LG 42LW4500 no picture no backlight but near all lines good

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  • disorder
    Senior Member
    • Sep 2015
    • 156
    • france

    #1

    LG 42LW4500 no picture no backlight but near all lines good

    hello, i have the same issue as this thread :

    https://www.badcaps.net/forum/showth...8668&styleid=8

    it's a 42LW4500 LG tv, previously it had the classical L407 classical issue : desoldering due to bad resin that gave me no 24V. work great for one month

    yesterday : sudenly, green light then screen shut down. white led ok

    open it and test : first put a resistor between 3,5 and PWR-ON, another one between 3,5 and inv_on : tv start, backlight ok and stable.

    but when the connector between motherboard and PSU is connected : i have 3,5V 12V 24V but NO INV_on, a_dim and p_dim.

    if i test ic1002 at inv_ctl : got 3.5V, r421 is good and Q405 no short.

    so i think my issue is due to P_dim (power dimming so)

    all pins joints were resoldered

    during all this tests, once the tv works for some seconds then shut down.

    thank you

    put here service manual .
    Attached Files
  • nomoresonys
    Badcaps Legend
    • Jan 2013
    • 12156
    • U.S.

    #2
    Re: LG 42LW4500 no picture no backlight but near all lines good

    Makes me think the same thing as the thread you refer to...that the mainboard is not sending the inverter on signal to the powerboard, I would try a replacement mainboard.

    Comment

    • zen
      Badcaps Veteran
      • Apr 2011
      • 511

      #3
      Re: LG 42LW4500 no picture no backlight but near all lines good

      I do recall working on one of these sets some time ago .

      The back light themselves can be very intermittent .

      In fact , on the one i had , set would run for 2 - 3 day without problem , before back lights cut out .

      Comment

      • disorder
        Senior Member
        • Sep 2015
        • 156
        • france

        #4
        Re: LG 42LW4500 no picture no backlight but near all lines good

        hello , some news : problem seems to be solved

        after analysed a-dim p-dim and inv_on, many filters missing compared to schematics.
        so it was easier to find the culprit

        removed Q405 by another NPN transistor and since the tv works normaly
        (i took a npn with same values except cutoff at 0,1A )

        hope this will help someone else later :-)

        Comment

        • zen
          Badcaps Veteran
          • Apr 2011
          • 511

          #5
          Re: LG 42LW4500 no picture no backlight but near all lines good

          Great result , & thanks for the information .

          Comment

          • disorder
            Senior Member
            • Sep 2015
            • 156
            • france

            #6
            Re: LG 42LW4500 no picture no backlight but near all lines good

            failure !!!

            the tv worked many time in my garage, just plug in in the house. worked for 20 minutes and then shut off and does the same weird issue.

            before shut off , screen goes like snow image as CRT screen

            maybe cold joints ? will look,

            Comment

            • disorder
              Senior Member
              • Sep 2015
              • 156
              • france

              #7
              Re: LG 42LW4500 no picture no backlight but near all lines good

              finally got it.
              i baked it in oven and now it works great , maybe for some days or years.
              i have tried it for more than 15 hours wthout any problem.

              some bad solder joint so.

              Issue solved :-)) !

              Comment

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