Hi guys,

I am trying to repair my TV's power board. A few days ago the bottom half of the TV went dim. It is a Samsung edgelit TV and the bottom half of both led strips are not working. The led strips are fine and the problem is somewhere on the power board.

I have traced the circuits for the top half and the bottom half and I think I may have found the issue and I believe it maybe one of the N-channel MOSFETs but my knowledge on electronic components is limited so would like some advice.
I have tested the MOSFETs in circuit, in diode mode when I placed the red probe on the gate of the MOSFET on the working side it climbs to 1.98v very quickly but the MOSFET on the non working side stops at .86v. Would this imply that it is the MOSFET that has failed?


Thanks in advance for any help.

 

I would suggest removing the mosfet to test it.

 

Measuring makes no differance if you don't know the polarity of the mosfet. Don't assume it's N-type depletion. Pull it out and look for the part number, then look for the data sheet for it.

 

Measuring makes no differance if you don't know the polarity of the mosfet. Don't assume it's N-type depletion. Pull it out and look for the part number, then look for the data sheet for it.

Don't assume that I assumed. I did identify the part and looked up the datasheet which is why I stated that it is N-channel MOSFET.

 

What the brand of TV and model?, so we can look at how the Backlight is driven
How did you test the leds?, have you removed the TV panel?

 

What the brand of TV and model?, so we can look at how the Backlight is driven
How did you test the leds?, have you removed the TV panel?

It's a Samsung UE40SE6300, the power board is a BN44-00518B.
I thought the problem was more than likely going to be something on the power board rather than the LEDs as they stopped working on the bottom half of both strips instantaneously, however I also purchased a new set of LED strips anyway as their cost was fairly low. The new strips act in the exact same way.

I do not have a picture of the underside of the power board to hand right now, I do have a picture of the front of the board which I shall try to attach which shows the pin out.

The MOSFET that I believe is the problem is an MDHT3N40.

 

What the brand of TV and model?, so we can look at how the Backlight is driven
How did you test the leds?, have you removed the TV panel?

I found a picture of the back of the board online if it helps.

 

I don't think it's the mosfet

 

I don't think it's the mosfet

It was the MOSFET, I just replaced it. I tested it in circuit and it matched the working side. I put the TV back together plugged it in and it was all working again.

About 60 seconds later the bottom half of the edge light went off again. So something else must be making the MOSFET go bad. Any ideas what could cause that?

 

A little confused, I left it on and it just lit back up and seems to be working fine now

 

It's off again, and every so often it flickers. Does anyone have any suggestions?

 

It could be a poor connection somewhere, including a broken trace on the board. Try wiggling connectors and placing pressure (with a wooden chopstick mabye) on various spots on the board.

They sell a cooling spray you can use to find temperature-related problems but I’m not sure that’s what you’re seeing.

 

Did you test the failed mosfet? how has it failed?

 

You have failing LED's

 

You have failing LED's

I think so too

 

You have failing LED's

I've tried replacing the led strips, I've attached links to videos showing it below:

https://1drv.ms/v/c/f13aa3123d36e9b9/EcHhjJJffZFGpecQbfi1haABLAEpI9JEN_RlYC4Pi_HFbw

https://1drv.ms/v/c/f13aa3123d36e9b9/EWuv-A3RBqRAgcIjjTCvfXwBnhwXliU4CU8_0LP4SDZCpA

https://1drv.ms/v/c/f13aa3123d36e9b9/EezQyv5BenlKt-m2YpVkEjYB-jFlzfpla4rdKS2j2c-eCQ

https://1drv.ms/v/c/f13aa3123d36e9b9/EesKRaBP4CZDpoizW3bKyocBb5XsukHN6v0rYRgY1CnMew

Not all of the flashing shows up on the videos. Both the original strips and the new strips are doing the same thing. Before changing the MOSFET the bottom halves of the strips did nothing, now that I've changed it , they do this. There must be something else on the board that has a problem but I'm unsure what it could be. The MOSFET I changed is the one on the right.

 

Check the IC and mosfet drivers
I believe you have checked the resistors and the diodes

 

Hi guys. A quick update for anyone interested. I've had several months of hospital stays, so the TV repair got shelved for a while.

I tested the IC, all inputs were spot on but the output voltage for the bottom led section was not working. I replaced the IC today, which was a real pain, the IC had been glued to the board.

The TV is now working properly. The problem was the IC and the MOSFET.