SMPS troubleshooting

[blowhard7]

I'm new at this so please go easy on me.

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

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

Unfortunately, the supply has a noticeable buzz coming from the center PQ3220 transformer when turned on. 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 find nothing wrong.

Suggestions are appreciated. I'll attach a picture of the supply. Schematic is avialable here: http://www.turuta.md/PSU/LCT-2016.pdf

[wrenchone]

The buzz does not necessarily mean a defect. The part you mentioned is the choke for the power factor correction/ boost converter. These guys will buzz audibly unless very carefullly wound and varnished. this looks like a fairly cheap power supply with an active PFC front end and several flyback supplies for output duty, hence the forest of output capacitors. If the output caps were undersizd/overstressed/cheaped out (perhaps all three) they would tend to dry out early. The air circulation is none too good in those monitors, which would accelerate the aging process.

[blowhard7]

The center PQ3220-1000C transformer located just about in the middle of the supply is the part producing the buzz. Believe that's a flyback for the output.

The buzz is annoying in that it's audible while watching the set. Although it's obvious Akia cheaped out on the supply, I can't believe it made that kind of noise when new. The old caps were junk CapXcon's made in 2005. Probably part of the "capacitor plague".

My concern is that perhaps there's still something defective in the power supply.

[wrenchone]

The power supply may have had marginal loop stability, depending overmuch on the ESR of the output caps to straigten things out. Adding better caps woth lower ESR would shift the ESR zero to a higher frequency, and could cause a marginal supply to oscillate. In that case, though, I would expect both transformers to sing, since they are both driven from the same controller/mosfet.

[blowhard7]

Yes, I was thinking along the same lines, as to why the other transformer is not buzzing. Maybe it's not under the same load??

Perhaps I'll desolder the transformers and swap their positions. That should confirm a poorly made transformer.

What has me puzzled is that about a year ago I recapped another one of these same power supplies. After recapping the supply, the same transformer made the same buzzing noise. That unit had a bad display and I ended up trashing the whole thing. Now wishing that I had not, could have pulled a transformer from it.

[blowhard7]

I've swapped the position of the two flyback transformers and it's now apparent that both transformers are buzzing.

Any ideas where to go now??