Help repairing ATX PSU

[Jechu182]

I recently found several old PC power supplies thrown away by someone. I hooked them up and measured its output voltages without load and they gave me very low voltages...

3V in the 12V line
1V in the 3.3V line

for example, was one of the PSU´s :shock:

So i went on and tested the basic components, fuse, capacitors, transistors, etc... But they all seem to be fine. What am I missing?
What could be the problem in them? I could really use them. I know I can just go buy some since they are cheap but i prefer to repair them for even cheaper.

Thanks in advance to anyone that helps!

 

[tpetar]

You are very rich if you want to fix this and to put pc components or something else including home in big risk.


Maybe product model ID or PCB photo should help people to help you.

 

[Genovator]

Mostly in PC psu's, the power mosfet gets damage. They shows as if they are working, but their efficiency decreases. Even if you repair them, they shows the same problem after few months. So people generally buy new psu's, since they are very cheap. Its good to recycle. Try changing all the mosfets.

 

[DeepOne]

Hi.
For repair quite well to have an oscilloscope. I usually connect faulty block to fault-free, giving 12, 5, 3,3V voltages (as well as feeding on PWM controller) and see form and value of signal (and its passing to key transistors) on entry and output of PWM controller as well as in voltage control chip. This if half-bridge topology is used.
If PSU is made on the base of UC384* chip, that approach looks little otherwise but same in principle.
In the first place, either as everywhere, cause of fault can be in electrolytic capacitors, also may meet defects in changeable resistors, transistors in driver, bad soldering, faults in auxiliary power source and so on and so forth.
Before that what proceed with repair is quite well to look scheme of this or like this PSU and datasheet for PWM controller.
During repair process high-voltage part better to connect to power network through underbar transformer and 40 - 100W incandescent lamp.

 

[Tipu606]

I recently found several old PC power supplies thrown away by someone. I hooked them up and measured its output voltages without load and they gave me very low voltages...

3V in the 12V line
1V in the 3.3V line

for example, was one of the PSU´s :shock:

So i went on and tested the basic components, fuse, capacitors, transistors, etc... But they all seem to be fine. What am I missing?
What could be the problem in them? I could really use them. I know I can just go buy some since they are cheap but i prefer to repair them for even cheaper.

Thanks in advance to anyone that helps!

try measuring the output voltages with a 100W bulb load