Electronic – Fixing an ATX PSU

atxpower supplyrepair

I have an 800W ATX power supply given to me by a friend because it doesn't work any more. It's quite a beast and would set me back a considerable amount new, so I'd like to see if I can fix it.

Well, here's the story. It has standby power at 5.15V. The power indicator on the back lights up (on the mains switch – it's an LED not a neon apparently, because when you turn off the unit it fades slowly, making me think it's not directly connected to the mains.) Anyway, when you turn it on (by grounding the PS_ON pin) it makes a faint clicking sound but does nothing else. With my scope I have found three short pulses of ~11.8V on the 12V rail for about 500 ms to 1.5 seconds before the supply goes dead, which makes me think some kind of protection system is being activated, as the power supply does work – it is producing the required output.

If it doesn't work it is going in the trash, so no loss there.

Best Answer

Bad capacitors. It's always bad capacitors (and it's never Lupus). They've dried out and become resistive. In the process they sometimes make life ugly for power transistors and diodes.

If you're not safe with some mains voltage (there may be some in the hot side of the supply) please don't open the box.

Look for swollen electrolytic capacitors (metal cans). Replace with reputable parts with same values (no consumer components supplier will sell parts as nasty as a cheap manufacturer can buy wholesale).

Check diodes and transistors on output side for blowage.

Main high voltage mains side capacitor might be dried out. Shows up as overload in feedback paths and shutdown. Replace it. Must be same voltage rating!

Go see the sci.electronics.repair FAQ.