Lg 1953t smps pwm

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  • inline79
    Member
    • May 2012
    • 31
    • Canada
    #1

    Lg 1953t smps pwm

    There's no switching happening on the main transformer on this LG 1953T. no 12V out, no 5V out.

    I put a scope on U101 which is a FAN7601 PWM controller:


    pin 1 -Start up- getting 160VDC off the bus (bus reads fine at 167VDC)

    pin 2 - Current Sense/Feedback - is 0.13V so it is not being shut down here (>1V will shut it down)

    pin 3 - Latch/SS - comes up to 1.5V as expected during start-up then goes to 0V (over 75ms)

    pin 4 - RT/CT - is clocking

    pin 5 - ground

    pin 6 - Out - nothing... flat line.. I want a square wave here to drive the FET. I can see some noise on here when it first connects to AC but then it goes away and never comes back.

    pin 7 - VCC - is going from 8V to 12V as expected, it looks like the Startup current is charging the cap. It drops from 12V to 8V over the 75ms startup time.

    pin 8 - Vref - steady 5V.

    Schematic attached. FET checks out - no shorts.

    pins 5/6/7 are not shorted so the internal transistors seem to be ok. Looking for some thoughts before I go to the trouble of ordering another FAN7601.

    THanks!

    PS. I do this in my spare time for a non-profit ethical computer recycling organization that provides these monitors to low income families, schools, etc.
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    • retiredcaps
      Badcaps Legend
      • Apr 2010
      • 9271
      #2
      Re: Lg 1953t smps pwm

      Originally posted by inline79
      pin 7 - VCC - is going from 8V to 12V as expected, it looks like the Startup current is charging the cap. It drops from 12V to 8V over the 75ms startup time.
      Check C103. It may be bad without visible bloating. I don't have an ESR meter so I routinely replace these startup caps unless it is a known good brand like Rubycon or Panasonic.

      If C103 is good, then see if D201, D202 and D103 to see if they are shorted.
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        Comment

        • Rtech
          Badcaps Legend
          • Jul 2010
          • 1095
          #3
          Re: Lg 1953t smps pwm

          The FAN7601,is one that I have seen fail on a number of occasions,so would not be surprised if that is the cause.

            Comment

            • tom66
              EVs Rule
              • Apr 2011
              • 32560
              • UK
              #4
              Re: Lg 1953t smps pwm

              Check your FET has no gate-source short.
              Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
              For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                Comment

                • inline79
                  Member
                  • May 2012
                  • 31
                  • Canada
                  #5
                  Re: Lg 1953t smps pwm

                  Originally posted by Rtech
                  The FAN7601,is one that I have seen fail on a number of occasions,so would not be surprised if that is the cause.
                  This is what I wanted to hear - that it is not an invincible NEV-R-Fail IC. Thanks!

                    Comment

                    • Th3_uN1Qu3
                      Believe in
                      • Jul 2010
                      • 6031
                      • Romania
                      #6
                      Re: Lg 1953t smps pwm

                      If the fet isn't shorted G-S as mentioned above, yes, definitely the IC. Looking at the datasheet, this is yet another variation of the classic UC3842, with a couple more bells and whistles. You could sub in a 3842, but this one looks a bit more work than it's worth it... since it's still available, buy it. I replaced a FAN7554 by a UC3842, but that one only differs by one pin.
                      Originally posted by PeteS in CA
                      Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                      A working TV? How boring!

                        Comment

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