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Funai LC32EM1F - blowing fuses

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    Funai LC32EM1F - blowing fuses

    I retrieved a LC32EM1F from the dump and found that it draws current as I slowly bring up the AC using an isolated xformer. At about 40 VAC, the current rises to 2A. Power transformer has an audible buzz so started looking there after disconnecting the inverter board and the main board. Made sure that the FET attached to the primary transformer was good and then looked at each winding on the primary transformer. The primary side is good (~2 Ohms), but the secondary side looks like they are all shorted together. Other thing I noticed is that when I disconnect the main board, the current drops. The transformer part number is ltt2pc0xb043, but I could not find specifications for it. Anyone help with this one?
    Attached Files
    Last edited by ahickman; 08-16-2012, 02:16 PM.

    #2
    Re: Funai LC32EM1F - blowing fuses

    This type of power supply is a self-resonating design so its current limiting is done via the main fuse! Very bad design in my opinion, hard to get cheaper than this. I would remove the fuse and briefly put a 100W bulb across it, then see if it stays lit. Check for shorted surge protection too. The secondary of a switching transformer will always read shorted.
    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


      #3
      Re: Funai LC32EM1F - blowing fuses

      Dis connect the Red/Black wires to the inverter board. Discharge that big cap first and then check the resistance reading between the +/- cap terminal to see if it shows really low resistance, the resistance should be rising as the cap is charging up by the meter. All those diodes in the secondary side are good? This looks the saem as SANYO TV (same company owns SANYO, EMERSON) I have seen in the forum before.
      What resistance do you get measure between black and white power cable attached to the board?
      Last edited by budm; 08-16-2012, 03:27 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


        #4
        Re: Funai LC32EM1F - blowing fuses

        With the inverter wires disconnected, I got about 20 ohms across the large filtering cap after discharging. The resistance was steady, without increase. When I measure the resistance across the AC plug, I get a very high value (500k ohm).

        Comment


          #5
          Re: Funai LC32EM1F - blowing fuses

          Tha20 Ohms indicated you have short circuits. Are you sure that the MOSFET and the rectifiers are good? try removing the MOSFET, lift one leg of each rectifer one at a time, etc. Closed up pictures of the primary side of both sides of the 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


            #6
            Re: Funai LC32EM1F - blowing fuses

            Replaced the FET with (K3563) with 65E6600 just to make sure. Still high current. I have not soldered the secondar side of the transformer back down yet either. Unsoldered each of the diodes on the primary side as well the the 10 or so closest to the transformer. All appear to be good. I also checked the glass/switching diodes nearest the FET. One (D612) looks shorted when soldered in, but turned out good when desoldered. Pictures below.
            Attached Files

            Comment


              #7
              Re: Funai LC32EM1F - blowing fuses

              Back at it again. I disconnected the peripheral boards and then desoldered the secondary pins on the primary xformer. Current immediately rises as I turn up the AC on isolated xformer. The transformer buzzes pretty loudly and the heatsink for the FET gets hot instantly. Measured across all the zener diodes and though I had found a shorted one (D612), but was not the case. Unsoldered one side and still high current. Suggestions on what to check next?
              Attached Files

              Comment


                #8
                Re: Funai LC32EM1F - blowing fuses

                That means the MOSFET is not swithcung on/off at high frequency like it should, so it can be that the Gate is getting steady voltage to turn it on instead of the pulses. I would suspect the IC next. When MOSEFT goes out, it will usually damaged the IC Gate drive circuit inside the IC also.
                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


                  #9
                  Re: Funai LC32EM1F - blowing fuses

                  There is no IC in this design bud -- it's a self-oscillating converter using one NPN transistor and the MOSFET. It oscillates around the properties of the transformer. A rather ingenious design, I must admit, but a very bad one for several reasons. It tends to be unstable; it can go out of control and make the output voltage shoot too high. It's a pain in the *@#! to debug.

                  In general, I'd say the only way the MOSFET could fail by getting hot would be:
                  1.) it's shorted but high ohms short (a couple kohm would do it)
                  2.) it's got a drain-gate short

                  But the circuit has many other ways it can fail:
                  1.) if R5 goes too high in value (it dissipates about 1/4W) the MOSFET will not be efficiently switched and voltage will not rise high enough
                  2.) if Q1 goes open, the circuit will not oscillate (gate will remain at 18V, which wouldn't be good!)
                  3.) if R7 goes open or high in value (it dissipates about 1/2W) the circuit will not regulate the output voltage (it will go high)
                  4.) if R3 fails by reducing resistance (rare), current limit characteristics will change
                  5.) if C3 fails, output will go out of control (2-3x rated voltage)
                  6.) any bad caps on output will cause the circuit to be unstable
                  7.) snubber failure (as with any flyback)
                  8.) ... probably other ways it can fail too, just some I can think of

                  On these Funai TVs and similar TVs, they also use the self-oscillating circuit for generating the ~135W backlight supply, only slightly modified. I wish I was kidding.

                  Short circuiting the output of a self-oscillating converter will normally kill it, the MOSFET fails short.
                  Attached Files
                  Last edited by tom66; 10-25-2012, 01:12 PM.
                  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


                    #10
                    Re: Funai LC32EM1F - blowing fuses

                    Thanks TOM, I just look at the bottom side of the board and see two transistors and an Opto connections.
                    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


                      #11
                      Re: Funai LC32EM1F - blowing fuses

                      With the isolated xformer barely turned up (1.5 amps @ 5 VAC) I am getting 7 VDC at the base of the Q1 transistor (Q602). Does this indicate that the Q1 is shorted?

                      Comment


                        #12
                        Re: Funai LC32EM1F - blowing fuses

                        bump

                        Comment


                          #13
                          Re: Funai LC32EM1F - blowing fuses

                          Is Q602 the small NPN transistor?
                          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


                            #14
                            Re: Funai LC32EM1F - blowing fuses

                            Yes.

                            Comment


                              #15
                              Re: Funai LC32EM1F - blowing fuses

                              This is possibly a drain-gate short. Test the FET out of circuit.
                              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


                                #16
                                Re: Funai LC32EM1F - blowing fuses

                                Will do.

                                Comment


                                  #17
                                  Re: Funai LC32EM1F - blowing fuses

                                  Que tal yo tengo una sylvania que no tiene nada de oscilacion en el transformador fet bien diodos bien en fin pienso que esta daƱado el transformador

                                  Comment


                                    #18
                                    Re: Funai LC32EM1F - blowing fuses

                                    El modelo es lc320em1f

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