Samsung 32" TV PSupply makes bubbly noise

[saddle_au]

I've got a strange one which has me stumped a bit.

I fixed a BN44-00260C supply that had been butchered by someone before me in an LA32B550 unit. The main fuse was gone and some one had tried to remove the bridge to fix it. I found the PFC fet shorted and the 12R driver resistor from the driver chip open circuit. The Fan PFC chip seems ok. I replaced the bridge, fet, fet resistor and up graded the main caps for good measure.

But,

When it's on, it makes a very soft bubbly/chirping/clicking noise from the power supply area. I remove the supply from the chassis (disconnecting the ccfls and the noise goes away, but when connected there's nothing else wrong other than the subtle noise. If it was a bad ccfl or ccfl connector, I'd expect to hear it constantly, but it disappears when the screen is displaying a dark image and is more pronounced for lighter images or the static of an un-tuned analog station. I disconnect the speaker and there is no difference. Its an LCD for heavenssake! The drive circuit for the ccfl is independant of the screen contents (isn't it?).



I'd live with it, but whatever is going on may have caused the original fault. Has anyone else come across this?

[Th3_uN1Qu3]

Re: Samsung 32" TV PSupply makes bubbly noise

The power supply is oscillating due to an unstable control loop. This may be an inherent problem of the design or it can sometimes be caused by bad/marginal capacitors.

[saddle_au]

Re: Samsung 32" TV PSupply makes bubbly noise

Thanks for the reply.

That did occur to me but I'm lacking a good schematic for the supply board to investigate it further. I was tempted to mock up a load to pretend to be the ccfls. I figure them to be equivalent of about a 600R 60W load total between 4 connections. I'd do this to rule out arcing in the chassis from the tubes. Either that or dismantle the screen panel to get at them (nerve wrecking on a kinda running machine).

I can find BN44-00260A schematics but not BN44-00260C ones anywhere, and they are different enough to be bothersome.

This game is a shot in the dark sometimes with info. Its like TCONs, cannot for the life of me find schematics for them beyond general panel pdfs with connector guides.

[selldoor]

Re: Samsung 32" TV PSupply makes bubbly noise

Not sure about mimicking ccfls - v high voltage at startup - and may do more harm than good. Depending on the circuit for a short test you could make up some test bulbs from
cfls - find a post by budm and look in his signature.

When you say you upgraded the MAIN caps did you mean the BIG cap/s 400/450v?

Did you use new ones? Perhaps carefully test across the BIG CAP legs from switch on
until the lamps light and monitor the voltage. Meter on 600vDC

EDIT link https://www.badcaps.net/forum/showth...er+testing+cfl

Also if you post good clear pictures of your board someone may spot something.