LG inverter

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  • sabre504
    Badcaps Veteran
    • May 2010
    • 449
    • United Kingdom

    #1

    LG inverter

    Hi have an LG 42 " LCD and the inverter had gone so i bought another inverter (second hand )
    It works but the screen pulses and the inverter buzz's

    I have tested all the transformers and have found that 2 of the transformers are reading different from the other 10

    1st pic i have under lined the 2 (i call them 6 and 7)

    Test i did analogue meter set to 1K

    1 red probe
    2= 45
    3 = 0.3
    4 = 55
    Had the same reading on 1,2,3,4,5,8,9,10,11and 12

    Trans 6 and 7
    1 red probe
    2 = 140
    3 = 0.3
    4 = 180

    Power supply putting out 24.2 volts steady on all 4 pins

    So i am right in thinking that the 2 transformers are failing
    as the are seprate lamps 6 on lamp 3 7 on lamp 4

    Any input welcome
    Attached Files
  • Dgtech
    E. Technician
    • Apr 2009
    • 1462
    • Steeler

    #2
    Re: LG inverter

    I'd look at the circuitry leading up to that transformer. Look at the driver chip(s) or the mosfets. I'd point them out but the picture of the whole board is fuzzy. Post a clear picture of the whole board if you can.
    The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

    Comment

    • PlainBill
      Badcaps Legend
      • Feb 2009
      • 7034
      • USA

      #3
      Re: LG inverter

      You aren't doing the measurement correctly. The pins you have labeled 1 and 3 are the ends of the primary. There is no point in measuring them, the resistance will be so low that your meter can't resolve the differences. The pins you have labeled 2 and 4 are the secondary. Measure the resistance across them for each of the transformers.

      Interestingly, it looks like the primaries are in series. It would be nice to have a better resolution over-all picture.

      Also, note that the manufacturer has already labeled the pins 1-6 (but skipped 3), and has numbered the transformers.

      PlainBill
      Last edited by PlainBill; 05-08-2011, 06:17 AM.
      For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

      Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

      Comment

      • sabre504
        Badcaps Veteran
        • May 2010
        • 449
        • United Kingdom

        #4
        Re: LG inverter

        Hi again right have better pics but had to split in 2 as it was too long a board to get decent pic all in one .

        I sussed i had measured wrong when i took one of to test off board and also found it had a small heat mark under the transformer so took the other off too and same mark underneath.
        So have taken 2 transformers off the other board i had soldered them on and remeasured ,all read the same now so resoldered all pins on all transformers .

        Also noted some poor solder on the transistors (??) i think they are or maybe drivers or mosfets and the one i have ringed in yellow measures different from the rest.
        Meter on 1K
        probe on each leg measured 6
        one i have ringed measured 0 open so am going to change it with one from the other board i probably not measured it right but def something wrong with it .

        Will resolder all those too and report back
        Thanks for the input from you both
        Attached Files

        Comment

        • Village Electronics
          OldPro
          • Apr 2011
          • 118

          #5
          Re: LG inverter

          Originally posted by sabre504
          Hi have an LG 42 " LCD and the inverter had gone so i bought another inverter (second hand )
          It works but the screen pulses and the inverter buzz's

          I have tested all the transformers and have found that 2 of the transformers are reading different from the other 10

          1st pic i have under lined the 2 (i call them 6 and 7)

          Test i did analogue meter set to 1K

          1 red probe
          2= 45
          3 = 0.3
          4 = 55
          Had the same reading on 1,2,3,4,5,8,9,10,11and 12

          Trans 6 and 7
          1 red probe
          2 = 140
          3 = 0.3
          4 = 180

          Power supply putting out 24.2 volts steady on all 4 pins

          So i am right in thinking that the 2 transformers are failing
          as the are seprate lamps 6 on lamp 3 7 on lamp 4

          Any input welcome
          You say that "it works but"..... To me that indicates the panel is lighting.

          Based on that assumption....

          I have found that an effective way to get a handle on inverter board transformers is to put the plastic shielded end of my scope probe against the transformers one by one. The signal induced should be approximately the same on each transformer. If one is much different than the others that is where you have to concentrate.

          To isolate a single transformer that you suspect: Assuming you have no sub handy, simply swap the suspect transformer with another. If the the bad indication swaps then suspect the transformer. If it does not swap suspect the circuit.

          Comment

          • sabre504
            Badcaps Veteran
            • May 2010
            • 449
            • United Kingdom

            #6
            Re: LG inverter

            Sorry i wasnt clear about the fault.

            Fault being the screen had picture and sound but would appear to flicker or pulse as i put was giving the impression that the brightness was ramping up and down but i assume it was lamps 3 and 4 flickering .
            Ihave swapped the 2 transformers and the other part resoldered everything i could and now appears to be working .
            My 3 year old is testing it now with a marathon of Toy story

            Thanks again all for the help and advice ,will just have to see how it goes and if it lasts

            Comment

            • Dgtech
              E. Technician
              • Apr 2009
              • 1462
              • Steeler

              #7
              Re: LG inverter

              Good job! Keep that other board handy though, just in case.
              The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

              Comment

              • sabre504
                Badcaps Veteran
                • May 2010
                • 449
                • United Kingdom

                #8
                Re: LG inverter

                I will you never when someone else may need a transformer

                Thanks again all

                Comment

                • sabre504
                  Badcaps Veteran
                  • May 2010
                  • 449
                  • United Kingdom

                  #9
                  Re: LG inverter

                  Still going strong ,
                  have got the brightness down to 25 and backlight on 50 and still clear and bright

                  Comment

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