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UN40F5500AF Doen't Turn On - Flickering

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    #1

    UN40F5500AF Doen't Turn On - Flickering

    When the power button is pushed, the front LED blinks but the TV doesn't turn on. Unplugging the main board from the power supply, the backlight flickers. Checking the strips with an LED tester, they all test good.

    PFC voltage on main caps is 392V and drifts a couple hundred of millivolts. B5V is stable at 5.2V. B13V is around 11V, not stable and drifts +/-1V. Since this supplies the LED driver IC, I believe either the PFC or 13V supply side is the main suspect.

    PS is BN44-00645D. BN44-00645A schematic should be close.
    Attached Files
    Last edited by neilc6; 12-02-2018, 05:27 PM.
    Tags: None
    #2
    Re: UN40F5500AF Doen't Turn On - Flickering

    "Unplugging the main board from the power supply, the backlight flickers." That will not be main board problem.
    What does the LED tester show as far as the Vf of each LED strings, you have two LED strings (they are no the same, one has more LED's than the other).
    BTW, LED tester only put out 30 mA instead of >200mA. Bad LED's will fail at higher current.
    Last edited by budm; 12-02-2018, 05:34 PM.
    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

      #3
      Re: UN40F5500AF Doen't Turn On - Flickering

      I unplugged the backlight connector with main connector disconnected and B13V is jumping around 10.5 to 12.5V. PFC voltage is stable at 392V DC. There are 4 diodes and one capacitor at the transformer output (not quite matching the schematic). None of the diodes are shorted.

      Comment

        #4
        Re: UN40F5500AF Doen't Turn On - Flickering

        Inspect the solder joints of that T81CS transformer (it is for 13v supply and LED power supplies) legs, especially those with riveted through hole solder joints, also inspect other solder joints as well. Those through hole rivet solder joints do fail a lot.
        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

          #5
          Re: UN40F5500AF Doen't Turn On - Flickering

          Solder joints look good and show good connectivity with ohmmeter. Measured resistance across B13V and seems a bit low at 180 ohms.
          Attached Files
          Last edited by neilc6; 12-02-2018, 06:20 PM.

          Comment

            #6
            Re: UN40F5500AF Doen't Turn On - Flickering

            Found a service manual online for this model but no schematics and flow chart solution is to replace the power supply board. Flickering is likely caused when the 12V supply drops below minimum threshold for the supply on the LED driver ICs. I'll have to check the components that drive TM801CS. The output cap on the 13V line tested good as well as the diodes. There are quite a few differences from the A to D versions on that board.

            Comment

              #7
              Re: UN40F5500AF Doen't Turn On - Flickering

              Gave up and ordered a new PS board. Both outputs of TM801CS weren't stable. Checked all the rectangular poly caps and ceramic disk caps in that section and they all tested ok. Got 3.4V on VCC of ICM801. New board should fix it.

              Comment

                #8
                Re: UN40F5500AF Doen't Turn On - Flickering

                New power supply arrived and tv is fixed. Bought from Aliexpress seller. Probably safer to buy on ebay with money back guarantee.

                Comment

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