common psu failure point, looking for fix

[Vielgus-Kutas]

i have few psus that just stopped turning on, nothing burned out, popped or smoked, it just stopped turning on, the motherboard is still getting power and everything except the ON trigger has failed, does anyone know which component that might be? i want to take them apart and fix them

 

[Tsumi]

Most likely culprits are usually the capacitors. Those are typically the easiest to replace as well.

Mosfets and transformers are trickier and more expensive to replace, I wouldn't bother with replacing those.

 

[larrymoencurly]

it just stopped turning on, the motherboard is still getting power and everything except the ON trigger has failed,

How do you know the on/off trigger is OK if the rest of the PSU doesn't work?

First of all, do NOT work on a PSU except when it's unplugged from the AC outlet (not merely turned off) and has been unplugged for at least 30 minutes. Beware that at least one of the big heatsinks may be attached to high voltage. Also after you repair the PSU, don't plug it in again until all the screws for the circuit board and cover have been reinstalled, and use only a GROUNDED 3-prong AC outlet (not all 3-prong outlets are grounded).

A glowing LED on the motherboard indicates the fuse is OK and the problem is most likely in the low voltage section. Bad capacitors there probably won't cause total failure, unless they're shorted. The failed component may be a blown diode pack or transistor on the big heatsink near all the wires that go to the computer. Changing one of those components usually requires removing the heatsink because things are so crowded in there, and that typically means unsoldering all the components. Then test each one with the diode-check or ohms function of a multimeter, and failures tend to be shorts. These parts are often installed to the heatsink on a small sheet of silicone rubber for electrical insulation, and any screw goes through a nylon shoulder washer. This is not a simple flat washer because it has to keep the screw threads from touching metal on the transistor or diode. Also beware. There may be thermal paste, and it has to be dielectric, that is, an electrical insulator, so do NOT use anything containing pure metal, like Arctic Silver. Regular pure opaque white transistor heatsink grease from Radio Shack or dielectric grease from a car parts store (used for cooling ignition modules or waterproofing electrical connectors, like tail lights) will work.

 

[Jorona]

If the PSU detects that it's not putting out the proper or receiving the proper voltages, it'll never active the Power Good Signal. If that doesn't happen, then nothing it's plugged into will ever turn on.

 

[Dead Parrot]

Assuming you have tried a known good PS and verified the point of failure is the PS rather then the MB, pick one and start disassembling it looking for the bad part. You won't be fixing this one unless you find the bad part(s) early in the process. Sadly, the easy way to get to a bad part is often found only after doing it the hard way the first time. Plus, you can use this one to practice reassembly. Look for leaking/bulging caps and burn marks on circuit boards.

Be sure to test your freshly repaired PS on a known good but worthless system. Letting the magic smoke out of a 1TB HD while testing a 'fixed' $50 PS won't make for a happy day.

 

[OrangeBic]

It's pretty scary to open one up.

 

[me4get]

I've repaired a few PSU's with bad caps, Never been shocked by one yet, but I know to discharge the high voltage caps.