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Topic: I need help to fix an old PSU (Read 3523 times) previous topic - next topic

I need help to fix an old PSU

I bought an really old (1988 or so) PSU from second hand store. It is a 13,X VDC 5A PSU for Mobira "mobile" telephone :D When I plugged it in, it only gives one volt or so, and screams with high freq. I don't even know what component it is that is screaming, some short circuit protection I guess. (My dad has similar PSU and it keeps the same noise when shoted, but it works...)

Image (more available on request):

Re: I need help to fix an old PSU

Reply #1
I was reading about the constructions of transformers earlier this week. The multiple layers in the core can break loose with age and vibrate. Maybe that is the screaming?
Got a question? Please ask in the forum for the fastest answers.

Re: I need help to fix an old PSU

Reply #2
Caps are usually the first to go...check if any of them expanded, or bloated....also could you provide some test voltages.. like dc voltages on all the caps, just stay away from the 220Vstuff if you are not comfortable with high voltages....
best regards FIlip.

Re: I need help to fix an old PSU

Reply #3
Like arakis said, check the caps.  The high frequency scream is likely because a cap in the switching circuit is blown and the frequency of switching is probably wrong...

Then again, I don't know a whole ton about switching power supplies...

Re: I need help to fix an old PSU

Reply #4
Yeah, I agree with arakis and colin.i, it is probably a capacitor that is whining. Some caps fail without having any visible signs, but since they are so old you might as well just change them all out with some nice new ones that have not dried up. It may be a coil too, try to get something insulating like a glossy piece of paper and press on the coils to see if the whining stops. If it does, dab on some hot glue and all should be well!

Ian, I have heard of that idea but never had it appear in the wild for me. Though, here are some awesome pdf's for transformers. I wanted to find one in particular but for some reason I can't find it. :( ... 0guide.pdf <-- very awesome app note from microchip on working with inductive and pics, I really recommend this one.

Maybe some pictures of the rest of the smps can tell us more? :O

Re: I need help to fix an old PSU

Reply #5
The image is full, but the forum is hiding it, just click the link above or "show image". So I'll just switch all the caps and hope the best?

Re: I need help to fix an old PSU

Reply #6
Ah, I see it now. I guess so, try to find capacitors that have the lowest ESR, rated at the same if not higher voltage than the currently installed capacitors, and don't mix up the polarity. :P

Good luck!

Re: I need help to fix an old PSU

Reply #7
Whining usually comes from coils, for example if you drive a motor with PWM, with low duty cycle values, you can hear a sound coming from the motors. The sound comes until the motors have enough torque to start turning.

I'd advise the same thing, renew all the caps, especially electrolytic ones. My guess is that this PSU is a full-wave rectifier (the part on the left side of the heatsink) connected to a flyback converter (I guessed this because of the second transformer). One of the caps on the right side is probably damaged, throwing the PWM frequency off which is causing the sound coming out from the right side transformer. Can you listen to verify where the sound is coming from?

Re: I need help to fix an old PSU

Reply #8
Thank you for your help! I've been busy lately, today I took backups, changed HDD to SSD and now I'm enjoying it.

/me very happy ;)

I'll get to this project as soon as I have time, I have all the other caps but I'm missing the fat black 400V 100uF.

Re: I need help to fix an old PSU

Reply #9
You get output and not total collapse, Try cleaning it?

D1 and D11 areas seem rather bad.

Transformer noise is distinct rattle and buzz. Capacitors whine.
If you pass a thick piece of cardboard/paper left to right and front to back over it as a sound baffle, You might pick out the noise location.

What does the other side look like?
What is with the holes next to C7 just to the right of the heatsink?
Why are the AC diodes smaller than the DC ones(D11-12,D7-8)?

Re: I need help to fix an old PSU

Reply #10
Status update: The PSU has been lying around unplugged for few weeks. Now I took the board out of the box, and...

First start: Beeeeeeeeeeeeeppp, just like before, output voltage around 1,3 Volts. Turning off. My 12V LED string (load) flashes after the shutdown.


Little bit of thinking, then second start: No beeeeps, LEDs light up, output 13,6V, steady.

Switch off. And on, and off... It seems to work, when not in the box. Now lets see what happens when I assemble it again.

Re: I need help to fix an old PSU

Reply #11
Hah, working out of the box, not always a good thing

Re: I need help to fix an old PSU

Reply #12
It works even after assembling! I must find those 4 small screws to hold the cover and this project is finished! Thank you for help, I learned a lot :)


Re: I need help to fix an old PSU

Reply #13
Works when outside the box, awesome! :P

That may mean that all the screws were not connecting the board to the "ground" on the case, or there was a conductive component between the board and case. Or when you took it outside of the case, there was a component that was not soldered correctly but since you moved it the component came in contact again! I suggest looking at all the solder joints and seeing if there may be a cold joint somewhere or something.

But, congrats on it working! :) May it continue to work for many many hours further!