Samsung LN40B500P3F backlight won't stay on

[Nexus1966]

I picked up a Samsung LN40B500P3F for free. It turns on for a few seconds, but then the backlight dies. It will come on again after about 5 seconds and stay on for about a second. It repeats this a few times, and then eventually the backlight stays off.

As far as I can tell the rest of the TV is working good, it is only the backlight that won't stay running.

I see there are numerous posts here about this model or similar Samsung LCDs, but it seems like most of them are completely dead and not the same issue as this one.

I get 392 volts DC across the 2 big capacitors at all times when the TV is on, even after the backlight dies that reading does not change.

Can someone point me in the right direction on what I should test to narrow down the problem please?

[Nexus1966]

Re: Samsung LN40B500P3F backlight won't stay on

More pictures of the back side of the board

[Nexus1966]

Re: Samsung LN40B500P3F backlight won't stay on

I've tested all of the diodes and all of the capacitors, all seem to be OK.

I also tested the resistance on the backlight output transformers, they all are fairly close.

Could this be a bad backlight bulb or bulbs causing the issue?

[tvtimmy]

Re: Samsung LN40B500P3F backlight won't stay on

I'm thinking it is a bad bulb , or more than one.
Black looking ends is a dead giveaway.
Easy to fix, if you have spares

[dick_barton]

Re: Samsung LN40B500P3F backlight won't stay on

Can you give the primary and secondary resistances for each of the inverter transformers.

[Nexus1966]

Re: Samsung LN40B500P3F backlight won't stay on

Primary is .6 ohms on all of them, but then again my multimeter shows .4 just touching the leads together, so make that .2 ohms corrected reading?

Secondary resistance starting with the top output of TI801S and then going down the board from there is:

1536 ohms
1531 ohms
1542 ohms
1541 ohms
1538 ohms
1536 ohms
1515 ohms
1514 ohms

[Andrew F. Ali]

Re: Samsung LN40B500P3F backlight won't stay on

Are these the measurements you made?
Secondary resistance readings (ohms):
TI801S TI802S TI803S TI804S
Across Pins 1-2: 1536 1531 1542 1541
Across Pins 3-4: 1538 1536 1515 1514

Primary resistance corrected readings (ohms):
TI801S TI802S TI803S TI804S
Across Pins 5-6: 0.2 0.2 0.2 0.2

[Nexus1966]

Re: Samsung LN40B500P3F backlight won't stay on

Sorry, I was not very clear on what readings were for what output there. Here are the readings as I took them from the top of the board to the bottom along with which inverter transformer and which pins they correspond to.

1536 ohms - TI801S between pins 3 & 4
1531 ohms - TI801S between pins 1 & 2

1542 ohms - TI802S between pins 3 & 4
1541 ohms - TI802S between pins 1 & 2

1538 ohms - TI803S between pins 3 & 4
1536 ohms - TI803S between pins 1 & 2

1515 ohms - TI804S between pins 3 & 4
1514 ohms - TI804S between pins 1 & 2

[Nexus1966]

Re: Samsung LN40B500P3F backlight won't stay on

The backlight tubes LOOK fine, at least the ends that I can see without taking the back of the TV apart part enough to expose the bulbs completely.

Is there a good way to test the bulbs? Or determine what part of the board is bad? I read a post about using an old CFL for testing. I have several of them around that work still. Which is a better test - using the old bulb circuitry to try to power the bulbs in the TV? Or trying to use the TV board to power the CFL tube?

[culcuhain]

Re: Samsung LN40B500P3F backlight won't stay on

http://www.lcdparts.net/TransformerD...ProductID=3991

Looks like the inverters are good.