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Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

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    #1

    Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

    Hi everybody, I have a Magnavox 20MF605T/17 here that had an intermittent backlight.
    I opened it up and found all 3 caps (CapXon GL 300uf 25v) on the inverter board had popped. Had some Nichicon PM series 330uf 35v caps on hand so I went ahead and put em in.
    I put the board back in and I still have no backlight.
    So I started testing further and while testing resistance on the 4 (small transformers?) labeled F2-F5 from leg to leg they all gave 98 ohms except one of them that doesn't even complete a circuit.
    I'm guessing my next step is to replace that component but I'm not even sure what it is or how to find a replacement, can anyone help me out?
    Thanks,
    Jared
    P.S. retiredcaps: I haven't dropped my other threads I'm just doing alot at once I'll pick them back up soon
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    #2
    Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

    Originally posted by itek View Post
    So I started testing further and while testing resistance on the 4 (small transformers?) labeled F2-F5 from leg to leg they all gave 98 ohms except one of them that doesn't even complete a circuit.
    A "F" on the pcb suggets a fuse. A good fuse should measure less than 1.0 ohms. At 98 ohms, all those fuses are open (blown).

    BTW, I can't see F2-F5 in these pictures. I'm using a public Internet computer and limited by "guest" privileges and other restrictions.
    Last edited by retiredcaps; 04-28-2011, 08:13 PM.
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      #3
      Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

      I think itek meant the coils L2-L5? The only fuse I see is F1 (marked with a "W") to the right of the ribbon cable. Also, check the four mosfets U3, U4, U5 and U6 for shorts, since the caps went bad.
      Last edited by jetadm123; 04-28-2011, 08:35 PM.

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        #4
        Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

        Oh gosh you are so right i'm sorry I meant L2-L5!!!! OK so they are called coils, should I replace that one?, and the mosfets seem to test out fine, thanks guys!

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          #5
          Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

          Originally posted by itek View Post
          OK so they are called coils, should I replace that one?,
          I always associate L with an inductor. I believe inductors should be less than 1.0 ohms.

          If you look at the picture, there is R47 which reads 2001 which is a 200 ohm resistor. Since this resistor is very close to the inductors, it may be affecting your readings and that is why the inductors read 98 ohms.

          If you have one inductor that isn't measuring 98 ohms, you can try desoldering it and measuring it out of circuit.
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            #6
            Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

            OK so I desoldered the two legs of the inductor in question and it gives similar readings to the rest while floating, I solder it back down and no circuit.... again on my fluke- it gives 2.0k ohms on those 2001 resistors, if i set it manually to ohms i get no reading,, is this thing working correctly? thanks for hanging in there with me

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              #7
              Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

              Arrghh, I made a typo. 2001 is 2000 ohms (or 2k).

              If the inductor measures fine out of circuit, I wouldn't worry too much about the readings "in circuit".

              BTW, my experience with measuring inductors and getting 1.0 ohms is with the smaller standalone coil ones. These yellow square ones with multiple legs are new to me so 98 ohms or thereabouts could be the "normal" reading.

              PS. If the backlights are not coming on, it could also be the main board not signalling the backlights to come on and/or there is no voltage going to the inverter board.

              Edit: I think you should post pictures of the power and main board.
              Last edited by retiredcaps; 04-29-2011, 02:27 PM. Reason: edit: request for pics
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                #8
                Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

                OK i'll take pictures of all boards and test voltages tomorrow morning and get back, Thanks!

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                  #9
                  Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

                  OK so here are those long ago promised pictures, I think I have narrowed it down to a series of surface mount capacitors as seen in the picture. Each backlight lead has one, and all of those show infinite resistance and no capacitance, while nearby caps that look the same show ~350-400nf capacitance and ~280k resistance. Should I go ahead and replace those? and if so how do I figure out which ones to purchase, thanks again for all the help and patience! PS., the leads to the inverter board are reading like this:

                  pin1 16.5v
                  pin2 16.5v
                  pin3 16.5v
                  pin4 3.2v
                  pin5 0.7v
                  pin6 0v
                  pin7 0v
                  pin8 0v

                  edit:: as I was typing this i may have cross pin3 and pin4 for a slight second, so I turned the set off then on again and now the backlight comes on for a second or two then shuts off! now I have this:
                  pin1 16.5v
                  pin2 16.5v
                  pin3 16.5v
                  pin4 1.6v
                  pin5 0.4v
                  pin6 0v
                  pin7 0v
                  pin8 0v
                  I am getting more and more lost,, does this mean I was wrong about the surface mount caps?
                  Attached Files

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                    #10
                    Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

                    Originally posted by itek View Post
                    edit:: as I was typing this i may have cross pin3 and pin4 for a slight second, so I turned the set off then on again and now the backlight comes on for a second or two then shuts off!
                    2 seconds to black can be

                    a) bad electrolytic capacitors
                    b) bad inverter transformer
                    c) bad ccfl and/or wiring

                    All discussed in detail starting with posts #13 and 14 at

                    https://www.badcaps.net/forum/showthread.php?t=10419
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                      #11
                      Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

                      Thanks retiredcaps- I got the inverter going- only thing is now there's no image, only a blank grey screen! I can't win with this thing. I know there was image before the backlight worked!

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                        #12
                        Re: Philips Magnavox 20MF605T/17 20" LCD TV Inverter Board Issue

                        Originally posted by itek View Post
                        Thanks retiredcaps- I got the inverter going- only thing is now there's no image, only a blank grey screen! I can't win with this thing. I know there was image before the backlight worked!
                        Doublecheck the seating of the ribbon cable that runs from the left side of the main board to the LCD panel.

                        PlainBill
                        For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                        Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

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