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Televisions made by the Chinese multinational electronics company, TCL.

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TCL 55C715 Having no backlight, but there is an image when its "ON"

Hello.

I would like to have the help of experts to help me troubleshoot a broken television.
This television is a TCL 55C715 UHD television and the backlight failed overnight.

Observation:

- It is possible to see the image on the screen if I place a powerful light on the screen, and the sound works.

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Actions already tried:

- Full TV reset.
- Measurements on the power board:
* Voltage measured on the motherboard power supply: 12Vdc.
* Voltage measured on the backlight power supply: 0v.
* Voltage measured from the motherboard on the "BL_ON" pin and ground:
> On: 3V.
> Off: 0V.

(So for me, the instruction for lighting the LEDs is given....(?))

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- Testing a new power board:
* Purchase of an identical card on the internet, it was received and then directly tested on the television. (No improvement). No voltage at the backlight pin outputs either.

- I tried to power a pair of LEDs at 3.3Vdc, 5Vdc, 12Vdc with my power supply but nothing visible (screen on)

- (Not sure of me) Attempt to measure LEDs (No resistance on the 4 pairs, between each red and black wire).

- (Not sure of me) Attempt to measure LEDs with the "LED" mode of the tester, no result.

-With an BackLight tester, I've try to lightup LED. Its worked ! Each pair lighting up different parts of the TV! (96Volts)

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Do you have any ideas about the breakdown or suggestions for measures to be taken?

Thank you very much for your feedback.

Have a good day.

Update (12/23/22)

After making the measurements, I've plugged the backlight connector to the power PCB.

I was surprised to see the back light working fine when I shut on the TV!

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I dont know why and how the backlight worked fine.

Think it the issue was solved, I reassembling the TV. However, whe I've rpowered the TV, the backlight where off...

I've noticed that there is a flash when I switch on the TV:

(Little flash appear in the little hole, during maybe 0,5 seconds)

When the where fine, I've make measurement:

Between L1+ & Ground = 20mv
Between L2+ & Ground = 20mv
Between L3+ & Ground = 115V
Between L4+ & Ground = 115V

I dont know why, but there is no voltage on L1 and L2. But the backlight where fully working at this point (No dark zone)

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Hi @johnythings

With the TV on and the images "seen" and backlights disconnected from the board, what voltage do you get between L1+ve and earth (not L1-ve) and L2+ and earth, L3+ve and earth and L4+ve and earth? Earth can be the head of a screw that secures the board to the chassis.

Did you disconnect the LED power leads from the board when trying to test for resistance and also did you reverse the meter leads at all when testing so that you tested "both ways" - LEDs are diodes after all, just in case you had the leads around the wrong way at first?

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@johnythings follow the instructions given by my esteemed colleague @jayeff first. See if you have a BL_Dim connection on your board. You should have around 3Volts on that one as well. You already replaced the power board (this is a new one correct?) and you tested your backlight strips (did you measure the voltage on the connector with the connector on or off?) You mentioned that you "tried to power a pair of LEDs" how did you do that? No voltage at the backlight pin outputs and everything works as it should (including the LED backlight strips), then in theory the only thing left is the main board.

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@johnythings, A problem I saw was that my backlight tester would light up both LED sets but with the LCD off it was obvious that several individual LEDs were out and the power board wouldn't light them. So just because your tester will turn them on doesn't mean they're good; replacing the strips fixed the issue in my case.

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Hi @jayeff. Thanks you for replying.

After I unpluged the LED connector, I've tested voltage between Lx+ and ground.

Voltage I found when PCB powered:
Between L1+ & Ground = 20mv

Between L2+ & Ground = 20mv

Between L3+ & Ground = 0.66V

Between L4+ & Ground = 0.66V

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Hi @oldturkey03 thanks you for replying.

After researchs, there is no "BL_DIM" but there is "P_DIM" and "A_DIM". I dont know what is those are for.

However, I have measuring P_DIM :

Between Ground & P_DIM = 0V (When TV off but PCB powered)

Between Ground & P_DIM = 3,3V (When TV ON)

I've replaced the power board by an another (Normaly Working).

I power a pair of LED with an "Backlight tester" device. Fully worked.

I think too that is probably the motherboard as you said. But I would like to be sure before invest money to brand new mother board.

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@johnythings that flash sure looks like your backlight strips failing. I really would consider that my next step.

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Thanks for your feedback. If the strips are thé issue, then why there IS non volts on pins when I unplug thé les connector?

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@johnythings some of the power boards recognize when there is a no-load situation and turn the power off. The larger issue here is that schematics are not available which prevent us from proper troubleshooting. You already checked and replaced what you can with the exception of the main board and the strips. The main board is what tells the power board to turn the backlights on.

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buenas tardes, tengo el mismo problema, tenéis algún vídeo de el proceso de desmontaje del panel en esta tv para sustituir las barras de leds

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@juanmiguel27849 in principle, they are all pretty much alike. Check something like this video to get a good idea of what is involved.

Transaltion

@juanmiguel27849 en principio son todos bastante parecidos. Mira algo como este vídeo para hacerte una idea de lo que se trata.

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John estará eternamente agradecido.
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