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LG 32LJ500V blown charred PSU board easy repair

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    #1

    LG 32LJ500V blown charred PSU board easy repair

    My grandpa has this TV, it turned off with a bang and flash one day. Opening it up revealed soot and burn marks on the heatsink of the primary mosfet, but no short circuit and the mains fuse intact.

    There is a 0.33 ohm resistor between the diode bridge and primary filter capacitors, that measured open. I replaced that with a ferrite bead and plugged the PSU in, it went bang again, but nothing blew open anymore. I then removed the mosfet and primary heatsink, cleaned up the soot and plugged it in again. Bang and flash. This time the source of the flash was easily identifiable - the snubber capacitor going between the drain and the source of the mosfet. The capacitor is 150pF 1kV and looks like an Y class cap, but isn't. There are no safety markings on it. Y caps should never fail short.

    I removed this capacitor, reinstalled the primary heatsink and mosfet and put the PSU board back into the TV. It powered up and works fine. Another fairly solid design let down by cheap parts...
    Last edited by Th3_uN1Qu3; 08-21-2021, 11:45 AM. Reason: Spelling
    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!
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    #2
    Re: LG 32LJ500V blown charred PSU board easy repair

    If you don't install the snubber cap it will likely blow the mosfet, Replace it with a 150pf 2kv cap. The 0.33Ω resistor acts like a fuse, replacing it with a piece of wire is not a good idea.

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      #3
      Re: LG 32LJ500V blown charred PSU board easy repair

      Board says R and FB above it, so either can be used. Having a fuse between the rectifier bridge and filter capacitors is not usual design practice. The mains fuse is there for a reason.

      The mosfet is rated 800 volts minimum and this increases to over 900 volts at the junction temperature it typically runs at. A single capacitor doesn't form a snubber - all it does is slow the mosfet down and reduce EMI somewhat. A proper snubber circuit is RC or RCD.

      Edit: Installing a 2kV cap would have the mosfet eating all of the spikes anyway... And there is a MOV on the AC line side. I have attached a PDF about snubber design, and with a single capacitor even a large value only reduces spike voltage by about 50 volts - basically nothing.
      Attached Files
      Last edited by Th3_uN1Qu3; 08-21-2021, 01:12 PM.
      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

        #4
        Re: LG 32LJ500V blown charred PSU board easy repair

        Looks like we'll have more of this PSU tomorrow and also some pictures. TV works fine but the power supply makes a constant unsettling whining noise... Feedback loop oscillating.

        Typical crapola Chinese design practice - "if it works, ship it".
        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

          #5
          Re: LG 32LJ500V blown charred PSU board easy repair

          hello, for this model, please nand software.

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