LT-42EX17 LCD backlight issue

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  • Nick's Tvs
    Badcaps Veteran
    • Jan 2014
    • 621
    • USA

    #1

    LT-42EX17 LCD backlight issue

    This is not just for this tv but a few other models I have encountered this problem and I just don't really know how to proceed for a repair.

    Originally had a bad backlight inverter. I fix the board, plug the set back in, and the backlight turns on for all of 2 seconds before going dark. I also noticed that the left side of the screen is considerably dimmer during those 2 seconds of being on. No clue as to why this is happening.

    I also had someone bring in a sceptre 42" LCD with the same back-light symptom maybe 2 weeks ago and he said it turns on for 2 seconds then goes dark. I check it, and sure enough it does and one side is much dimmer than it should be. For that one, I swapped the backlight inverters with my known good ones from that same set I was selling and no difference. I ended up just trading him my sceptre set for his and some cash, but now I have 2 tvs that have this same problem.

    Back to the JVC I currently have open.
    Some of the CCFL's are the same brightness from left to right, and some are dimmer on the left side, so I swapped one of the dim ones with the top one which seemed to be at a good brightness, but it seems like the top one got dim and the dim one when in place of the top one was normal again. I checked continuity, I cleaned with isopropyl everything seems like it should work but obviously it does not.
    To me it sounds like the CCFL's are just old and going out, but swapping a bright one with a dim one"When I say dim I mean has the left side being dim" just makes the bright one go dim and the dim one go bright. So now I think its not the CCFL's but nothing else would make sense. Also can someone explain to me how a CCFL can have one side dim and the other bright? Also I checked the boards that the CCFLS plug into and connectors on those and they are fine in terms of continuity.

    Super confused and would like to understand this phenomenon.
    Thanks
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: LT-42EX17 LCD backlight issue

    Do both TV have Master/Slave Invereter boards?
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

    Inverter testing using old CFL:
    http://www.badcaps.net/forum/showthr...er+testing+cfl

    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
    http://s807.photobucket.com/user/budm/library/

    TV Factory reset codes listing:
    http://www.badcaps.net/forum/showthread.php?t=24809

    Comment

    • Nick's Tvs
      Badcaps Veteran
      • Jan 2014
      • 621
      • USA

      #3
      Re: LT-42EX17 LCD backlight issue

      Budm
      Yes both of these sets have Master and Salve inverter boards. I will take pictures when I get back to the shop in an hour or so. Ill post part numbers as well.

      Comment

      • budm
        Badcaps Legend
        • Feb 2010
        • 40746
        • USA

        #4
        Re: LT-42EX17 LCD backlight issue

        The MASTER/SLAVE Inverters are set up to drive the CCFL in push-pull mode, the drive signal for the Slave inverter is in the Master inverter board. If the MASTER Inverter output is bad but the drive circuits are still good and the Slave inverter board is good then the Slave Inverter will drive the lamps half way of the lamps, if the MASTER inverter is good but the Slave is bad then the MASTER will only drive half way of the lamps.
        Never stop learning
        Basic LCD TV and Monitor troubleshooting guides.
        http://www.badcaps.net/forum/showthr...956#post305956

        Voltage Regulator (LDO) testing:
        http://www.badcaps.net/forum/showthr...999#post300999

        Inverter testing using old CFL:
        http://www.badcaps.net/forum/showthr...er+testing+cfl

        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
        http://s807.photobucket.com/user/budm/library/

        TV Factory reset codes listing:
        http://www.badcaps.net/forum/showthread.php?t=24809

        Comment

        • Nick's Tvs
          Badcaps Veteran
          • Jan 2014
          • 621
          • USA

          #5
          Re: LT-42EX17 LCD backlight issue

          Budm,

          In the JVC model, that would make sense, it is possible that there are additional faults on the inverter that I "fixed". However for the sceptre, the same exact symptom is occurring, and I swapped out the boards with known good ones.
          anyway, here are pictures of the JVC CCFL's and the back of the panel.
          The board I supposedly fixed is part number 6632L-0449C

