how can I repair my computer PSU?

[Delphianrex]

hi,
my computer PSU is outputting invalid voltages on +12, -12 and -5 voltage rails.
it has +11v on +12rail, -8 on -12 rail and -3 on -5 rail

is that possible that the filtering caps to be problem while they seem normal?

thanks in advance

 

[(*steve*)]

Do you have a load on the 5V rail?

 

[Delphianrex]

nope, there is not any load on the PSU. just whenever i short the PS-ON and Gnd wires of the psu, it turns on with these invalid voltages. i guess there is a problem with +12v rail. coz the schottky diode of the +12 rail, is outputting this invalid voltage either. but when i tested it out of board, it was ok.

 

[davelectronic]

Hi there. I think fault finding a modern atx psu is going to be an easy task if you can find the component fault in the early stages, you could look for ever, there are so many SMD's and IC's in todays supply's i think it a mind field, it could be any of the switching transistors or just about any other component, if its not apparent easily, for cost, as little as 25 quid in the uk i would bin it and get a new one, is it worth damage to your other hardware ? but thats just me.

 

[Resqueline]

There's not neccessarily any fault with the PSU, but those types are just not made to be run w/o any load at all, and the load needs to be on the 5V.
They regulate only the 5V, and all the other voltages follow suit. If there's no load on the 5V then there's simply not enough juice on the rest to "hop them up".
A 10 ohm 3W resistor placed on the 5V usually takes care of this issue.