SMPS troubleshooting

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Topic:

Topic author: blowhard7
Subject: SMPS troubleshooting
Posted on: Dec 04 2008 11:55:34 AM
Message:

I'm relatively new at electronics repair and have a difficult problem.

I have an Akia 200watt SMPS that I have partially repaired. The supply powers a 27 inch lcd tv display. Originally it had 5v standby power only and would not turn on. Almost all of the low voltage output capacitors were domed and leaking. I replaced all of the output caps with new low esr, high frequency caps of the same voltage and capacitance. Both main filter caps were replaced also as a precaution.

The display now functions properly. 5v, 12v and 24v outputs appear good.

Unfortunately, the supply has a noticeable buzz coming from both PQ3220-1000C transformers when turned on. Annoying to the point that the set is not usable. I do not know if the supply buzzed prior to the cap failure since the unit was given to me dead.

Perhaps the bad caps caused damage to other components?? I see no other visible damage. I have checked almost all other components in circuit (to the best of my ability) with a digital VOM and cannot find any bad components.

Suggestions are appreciated. Not sure what to look for next. I'll attach a picture of the supply. Schematic is avialable here: http://www.turuta.md/PSU/LCT-2016.pdf


Download Attachment: 200W SMPS.JPG
70.58 KB

Replies:

Reply author: chandan.ahluwalia
Replied on: Dec 04 2008 5:34:29 PM
Message:

This is not a solution but that means a lot of current is being drawn from the transformers causing humming sound.

Reply author: audioguru
Replied on: Dec 04 2008 7:14:13 PM
Message:

A very cheap and poorly made transformer buzzes.

Reply author: blowhard7
Replied on: Dec 04 2008 8:03:36 PM
Message:

Thanks, yes, I'm aware of the poor design and high current possibily. It's also been suggested there is a possible control loop instability??

Cannot believe the transformers made that kind of noise when new. The set would have been returned under warrenty.

Just not sure what to look for next.

Reply author: chandan.ahluwalia
Replied on: Dec 05 2008 05:34:18 AM
Message:

Aaron or Audioguru can definately give an answer for your problem.

Reply author: j76bravo
Replied on: Sep 04 2012 11:49:31 PM
Message:

I don't consider myself an expert, but I bought a diode, transistor, etc... tester from radioshack around 2001. Any how, I also replaced the caps, and at first the buzzing sound was horrible when on stand-by and while it was turned on it went away. couple of months passed and now it's pointless to even have it plugged in. So I felt that there was too much current going through the transformers. So I took out eight transistors along with the heat-sink they where attached to and tested all eight of them and about half were bad. I haven't corrected the problem yet but if you like I will keep you posted. Oh, thanks a lot for the schematic, I couldn't find one any where.

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