UN55C6400RF Dim half side of screen

[neilc6]

This TV originally didn't power up but had a standby light. Youtube video shows replacement of transistors on LED driver portion of power supply board could be shorted. I found Q9101 shorted (and previously replaced) and replaced all four anyways (Q9101, Q9104, Q9201, Q9204).

TV powered up but one side is dim. Checking the strip connector unplugged with an LED tester showed 121V on all 4 pairs.

With connector plugged in, I measured
Pin 1 280V
Pin 2-3 182V
Pin 4 85V
Pin 5 307-308
Pin 6-7 210-211
Pin 8 113-120

The dim side is the unstable side as expected on pins 5-8

Checked D9101 and D201 and both were good. Since it is the same circuit for both sides, I can compare the working circuit with the non-working one from the top side.

Board is BN44-00358B and pretty expensive on ebay since they have a high failure rate.

[budm]

Re: UN55C6400RF Dim half side of screen

The second LED power supply show high Anode Voltage because it is not seeing the proper load so it went up to 308V.
Did you verify that Q9204, R9221 are good and Q9204 is getting the Gate drive?
Did you verify for sure that you do have good connection for the solder side of the male pins on the power supply board LED connector all the way to the LED strips?

[neilc6]

Re: UN55C6400RF Dim half side of screen

Poking around Q9204 and the connector with sharp probes, the voltages became normal and LEDs light up properly. Possibly a loose connection. Will run it for a few days.

[budm]

Re: UN55C6400RF Dim half side of screen

May be the female pins on the LED connector are expanded too much.

[neilc6]

Re: UN55C6400RF Dim half side of screen

Dimness came back. You know what they say about problems that go away by themselves. Diificult to get at the strips since it is edge lit. Turned the backlight way down and its still dim so think the issue is in the driver circuits.

[neilc6]

Re: UN55C6400RF Dim half side of screen

I ordered an as-is PS on ebay and this one doesn't even have standby voltage. All three fuses are good as well as all FETs. Can't figure out where the standby voltage is generated. Shopjimmy repair kit lists Q9101, 9104, 9201, 9204, QP801, 802, FS801S, 804S, D9201, DM810, DP802, 809 and ICS801. These parts test fine.

Recall seeing a training manual posted for similar model but can't find it.

[neilc6]

Re: UN55C6400RF Dim half side of screen

Studying the schematic, STBY5V is at CS802 and is switched through a FET for 5.3V. Checking the as-is board, it was missing VX801S and then realized the bridge rectifier is missing as well. Back to the LED driver circuit on my original board.

[neilc6]

Re: UN55C6400RF Dim half side of screen

Suspecting the LED driver IC, I transplanted the MAP3201 IC from the as-is board to my semi working board. Right side is still dim but voltages are lower and stable.

Pin 1 275V
Pin 2-3 160V
Pin 4 47V
Pin 5 203V
Pin 6-7 107V
Pin 8 18V

[neilc6]

Re: UN55C6400RF Dim half side of screen

Transplanted the second IC from the other board and get same result as above. Checked all surface mount resistors and diodes around IC. Retouched all solder joints as well. Don't really want to give in to buy another power supply board.

[neilc6]

Re: UN55C6400RF Dim half side of screen

Ordered a substitute board from buyqual from China and it came in today (just over 3 weeks for delivery, total cost $49 with shipping). Substitute board is narrower so they have included a longer cable to attach to the main board. Seems to be ok.

https://www.buyqual.com/samsung-bn44...board-new.html

"We send the Samsung brand substitute for BN44-00358A / BN44-00358B / BN44-00359A / BN44-00359B , not original board. we send the rightside of the picture, same use and install. Your TV have holes to install, over 400 pieces sold, stable and safe product."