HISENSE F42K20E Blown Main Cap and Fuse

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  • 1beertap
    Badcaps Veteran
    • Mar 2009
    • 219
    • USA

    #1

    HISENSE F42K20E Blown Main Cap and Fuse

    Hey all,

    I found this tv at a local recycler and took it home. Upon trying it out, I noticed that there was no power at all. I took the panel apart and I checked the fuse, which was blown. Next, I then checked the rectifier to be good. I noticed there was a bulge in the main cap and tested it to be bad. I replaced the main cap and fuse. Now, I turned on the tv and yes the tv came on, but briefly. I tried it again and heard a "sizzle" type sound coming from the area the new main cap. Next, I took a voltage reading on the main cap and observed that the voltage, when on, was exceeding the 450vdc that the cap is rated. Also, I took a reading on the plus and minus legs of the rectifier and observed the same results. At one time I saw almost 800vdc on my meter!!! I really don't want to blow a cap. My question is what could cause the overvoltage?
    Attached Files
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: HISENSE F42K20E Blown Main Cap and Fuse

    "At one time I saw almost 800vdc on my meter!!!" Impossible! What did you use for the ground pointy for the meter? Did you verify the meter that it is OK? You will see big explosion if that cap is exposed to 800V.
    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

    • 1beertap
      Badcaps Veteran
      • Mar 2009
      • 219
      • USA

      #3
      Re: HISENSE F42K20E Blown Main Cap and Fuse

      budm, It is randomly going all over the place. My meter is okay. I saw 753vdc to be exact. The voltage seems to be hovering around 540vdc, but again it's not stable. I'm too afraid to leave it on for any length of time because of the risk of an exploding cap. When the tv is cool, the voltage on the cap seems good at around 380vdc and holds steady for a few seconds, but it doesn't take long after startup and the voltage increases and goes all over the place. I know that there is a part that is thermally failing.

      Comment

      • budm
        Badcaps Legend
        • Feb 2010
        • 40746
        • USA

        #4
        Re: HISENSE F42K20E Blown Main Cap and Fuse

        What voltage do you get if you have the power supply board by itself without having the cable connected to the main board? it should be around 160~170VDC.
        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

        • 1beertap
          Badcaps Veteran
          • Mar 2009
          • 219
          • USA

          #5
          Re: HISENSE F42K20E Blown Main Cap and Fuse

          I get 165vdc on main cap with the main board disconnected.

          Comment

          • ben7
            Capaholic
            • Jan 2011
            • 4059
            • USA

            #6
            Re: HISENSE F42K20E Blown Main Cap and Fuse

            It's not as impossible as you think!

            Try disconnecting the feedback sense on a poorly designed smps.... this is the result. You might even be able to get thousands of volts out, but I think that cap would blow quickly.

            In fact, I am surprised your new cap didn't vent. Did it get hot? Check to see if it is bulged, it might be damaged now.
            Muh-soggy-knee

            Comment

            • tom66
              EVs Rule
              • Apr 2011
              • 32560
              • UK

              #7
              Re: HISENSE F42K20E Blown Main Cap and Fuse

              I've had a PSU PFC circuit go crazy. Result was an explosive venting of the 450V capacitor. Voltage exceeded 650V for some time.
              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

              • Th3_uN1Qu3
                Believe in
                • Jul 2010
                • 6031
                • Romania

                #8
                Re: HISENSE F42K20E Blown Main Cap and Fuse

                Yes this does sound like a bad PFC circuit. I would try replacing the PFC controller IC. It could also be from the passive components in the feedback loop, but an IC is more likely to blow than a few resistors.
                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

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA

                  #9
                  Re: HISENSE F42K20E Blown Main Cap and Fuse

                  I have seen when PFC lost its feedback and the Voltage on the cap went up to around 600V but the cap blew up badly. I can see the cap goes up as high as 800V if it is operating from 230VAC power line.
                  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

                  • tom66
                    EVs Rule
                    • Apr 2011
                    • 32560
                    • UK

                    #10
                    Re: HISENSE F42K20E Blown Main Cap and Fuse

                    I know the voltage exceeded 650V on my set as there was a 650V CoolMOS in that area of the circuit which failed after the cap vented. Drain-gate short, drain-source ok, blew the arse-end out of the driver IC. Rare fault.

                    PFC circuits can pump huge amounts of energy into a circuit when they go bad - they are essentially wide-range boost converters...
                    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

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