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70" LG TV Backlight turns off (new inquiry)

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    70" LG TV Backlight turns off (new inquiry)

    I made a post regarding my 70" LG TV a few weeks ago, I'm making a new post because I have a side inquiry. The following is from the original post:

    "I have a 70" LG 4k TV. The model is 70UH6350-UB.BUSMLJR
    I got it from someone who said it started to turn off on its own. When I got it, it worked fine for around 3 hours, then the backlight went out. You can still hear the sound and I did the flashlight test on the display and it was still working as well, so it was evident that the backlights went out.
    I thought it would be a bad LED, so I successfully opened up the TV, got to the LEDs, tested one by one with an LED tester, and they all work just fine, there was nothing wrong with them, all showed the same voltage. So I cleaned the connections all around as I put it back, hoping for some reason it would work or something, but sure enough when I turned it on, it only worked for a few minutes and then the backlights went out again. I had accidentally burned out the power supply as I was doing some testing (whoops, I know) so I replaced with a new one from a trusted ebay seller and the same problem was there, so it can't be the power supply."

    The update is that I bought a new set of LEDs for it (got very lucky since no one is really selling these). Before I changed the LEDs I wanted to take a look at the boards one last time just in case... and I noticed that the main board has a tiny little round button on it (see picture). I assumed it's some type of reset button. So I turned on the TV, was connecting my fire stick and sure enough the backlight went out. So I turned it on again, pressed that button on the main board and it turned off and on the TV. Doesn't look like a factory reset since it took me back to where I was before reseting it. The backlights have been working for the past 30mins, they have not turned off.

    Do any of you have more information on that button on the main board? Do you think it's capable of fixing my issue or do you think the backlights will soon enough turn off again? I really want to avoid opening it up and replacing the LEDs.
    Attached Files

    #2
    Re: 70" LG TV Backlight turns off (new inquiry)

    By the way, I have done regular resets before as well (unplugging, pressing the power button for a while, leaving it disconnected for days, plugging it back in and turning it on) but I still had the same issue. With this main board reset button is the first time I see it last.

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      #3
      Re: 70" LG TV Backlight turns off (new inquiry)

      NEVERMIND, backlights went out -__-

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        #4
        Re: 70" LG TV Backlight turns off (new inquiry)

        Yea that is a hard reset for the microcontroller, and I suspect it is used during factory debug / programming. I am not enough of a software / firmware guru to tell you if it is that much different than the power up internal reset.

        Check the voltages as suggested in your other thread just as a learning experience and to be sure. In particular look at the "error" pin on the Power Supply to see if it is changing state when the LED's shut off indicating a fault was detected.

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          #5
          Re: 70" LG TV Backlight turns off (new inquiry)

          I replaced the backlights of a 50UH recently. Symptom was flash of LG logo. Replaced all with new. When I tested the old strips afterwards, they all tested good with my LED testers both low current and my 300mA tester.

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            #6
            Re: 70" LG TV Backlight turns off (new inquiry)

            And after you replaced those backlights the TV worked?

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              #7
              Re: 70" LG TV Backlight turns off (new inquiry)

              Flash of backlights is a symptom of bad backlights and replacing them all with new and turning down the backlight level is the proper repair procedure. I don't bother testing before replacement any more but will test once installed by turning on the tv with the panel still removed. I may use my LED tester to power the backlight independent of the power supply to check the panel for damage before ordering new strips.

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                #8
                Re: 70" LG TV Backlight turns off (new inquiry)

                Yeah, the thing is that for my TV the backlights turn off at random, there is no pattern. Sometimes they turn off within a minute, other times they'll turn off after 10mins or more. One time they kept turning off after 1min, and after about 8 times turning it on they stayed on for 8mins and then turned off again. And it's also random if I have the backlights at low level. Nothing I do can give me an indication of exactly what could be wrong. Do any of you happen to know exactly where is the section that controls the LEDs is? It doesn't have a separate LED Driver, so I'm thinking it's somewhere in the main board?

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                  #9
                  Re: 70" LG TV Backlight turns off (new inquiry)

                  My apologies about the voltage reference above, I got confused with another thread. That said though, checking the voltages on the Power Supply connector to Main board may reveal what is going on when working or not.

                  The bulk of the LED control is done on the Power Supply. I can't make out the IC number on the shopjimmy website but Q801 through Q812 are the Mosfets that are controlling the current supplied to the LED's. You can see those outputs on the LED connectors labeled Vc-1 through Vc-12.

                  The Main board is going to determine how hard the LED's are driven and that should be dictated by the P-DIMM1 and P-DIMM2 signals. These are likely PWM drive signals but checking with a meter when not working and working may be beneficial. I am not sure if they were to be zeroed out (minimal drive) if the LED's would go dark.

                  I think I have a 60 inch Power Supply that uses the same controller. I can look at it later but I seem to recall going down that rabbit hole before and not finding any kind of data sheet on that IC

                  Good clear pictures are always welcome of each board and of the entire set showing the interconnect of the boards.

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                    #10
                    Re: 70" LG TV Backlight turns off (new inquiry)

                    Are you sure the LEDs you bought are new?
                    Measure the voltages at LED connector when back backlight is on and off.
                    I wouldn't trust LED tester in your particular case.
                    Please just follow step-by-step if you want this to be solved.

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                      #11
                      Re: 70" LG TV Backlight turns off (new inquiry)

                      Yeah, when the backlights turn off the voltage at the connector is 0, but that could be because most likely the LED driver is cutting the power to the LEDs to prevent further damage since it detected a problem with the LEDs or possibly it is malfunctioning?

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                        #12
                        Re: 70" LG TV Backlight turns off (new inquiry)

                        And no, unfortunately the LEDs I bought were from a broken TV (cracked screen) of the same model. I haven't replaced the LEDs with the ones I bought yet, I just wanted to troubleshoot and see if maybe it was the boards that were causing the issue before moving to replacing the lights.

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                          #13
                          Re: 70" LG TV Backlight turns off (new inquiry)

                          Update: I changed the backlights for the used ones I bought... seems to work so far, just gonna cross my fingers it lasts.
                          Does anybody have any information as to reasons why LED arrays may fail in the first place? The root cause?

                          Comment


                            #14
                            Re: 70" LG TV Backlight turns off (new inquiry)

                            LEDs will heat up, wear and burn out just like light bulbs. They are typically overdriven and running at their max specs if the backlight level is set to 100% which many of the default picture modes do. 8 out of 10 repairs I do are for backlights. If you want your replacements to last turn down the backlight level for all inputs and all apps. https://techhometravel.com/lg-ln-ser...ed-backlights/
                            Last edited by neilc6; 06-02-2021, 04:01 PM.

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                              #15
                              Re: 70" LG TV Backlight turns off (new inquiry)

                              Thank you for the information. I turned down the backlight to 50% for all inputs and I'm just using my fire stick. This nightmare is finally over, I've been dealing with this tv since April 13th. I think I'm going to take a break from fixing tvs, haha

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