Repairing Desktop PC Power Supply

googler

Came back from a few days away to find one of the Desktops as dead as a doornail, wouldn't power up at all, no response to front panel switch, no blinking LEDs or Beeps.

Removed HDD, placed in caddy, and that works fine with another PC connected through USB, so data is safe.

All symptoms seem to point to a failed power supply. There's an LED on the rear panel showing mains power is getting to the PSU itself. An identical PSU has been ordered from an eBay supplier, and I intend a straightforward swap.

Does anyone know of any references, sites etc dealing specifically with repairing a PSU? Which component within the PSU is likely to have failed? Do the manufacturers tend to provide spares?

gonzo127

probably a silly question, but have you checked all the switches, for example most psu's have a 'extra' on off switch next to where the power cable goes in? and of course all the calbles are connected and havent been knocked?

Notmyrealname

googler wrote: »

Does anyone know of any references, sites etc dealing specifically with repairing a PSU? Which component within the PSU is likely to have failed? Do the manufacturers tend to provide spares?

There are no manufacturer specific components and you can get them from CPC Farnell or Maplin.

There is no specific component that is likely to have failed - it could be a number of things.

You will need a capacitor tester, transitor tester, multimeter, soldering iron, desoldering tool and some knowledge fault finding electronics and also in how a switch mode PSU works.

If you've got all that, you're good to go.

I'd start at the feedback circuit because if that isn't getting what it expects, it'll kill the output. I'd also check the switching transistor.

googler

gonzo127 wrote: »

probably a silly question, but have you checked all the switches, for example most psu's have a 'extra' on off switch next to where the power cable goes in? and of course all the calbles are connected and havent been knocked?

Thanks, but it was working just before we went away, and nobody was in the house whilst we were away, so it couldn't have been knocked in any way.

There's no on-off switch on the back, only the momentary-contact front panel switch.

The back panel has the LED built in to the power supply which shows that mains power is reaching it.

googler

Notmyrealname wrote: »

There are no manufacturer specific components and you can get them from CPC Farnell or Maplin.

There is no specific component that is likely to have failed - it could be a number of things.

You will need a capacitor tester, transitor tester, multimeter, soldering iron, desoldering tool and some knowledge fault finding electronics and also in how a switch mode PSU works.

I got 3 outta 6.

tonyh66

and opening up a psu can be fatal due to the capacitors within storing electric charge, so beware poking around inside it.

googler

tonyh66 wrote: »

and opening up a psu can be fatal due to the capacitors within storing electric charge, so beware poking around inside it.

I've built my own valve amplifiers before now, so I'm no stranger to high voltage devices. Thanks.

letsbehonest

Bin it and buy a new one trying to repair them is a pain and uneconomical

macman

Agree, a new ATX 400W PSU is about £20 if you go for a cheapo generic one. Not worth trying to home repair, even if you have the skills.
If you really want to test it, a PSU tester will not cost you more than about £15. I find mine invaluable.

Notmyrealname

macman wrote: »

If you really want to test it, a PSU tester will not cost you more than about £15. I find mine invaluable.

Those do nothing but empty the wallets of the stupid. They tell you nothing a multimeter wouldn't.

skivenov

Sounds daft, but pull the mains lead out the back of the PC, leave it a while, then plug it in and give it a try. The LED might be running off some other latent voltage, so test your mains lead or try a known good one. Dafter things have happened.

It's bizarre, it doesn't always work, but it's worked for me a couple of times.

If you're got a multimeter (or an LED), whip the PSU out (remember there a lot of electrostatically sensitive bits in the box).and test for the 5v standby voltage coming out of pin 9 (purple). In the good old days, I haven't worked in IT for years, we used to take the PSU out of the PC, plug it in on the bench and earth pin 9 (purple) to a black to simulate the power button being pressed. If you Google "ATX pin out" and you'll find loads of hits showing what wire should kick what out. Here's one http://www.pcguide.com/ref/power/sup/partsMotherboard-c.html

Seeing as you're competant with HVAC, open the PSU up, having taken the normal precautions. Sometimes they have a fuse inside.They're normally pretty obvious and in a holder on the PCB. To anyone else reading who's not competant at playing with things that can have fatal voltages in - don't!

Beyond the fuse though, they're not worth fixing unless there's an obvious dry joint.