a strange ailment: when I turn it on, the 5V power line from the PSU is very low, hovering at around 4.2V. This is intermittent however, sometimes it jumps back up to 4.98V.

I thought the PSU was dead, but now I'm not so sure. I tested it with a multimeter, under load (a fan), but not connected to the motherboard, and was unable to get it to misbehave. The 5V line is rock steady at 5.02V. I'm inclined at this point to think that the motherboard is leaking current somehow, as I have never found it to be very stable.

The PSU is a Corsair HX520W. The motherboard is a Asus P5K Pro, and the graphics card is a Geforce GTX 285. Everything else has been unplugged from the motherboard, which is resting on the backplate from its box, and is otherwise removed from said box.

Sometimes I can get it running all the way to Linux, or Windows, and there as far as I can tell, putting it under load has no effect on this issue. I recently moved the computer across town, and the problems started when I plugged everything back in again, so is may be something to do with that, although I can't see what, as I had it all properly packed, and so on.

Does anyone have any ideas as to how to proceed? I would like to isolate the component that is causing this, so that I can replace it and be sure of actually curing the problem.

I think I have included all relevant details, but if anyone has any questions, fire away!!

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First test would be to try a known good PSU. –  Moab Oct 15 '12 at 18:56
    
Did you measure 4.2V on the 5V line with a multimeter? –  David Schwartz Oct 15 '12 at 19:30
    
Yes. I have confirmed all of the voltages I mentioned with a multimeter. –  Tom Macdonald Oct 15 '12 at 19:41
    
Then I agree, 4.2V is way out of spec. The power supply should never output that voltage on its 5V line. The power supply is bad. –  David Schwartz Oct 15 '12 at 20:25

2 Answers 2

up vote 4 down vote accepted

If you are sure you are measuring it correctly, then you have a bad PSU. It should never allow that to happen. You should contact Corsair for an RMA.

In the event of a current overload, the PSU is supposed to shut down rather than allow the voltage to sag.

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Well I'd like it to be that, in a way, as the PSU is still under guarantee, but I want to be sure first. –  Tom Macdonald Oct 15 '12 at 19:20
    
I contacted Corsair, and sent the PSU back. They replaced it with a HX650 Pro, which costs 50€ more now than the original one did then (April 2011) so kudos to them! –  Tom Macdonald Oct 29 '12 at 12:44

The PSU is probably broken, 4.2V is way out of its allowed range (5V±0.25V), contact your manufacturer!

A Fan is not much of a load really, they are easily driven compared to other computer peripherals.

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