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.
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....(?))
- 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)
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!
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)
6条评论
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?
由 jayeff 完成的
@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.
由 oldturkey03 完成的
@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.
由 Jerry Wheeler 完成的
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
由 John 完成的
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.
由 John 完成的
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