Samsung LN52B750U1FXZA - Backlight Issue

[fuzzyrunner]

Hello All!

So here is the issue I am having. When everything is plugged in as normal, the TV turns on and I get a quick flash of the backlight. After that the backlight never turns back on but the rest of the TV works fine. If I pull the PWM pin from the cable between the main board and power board the backlight turns on and the tv works "normally". If it is plugged in I get nothing. I did do a factory reset but that didn't solve my issue. When everything is plugged in normally I get .6v at start up and continues rise over about 10 seconds. (~.6v then ~.8v then ~1.8v then settles on 3.3v). I feel like the voltage is correct (though I don't actually know). My guess is that the power supply isn't "reading" the varying voltage correctly and shutting the backlights off but that's just a guess. Thanks!

[CapLeaker]

We need straight shot pictures of the whole board and measurements on the connectors. Pay attention to BLon, PWM / DIM signals.
Try to disconnect the main board from the PSU and plug the TV into power mains. It should turn on. If it doesn't take 1k resistors and jump PSon and BLon to STBy. See what that does.
There isn't any clicking or something going on when everything is hooked up like it is supposed to?

[CapLeaker]

Also read this:
https://www.badcaps.net/forum/troubl...guides?t=25810

[fuzzyrunner]

We need straight shot pictures of the whole board and measurements on the connectors. Pay attention to BLon, PWM / DIM signals.
Try to disconnect the main board from the PSU and plug the TV into power mains. It should turn on. If it doesn't take 1k resistors and jump PSon and BLon to STBy. See what that does.
There isn't any clicking or something going on when everything is hooked up like it is supposed to?

Backlight turns on when PSU is plugged in to mains but not connected to the main board.

Voltage measurements on the connector between PSU and main board.
1: .894v
3: 5.2v
5: 0v
7: 12.65v
9: 0
11: 0
13: 5.1v
15: 5.1v
17: 0v
19: 12.1v
21: 12.1v
23: 5.1v (Blon/Inv on/off)
25: 3.3v (PWM)
27: -.031v
29: -.031v

Yes, everything works as normal when it is hooked up the way it is supposed to except for the backlight (only get a quick flash).

[CapLeaker]

There is something wrong with pin 27 DET 5V (detect) for the backlights. You got zilch and there should be 5V. Are all CCFL’s ok?

[nomoresonys]

Guess I still don't or forgot what exactly the DET_5V is suppose to be, ran across this but don't know if he is correct, he states it isn't suppose to be there, he says/thinks it only has 5v there if the inverter is broken, I had one with the det-5v missing a long while back but completely forgot what happened with it but it did always seem like it SHOULD be there on properly working tvs: https://www.ifixit.com/Answers/View/...missing+5V_DET

[nomoresonys]

Then here in post 12 it says there's probably a fault in the inverter section and that's why there's no det_5v, so I guess the guy in the ifixit thread had it backwards: https://www.badcaps.net/forum/troubl...roubleshooting

[fuzzyrunner]

There is something wrong with pin 27 DET 5V (detect) for the backlights. You got zilch and there should be 5V. Are all CCFL’s ok?

When I pull the PWM pin from the connector and turn the TV on I get 5.2v on Pin 27. Here are two pictures of the screen with the backlight lit up. I guess one could be starting to fail but not showing outwards symptoms yet.

[fuzzyrunner]

Thanks for the links! I'll take a look into both of them a bit.