LN40B540P8F Flickering Backlight

[pvfjr]

Hello, our Samsung TV has started flickering. I've got the back off, and can see the backlight flickering on and off through the small holes in the sheetmetal. Here are some things I've noticed so far:

-I can hear a light buzz when the lights are on, and it's silent when it goes out. Possibly just normal inverter noise?
-I have 386VDC on the main filter caps
-I don't see any bulging caps
-I don't see any overtly burned components
-T1803 has some very light discoloration on the tape around its core
-Energy saver mode is already off
-If I turn the backlight down to "2" in the settings, they stay on. That's a bit dim for our tastes, and obviously, doesn't fix what is actually wrong with it.

How should I proceed from here? Thanks in advance!
BTW, I do have an o-scope, just in case it's needed.

[budm]

Re: LN40B540P8F Flickering Backlight

Start by uploading good clear pictures of the whole back side of the TV to show all the boards, then the pictures of the T1803.

[budm]

Re: LN40B540P8F Flickering Backlight

Start by uploading good clear pictures of the backside of the TV showing all the boards, and the pictures of T1803.
What is the power supply board P/N? BN44-??????

[pvfjr]

Re: LN40B540P8F Flickering Backlight

Start by uploading good clear pictures of the backside of the TV showing all the boards, and the pictures of T1803.
What is the power supply board P/N? BN44-??????

BN44 sounds right, I'll check the other digits and get pics when I get home. I think it ended with a 'B'...

[pvfjr]

Re: LN40B540P8F Flickering Backlight

It's a BN44-0264B. I pulled it out to inspect the solder joints, and found nothing obvious. Unfortunately, I'm not sure what the best way to reinstall the board is. The ends of the CCFLs are quite floppy, and don't want to engage with their connectors. Is there a quick and easy way to access them from the other side, or must I dismantle the entire TV now?

[budm]

Re: LN40B540P8F Flickering Backlight

You have bad inverter transformer, the one with the brown markings on top.
What ever you do, do not try to remove that black plastic strip when the board is plugged into.

[pvfjr]

Re: LN40B540P8F Flickering Backlight

I'm curious to know how these transformers are hooked up. With there being multiples, do they each power a portion of the backlight? If so, why does the bad one disable the entire backlight? Unless it were shorting and causing an over-current condition that tripped the protection circuit. Even then, why would it stop shorting when the backlight is turned down to 2? Turning down the backlight setting just changes the modulation percentage, not the voltage...right?

It's obviously a damaged transformer, I'm just curious about why it went bad. There was a guy on Youtube to appeared to replace the same one. Afterwards, the new one continued to run hotter than the others.

https://www.youtube.com/watch?v=PflyJ4H51Fg

[budm]

Re: LN40B540P8F Flickering Backlight

The protection circuit will detect if ALL the lamps are on or not otherwise it will shutdown the inverter circuit, the protection circuit also detect over current and will go into shutdown. So in this case it is the over current shutdown due to bad transformer when it is driven hard.
There are two high Voltage windings on the transformer, the Blue one has only one high Voltage winding.
http://www.shopjimmy.com/catalogsear...?q=BN44-00264B

[pvfjr]

Re: LN40B540P8F Flickering Backlight

Well replacing that transformer seems to have done the trick for now. I'm not sure what made it go bad in the first place, but all the CCFLs seem to work fine. I left the back light turned down just a bit, maybe it'll last longer this time.

The trickiest part was getting the CCFLs plugged back in after removing the power supply incorrectly. Luckily I was able to create a method for getting them plugged back in to their sockets without disassembling the panel any further. I just looped short pieces of waxed string around each lamp's end, put the connector strip into place, then tugged on each string to seat it's respective bulb into place. Afterwards, pulling on one end of the string got them out of the way.

[budm]

Re: LN40B540P8F Flickering Backlight

Glad to hear you got it working.
Worn out lamp will draw more current but may not be enough to trip the protection circuit but high enough to over heat winidings of the transformer, if you have thermometer you should check the temperature of each transformer.
Worn out lamp will have blackened ends just the same as you see on the home fluorescent lamps.
http://s807.photobucket.com/user/bud...?sort=3&page=1

And yes, it is good idea to turn down the backlights level (especially the one using LEDs for backlights) to as low as you can but still get good enough pictures.

[pvfjr]

Re: LN40B540P8F Flickering Backlight

Thanks for sharing those pics. I didn't see anything like that with my lamps. Perhaps they're just not far gone yet. I may just wait and see how long this lasts with the back light turned down a bit. The transformer came in a two-pack...

I've gotten the impression that the Samsung protection circuit used here it just extra touchy. I've noticed that with some models, people are disabling it altogether by clipping a jumper. Thankfully, I haven't gotten that desperate yet.

[ngth82]

Re: LN40B540P8F Flickering Backlight

Sorry for bumping an old thread, but i picked up a similar model 40" Samsung TV with a flickering CCFL backlight. I ended up taking it apart to find charring/black ends on almost all of the CCFL's. At $10 a piece it was going to cost $100 shipped to get replacements.

I opted for installing a 100 ohm 1/4 W resistor from the diode shared connection to ground. It works and has worked well for the past week or so.

It's an option, and I've read about others doing that on this forum, but i'm curious if this causes a fire safety issue. Should I just part TV out instead?

[budm]

Re: LN40B540P8F Flickering Backlight

Worn out blackened ends lamps will draw more current than normal.
What happen when people bypass the circuit and called it a fix.
I told those DIY's that like bypassing the protection circuits as how lucky they are that their houses did not burn down.
The first picture is from the unit that the backlights were still on, DIY said the backlights are still on after bypassing the circuit per YouTube, it is working but has arcing sound.