Samsung UE75TU7090 backlight or PSU fault?

[Gjackson]

Hi all,

I have a Samsung UE75TU7090 with PSU board BN44-01056A (L75S6N_THS) with a backlight fault. With both backlight connectors unplugged the TV boots normally with a stable 12V rail. With the backlight connected the TV fails to start, Channel 1 (rated 115V) folds back to around 20V, Channel 2 (rated 70V) drops to 0V, and the 12V rail collapses to 3.1V. With the backlight disconnected and powered on, Channel 2 ramps up to around 230V open circuit while Channel 1 sits at 0V.
Since the TV boots fine with the backlight unplugged I'm leaning towards a shorted LED strip causing OCP which brings everything else down with it. Is it safe to assume the fault is in the LED strips and proceed to open the panel, or does Channel 1 reading 0V open circuit suggest a PSU fault that needs investigating first? I'd like to avoid removing the LCD on this behemoth of a TV if I can...

I found no indication of shorted FETs or diodes.

Thanks!

[tom66]

I suspect shorted LED strip too; if you have a power supply capable of higher voltage (115V DC+) you can try to test the LEDs. Does channel 2 voltage go up and then back down again?

[Davi.p]

Hi, i some times suggest a test, but i always meet lazy or uncapable people that don't do that, in years and years no one did it, i never did it, because i no more repair tv sets, i don't know if it works, anyway requirement: few cm of thin cable and a spare working 3v led bar or even some 1n4007 diodes... let me know in which shore you stay, bye..

[lotas]

There was a breakdown of the mask (substrate) on the radiator (aluminum base) of the LED strip, and the strip itself comes into contact with the TV case, and because of this, a short circuit occurs to the case and the power supply goes into protection.

[Davi.p]

The test is not possible with the connector i see now, no cables, in any case you need to open the leds tray

[Repairtechlcd]

You may have one of ther led strips shorting to chassis. Get a led tester and put the negative lead to the metal chassis and probe with the possitive lead the backlight connector. See if you get any readings which of cource would indicate a fault.