Hisense 42K20E Backlights fade to off - Had image

[x_orange90_x]

So the backlights light up on power on, then fade to off. I can see an image on the screen (No Signal). I've checked voltages of the LED connector and nearly all of the caps. None of the capacitors have any where near the rating including the main. The LED strings don't even have the same voltages. I've attached a pic of the power supply.

Here are the voltages:

C810 = 155v

All of the following are 128v-130v
C910
C915
C953
C952
C955
C951

The following are at 11.8v
C922
C907
C923

C908 = 14.66v
C905 = 14.7v

The LED connector has
1+ = 130v
2+ = 3.8v

[CapLeaker]

Re: Hisense 42K20E Backlights fade to off - Had image

bad backlight leds in the second string.

[budm]

Re: Hisense 42K20E Backlights fade to off - Had image

Looking at the board, it looks like The LED strings are connected in series so the 1+ (Anode) will not be same as the 2+ (Anode).
1+ (Anode) is fed by the LED V boost converter, the 1 - (Cathode) should be connected to the 2 + (Anode), the 2 - (Cathode) goes to ground via PWM controlled MOSFET and current sensing resistor.
Please upload good clear pictures of the whole backside of the board.

[x_orange90_x]

Re: Hisense 42K20E Backlights fade to off - Had image

bad backlight leds in the second string.

I forgot that I posted this same issue over 2 years ago and hadn't continued with diagnostics. Please refer to the following link, especially the last few posts about the BRI voltage. I never continued because I didn't have a 1k resistor, and I still don't. Is there an alternative to doing this without the resistor, or is it absolutely necessary?

https://www.badcaps.net/forum/showthread.php?t=63913

[budm]

Re: Hisense 42K20E Backlights fade to off - Had image

You should cancel this thread then and go back to the original thread.

[x_orange90_x]

Re: Hisense 42K20E Backlights fade to off - Had image

You should cancel this thread then and go back to the original thread.

I was thinking the same. How do that?

[x_orange90_x]

Re: Hisense 42K20E Backlights fade to off - Had image

@budm please check the old thread. I've made some very interesting discoveries.