LG 55LA740V Intermittent backlight band troubleshooting help.

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  • Kastcool
    New Member
    • Nov 2020
    • 5
    • España

    #21
    Re: LG 55LA740V Intermittent backlight band troubleshooting help.

    Hi. I am new here.
    I don't know if I'm doing it right, putting this thread back. If it's wrong, forgive me and put this answer where it should be.
    I have a LG55LA740s tv with more or less the same problem as Mano84 and Heder.
    In my case, the vertical dark areas are almost always the same as on their TV, but on other occasions, if we think of 6 vertical columns and we count them from left to right, the dark areas can be 3 and 5 or 2 and 3 or 1 and 6, etc ...
    To make it more complicated, I have noticed that the failure increases in winter. The screen turning off leaving only the sound (I don't know if it is a protection system or another failure).
    In summer, however, it almost does not fail.
    I have seen that Mano84 solved her problem by changing a led strip but in her case it seems that the fault was always in the same place. In my case it is intermittent and can be any strip.
    The black and white connectors heder talks about are listed as follows:
    Black connector on the left central side of the power board, from left to right: led +, NC, NC, NC, NC, VC4, VC5, VC6 View the front screen, this connector handles the right half.
    White connector on the central right side of the power supply board, from left to right: VC1, VC2, VC3, NC, NC, NC, NC, led +. Viewed from the front, this connector handles the left half.
    I measured as best I could the voltages on the black and white connectors on the top of the power supply and the most relevant was:
    The voltage on the led outputs, with the television being good or bad (with some shadow) is always approximately 87 volts except when the screen turns off completely, then it drops to 71 v.
    Outputs VC3 and VC5: The normal voltage is around 10 to 15 volts but when it fails it goes up to 30 volts and when the screen turns off (the sound remains), it seems to be stable again.
    I have seen that finally Heder did not tell us if he solved the problem.
    Can you give me some idea.
    Thanks and regards.

    Comment

    • neilc6
      Badcaps Legend
      • Nov 2015
      • 1550
      • Canada

      #22
      Re: LG 55LA740V Intermittent backlight band troubleshooting help.

      Failing edge lit LED strips that are mounted horizontally will cause dark vertical bands. The LEDs will burn out especially if the backlight level is set to maximum. If it is intermittent, you can try turning down the backlight level and turn off energy saving mode.

      Comment

      • Kastcool
        New Member
        • Nov 2020
        • 5
        • España

        #23
        Re: LG 55LA740V Intermittent backlight band troubleshooting help.

        Okay. I'll try it and say something. Thank you

        Comment

        • Kastcool
          New Member
          • Nov 2020
          • 5
          • España

          #24
          Re: LG 55LA740V Intermittent backlight band troubleshooting help.

          Hi. Indeed, as Neilc6 said, the backlight was at maximum and a power saving mode was activated.I turn the light down to half and turn off the economizer. I turned off and on and the screen came out completely black. I turned off and on several times until the screen came out fine.
          It was like that all night and now comes the second part: At the end of the night there was a very good program and, since I have a disc connected to the TV, I put it on record.
          This morning the tv appears with black screen and there is no way it works. I observed that last night when I left it off and recording the red led that it has on the power supply flashed 5 times.
          This is not the first time this has happened. I have observed that the recordings on the hard disk do not go well either.
          I have seen that there are people who talk about failures in the soldering of the microprocessors and put the circuit boards in homemade ovens and say that they fix them.
          I'm not going to do this, but could the failure be due to a main board microprocessor soldering problem?
          Does anyone know if when the screen turns off completely it is a system protection or it is another failure?
          Last edited by Kastcool; 12-01-2020, 03:50 AM.

          Comment

          • Kastcool
            New Member
            • Nov 2020
            • 5
            • España

            #25
            Re: LG 55LA740V Intermittent backlight band troubleshooting help.

            Problem solved.
            I write these lines to help anyone who has the same problem.
            The LG 55LA740S TV has a single strip of LEDs mounted on an aluminum plate located at the bottom and on top it has a sheet of some type of plastic that is what brings the lighting to the entire screen.
            That strip is the one that fails.
            When this type of failure occurs to you, stop worrying about it. It is the led strip.
            Greetings.

            Comment

            • allrounder
              New Member
              • Jul 2021
              • 1
              • romania

              #26
              Re: LG 55LA740V Intermittent backlight band troubleshooting help.

              Originally posted by Mano84
              Hello,

              I have replaced the LED Strips on my 55LA790V, now the black bar problem is fixed. Thanks for the help.
              Hi Mano,

              I have exactly the same TV model and the same problem. Could you tell me where did you find replacement led strips for your tv?. I was looking all over the internet, no chance, only for 55LA7900 which is US model, another chassis. On the other hand maybe someone of you guys can recommend me a link or something, I don't want to disassemble the LCD panel without having the necessary strips. Any help will be highly appreciated. Thank you.

              Comment

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