VT420M zombification

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  • SirDaddySir
    New Member
    • Mar 2014
    • 5
    • United States

    #1

    VT420M zombification

    Trying to bring this thing back from the dead, and I am new to LCD repair but I have fixed some things before. I am hoping to attempt a component-level repair but I need to narrow down the issue a bit.

    This Vizio VT420M (ccfl) was a pawn shop as-is buy: "no sound, HDMI input only". (I don't care about the sound, but it never provided sound from either speaker or output jack. Getting it home, I was able to use all video inputs and even the digital tuning was fine. Score!)

    A month later, in the middle of a show, the picture froze and scrambled into the repeated artifacts I have now seen as typical of bad TCon. I restarted the tv and show, and about 30 seconds later the same thing happened. Tried again, and after the freeze, the backlights went and it was done trying.

    Symptoms: No backlight, no picture, and (probably) no sound.

    My findings are:
    In the past, someone had cut/badly patched one set of speaker wires (presumably to check out the sound problem), they may have been shorted somewhere inside their cocoon of elec. tape.
    Capacitors look fine. Everything visible looks fine.
    Voltage comes out of the PS in its various forms. Standby 5v is fine and logo lights up with power button. PS_on gets 3 volts or so. My 12v is only at 11.4v.
    Saw nothing on the flashlight test.
    BL_on (labeled O/F on this board) gets 0v. When jumpered to the 5v, the backlights are bright, responsive, and alluring.
    LVDS wires test to 0v or 1v, no real power seems to be heading to the TCon. TCon fuse has continuity but shows 0v.
    I only found 2 regulators on the main board, one was 5v-3.3v-0v and the other was a fixed ...09 that read 11.4v-0v-9v.

    I can provide more details if needed.

    So is it the weak 12v of the power board? Or the TCon disabling everything on a whim? Or the main board stepping on the necks of my dreams in unabashed mockery? And where do I splash the solder to make the pictures show up again?

    Thanks.
    Last edited by SirDaddySir; 03-29-2014, 04:57 PM. Reason: Omitted vital info
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: VT420M zombification

    Pictures of the main board?
    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

    • SirDaddySir
      New Member
      • Mar 2014
      • 5
      • United States

      #3
      Re: VT420M zombification

      Let's see if these will do. And one of the power board as well. I'm still waiting to hear there's an easy fix for all my troubles on that side.
      Attached Files

      Comment

      • budm
        Badcaps Legend
        • Feb 2010
        • 40746
        • USA

        #4
        Re: VT420M zombification

        Hmm, there are more than one version of your model, one has power supply-inverter combo, one does not. You have the combo one.
        The 12V is not tightly regulated power supply, it is used mainly for sound.
        http://www.shopjimmy.com/catalogsear...q=Vizio+VT420M

        It does sound like you do have main board problem since it is not sending out the BL_ON either, and it also not sending the signal to turn on the switched MOSFET (on the main board) to supply the B+ to the T-CON.
        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

        • SirDaddySir
          New Member
          • Mar 2014
          • 5
          • United States

          #5
          Re: VT420M zombification

          So is it failing to provide the BL_on because the B+ is missing? Vice versa?

          Which mosfet? Is there somewhere I can stick my probes?

          Oh, it may have no significance, but I also noticed that the two metal cages on the main board, if I leave the tv "on", the bottom one gets warm and the top one, by the LVDS connectors, stays cold.

          Comment

          • budm
            Badcaps Legend
            • Feb 2010
            • 40746
            • USA

            #6
            Re: VT420M zombification

            Without the diagram it will be real difficult to trace out the circuits.
            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

            • SirDaddySir
              New Member
              • Mar 2014
              • 5
              • United States

              #7
              Re: VT420M zombification

              After installing a used mainboard, the backlight and picture returned. Probably the sound, too.

              As for the original mainboard: I noted before that the top cage was staying cold on the original board. On the new one, whatever is in there warmed it up appropriately, so its on the same circuit as my mystery problem. I may go spelunking in there and see what's shaking once I'm done catching up on my show.

              Comment

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