          See attached pictures for CCFL's and the back of the panel.
          Attached Files

          Comment

          • budm
            Badcaps Legend
            • Feb 2010
            • 40746
            • USA

            #6
            Re: LT-42EX17 LCD backlight issue

            I see that some of the lamps do not light up the same as the others, and the lamps look to be EEFL type.
            https://www.badcaps.net/forum/attach...7&d=1457368574
            #6 (counting from the bottom looks strange, sagging lamp?
            You can also try disconnecting the power to the Slave inverter to see if you will see the same symptom.
            Never stop learning
            Basic LCD TV and Monitor troubleshooting guides.
            http://www.badcaps.net/forum/showthr...956#post305956

            Voltage Regulator (LDO) testing:
            http://www.badcaps.net/forum/showthr...999#post300999

            Inverter testing using old CFL:
            http://www.badcaps.net/forum/showthr...er+testing+cfl

            Tear down pictures : Hit the ">" Show Albums and stories" on the left side
            http://s807.photobucket.com/user/budm/library/

            TV Factory reset codes listing:
            http://www.badcaps.net/forum/showthread.php?t=24809

            Comment

            • Nick's Tvs
              Badcaps Veteran
              • Jan 2014
              • 621
              • USA

              #7
              Re: LT-42EX17 LCD backlight issue

              #6 from bottom is my fault, I didn't place it back in the plastic holder in the middle properly. Does the fact that they are EEFL vs CCFL's affects the set in any way?

              Disconnected the slave power but left the ribbon and the backlight stayed on. Now the lamps are just pretty much all off equally on the left side and are off for a solid third of the set.
              So does that pretty much guarantee that the inverter was not entirely fixed and still has a fault? Or could this still be caused because of another fault?

              On the slave inverter, I replaced the large/long IC since it appeared physically damaged from burning out. The copper trace also burnt out and since the trace goes under the IC, I had to bridge it over the IC with a wire. The diodes all checked out ok, so I didn't replace them at first. When the set started acting up as it is, I decided to replace them as well but obviously that didn't make a difference.
              Could the transformers be bad?

              Pics will be added in 2 minutes
              Attached Files
              Last edited by Nick's Tvs; 03-07-2016, 11:56 AM. Reason: pics added

              Comment

              • Nick's Tvs
                Badcaps Veteran
                • Jan 2014
                • 621
                • USA

                #8
                Re: LT-42EX17 LCD backlight issue

                Any thoughts on what to check on the inverter?
                I did basic check on transistors, resistors, diodes but all look good. I replaced caps as well on both boards since both had very bulged caps. Also want to point out that the slave does have 24 volts going to the transformers, but there is one line that goes to I think 3v when the backlight turns on and then down to 1.5 or so once the backlight goes off.
                I just re-flowed the entire left board that holds the lamps just in case, and cleaned both the lamps and the holders to ensure proper connection.
                I am putting the screen and the diffuser's back on. Going to assume bad inverter and go from there. That is odd however, because with the scepter TV, I had a very similar issue and replacing the inverters with known good ones didn't do the trick. These tv's are driving me crazy .

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA

                  #9
                  Re: LT-42EX17 LCD backlight issue

                  Usually when that IC goes out it will take the 4 Power MOSFET along with it.
                  Never stop learning
                  Basic LCD TV and Monitor troubleshooting guides.
                  http://www.badcaps.net/forum/showthr...956#post305956

                  Voltage Regulator (LDO) testing:
                  http://www.badcaps.net/forum/showthr...999#post300999

                  Inverter testing using old CFL:
                  http://www.badcaps.net/forum/showthr...er+testing+cfl

                  Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                  http://s807.photobucket.com/user/budm/library/

                  TV Factory reset codes listing:
                  http://www.badcaps.net/forum/showthread.php?t=24809

                  Comment

                  • Nick's Tvs
                    Badcaps Veteran
                    • Jan 2014
                    • 621
                    • USA

                    #10
                    Re: LT-42EX17 LCD backlight issue

                    I did replace those without a difference, and I do get 24 volts now going out of those to the transformer.

                    Comment

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