Samsung LN46A650 backlight turns off after a while

[tibimakai]

I have picked up this TV in very nice condition, pretty cheap.
It would turn on and after a while the backlight would turn off, with sound still on and image still on.
First thing that I did, I have replaced all the capacitors with Panasonic capacitors.
It seemed like it took care of the problem. I have left it for hours to run for like two days and it was fine.
Then, on the third day, after a while the backlight turned off again.
TV still runs fine with sound and image, but no backlight.
I'm measuring some of the ICs and they are pretty hot ~52 C*.
I'm wondering if this is heat related or is something else.
The whole power board and mainboard has a big steel perforated cover on them.
This on it may block even more the ventilation.
Tmorrow, I will take some measurements.
What should I look for?
Any ideas would be appreciated.

[capkid]

Re: Samsung LN46A650 backlight turns off after a while

You could try a hair dryer and cold spray to see if you can consistently reproduce the issue.

[leonard]

Re: Samsung LN46A650 backlight turns off after a while

what is the psu type, example bn44-***
and is the invertor separate from the psu pcb

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

I'm not home right now, but I think that this is my board:
http://www.shopjimmy.com/samsung-bn4...wer-supply.htm

[tom66]

Re: Samsung LN46A650 backlight turns off after a while

Schematic


Related service documents

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

It's 100 degrees in my garage, I can't work in there.
I have left a 20" fan pushing air on the TV from the back side without any covers on it and it did the same thing, so maybe it's not heat related. I don't have any sprays at the moment.

[aaronwt6]

Re: Samsung LN46A650 backlight turns off after a while

Looks like PS and Inverter all on one board. I see one on ebay as cheap as $40 shipped. I'd probably take the plunge for a TV that size.

Of course I guess it could be one of the CCFLs but seems unlikely if the picture is uniformly bright when its on.

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

I should probably take it apart and look at the tubes, to see if any one of them is acting up.

[tom66]

Re: Samsung LN46A650 backlight turns off after a while

I wouldn't immediately consider that; I'd look at the main board and PSU/inverter first.

[aaronwt6]

Re: Samsung LN46A650 backlight turns off after a while

My first guess is the power feeding the inverter transformers is dropping out. The $40 board just seemed pretty nice to me given the value of a 1080p 46" set in good condition.

Of course troubleshooting the board is never a bad thing. I've learned a ton here in the last few months.

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

I have taken some connector measurements before and after and these are the differences:
Small connector gong to the balancer board on the left:
LD: before= 9.84V after= 9.95V
Small connector going from PS to main board:
PWM: before= 3.87V after= 2.5V
No changes on the main connector between the PS and main board.
I have noticed a familiar diode on this PS board MUR460. If I remember well, this diode used to fail on some other devices no?

[tom66]

Re: Samsung LN46A650 backlight turns off after a while

MUR460 can fail but causes no backlight at all (ever), would not be likely to cause your fault but you can rule it out by checking PFC cap voltage (450V cap) it should be around 380V to 400V. If it is near 170V there is a fault.

I'd try forcing VPWM high (to 5V thru 1kohm resistor); may be that main board is dimming backlight (energy save if no signal, or dark scene), causing a bad bulb or the inverter to struggle to maintain correct output.

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

Today I have put a small(turbo) fan close to the main board and it did not turn off. There is another metal cover that should be over these boards and at one point there is a heat transfer sponge like material, but I don't think that it would matter that much.
I have took a picture with the board, still mounted.

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

384V stays on, when the picture goes out.

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

Any other ideas?

[budm]

Re: Samsung LN46A650 backlight turns off after a while

Where is this heat transfer pad?

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

Which one?

[tom66]

Re: Samsung LN46A650 backlight turns off after a while

PSU SCH

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

I'm not home now, but I remember that there is only one small sponge like transfer pad somewhere in the middle of the cover which covers the PS and mainboard PCBs.
I belive that the heat transfer pad is for the mainboard. I will ckeck it when I'm home.
There are at least two processors that run around 55 *C and they don't have any heat sinks.

[tibimakai]

Re: Samsung LN46A650 backlight turns off after a while

The main processor on the main board, has a heat sink on it and the heat transfer pad is between the heat sink and the metal cover.
I have replaced that sponge like heat transfer pad, with the other "wet" type and it didn't fix the issue.