Fujitsu B23T-7 no backlight

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  • Hombre
    Badcaps Veteran
    • Jul 2013
    • 231
    • Croatia
    #1

    Fujitsu B23T-7 no backlight

    Hi guys

    I need help with this one.
    When i turn it on it has no backlight, but with source connected image is visible with help of flashlight .
    So, 19V going to the mainboard are present, on/off pin is going from 0V to 2.8V when power button is pressed. DIM pin is 3.2V all the time.
    I dissmanteled LCD to reach LED string. Measured one by one LED and they are all OK. From power supply to the strip is going 19V, 2 lines, each feeding 18 LEDs and it has 2 current sensing lines going back to LED driver OZ998B.
    Q801 voltages with refference to GND are G:0V, D:19V, S:0V, does not change with pressing power button.
    Same is happening with LEDs connected or disconnected, does not change anything.
    Pins 8 and 10 of U801 on turning on goes from 0 to 6V.

    The schematic is more or less this one, difference is 19V power supply and 2 led strips:
    http://hsto.org/getpro/habr/post_ima...93e1b46374.gif
    Attached Files
    Tags: None
    • budm
      Badcaps Legend
      • Feb 2010
      • 40746
      • USA
      #2
      Re: Fujitsu B23T-7 no backlight

      Can we see good clear straight shot pictures of the top and bottom side of the board? And pictures of how all the boards are connected together?
      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

        • Hombre
          Badcaps Veteran
          • Jul 2013
          • 231
          • Croatia
          #3
          Re: Fujitsu B23T-7 no backlight

          I hope these are good enough

          Thanks Bud!
          Attached Files

            Comment

            • Hombre
              Badcaps Veteran
              • Jul 2013
              • 231
              • Croatia
              #4
              Re: Fujitsu B23T-7 no backlight

              OK, diode D802 SB5100 is shorted (conducting in both directions)
              Why and how it happened i have no idea. I will try to change it tomorrow and then hope it will not go bad again

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA
                  #5
                  Re: Fujitsu B23T-7 no backlight

                  MOSFET Q801, D802, L801 are part of the Voltage booster circuit to boost the incoming 19VDC to higher Voltage level to feed the Anode of the LED strings, so when D802 shorted out you then will not have the Boosted Voltage only an incoming Voltage.
                  Last edited by budm; 05-05-2017, 10:22 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

                    • Hombre
                      Badcaps Veteran
                      • Jul 2013
                      • 231
                      • Croatia
                      #6
                      Re: Fujitsu B23T-7 no backlight

                      Fixed!
                      Replaced diode and everything is as it should be.
                      Only thing is that i couldn't find proper diode, not even near replacement, because i am far from home and am happy that i could at all find some 30W soldering iron
                      So, i gambled and took out D901 and now this line is relying only on D904.
                      I don't know how long it will last but so far so good xD

                      Thanks Bud

                        Comment

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