Samsung UN55H8000AFXZA No Backlight, LED Strips OK

[alfredomfr1]

TV show image with the help of a flashlight but no backlight. I can confirm that all led are OK.

CNL802A
TV OFF
0v

TV ON
Pin 11 and 12: 16v
All others: 0v

CNL802B
TV OFF
0V

TV ON
Pin 9 and 10: 16v
All others: 0v

[budm]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

1) Do you get backlights if the cable between the power supply board and the main board is not connected?
2) What did you use for ground for the meter when you measure 16V on the + pins?
3) What are the Voltages of each pin of the connector of the power supply that goes to the main board? Notes: measurement is done with all cables in place and you need to put down the pin name for each pin so we can easily see what you have on each pin..

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

1- Not backlight with the cable disconnected.
2- TV Chassis
3-

2 SD: 0.3v
4 STT: 0.2v
6 DATA: 0.1v
8 VSYNC_IN: 3.2v
10,12 LD: 3.2v
14 BLUONOFF: 4.97v
16 PWRONOFF: 3.3v
18,20 GND:
22 Vamp: 12.68v
24 GND:

1 LD: 3.28v
3 CLK: 1.69v
5 CONT: 0.07v
7 GND:
9,11,13,15,17 A13V: 12.68v
19 GND:
21,23 V AMP: 12.68v

[R_J]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

Monitor the voltage between chassis ground and J816 when the tv is first turned on, What is the voltage?

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

J816: 16.2v

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

J876, J864: 16.2v

[R_J]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

That voltage is low, What is the voltage across one of the main filter caps, CP811S or CP812S ? If this voltage is about 385vdc then the pfc circuit is working ok.

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

CP811S: 387v remember I can see image in the dark screen

[budm]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

What is the power supply board P/N? BN44-?????

[R_J]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

CP811S: 387v remember I can see image in the dark screen

I know that, but if the pfc was not working properly the led boost circuit would not be getting full voltage.
It seems like the problem is that the Led voltage is not being boosted to 34v. This could be caused by a bad led in one of the strings causing the led controller to shut down. 1 bad led in any of the 16 strings could cause the issue.

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

Bn44-00740a

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

R_J I already open the screen and tested all the LED

[R_J]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

And how did you test all the led's?
You tested all the led's in all 16 seperate strings?

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

With my led tester, every string same voltage: 30v

[R_J]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

Check the fets Q9951 & Q9952, and the gate drive resistors, this is the led boost circuit, what is the number on the controller IC995?

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

I will check the FETs tomorrow, thanks to budm and R_J for your help.

[vinceroger69]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

not sure what power supply this is in this video but maybe worth a watch
https://www.youtube.com/watch?v=2BCdAz0xE70

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

Q9951 and Q9952 OK, R9988 and R9958 OK
IC 9951: NCP1034DG
10k R9972 and R9973 are OK when tested out but R9973 only read 68 ohm in the circuit. All components on the picture are OK

[budm]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

https://cdn.badcaps-static.com/pdfs/...2b3c1b8a2d.pdf
Start checking to see if the IC has the VCC for it to run.

[alfredomfr1]

Re: Samsung UN55H8000AFXZA No Backlight, LED Strips OK

VCC: 11.92v
FET Q9951 Gate: 76v
FET Q9952 Gate: 16.7v