42LG50 no backlight

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  • tom66
    EVs Rule
    • Apr 2011
    • 32560
    • UK

    #81
    Re: 42LG50 no backlight

    Check the cables to the backlight are properly secured.
    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

    • Audioplus
      Senior Member
      • Feb 2012
      • 157
      • USA

      #82
      Re: 42LG50 no backlight

      Will do, thanks Tom, will let you know late tomorrow!
      Dan C - Audioplus

      Comment

      • dpal
        New Member
        • Aug 2012
        • 3
        • USA

        #83
        Re: 42LG50 no backlight

        Be sure to check the fuse that member rb0746 posted on the first page. I have a bad fuse on the master inverter as he described. I replaced the fuse and the TV works. Time will tell. All my voltages seemed correct but I had missed the fuse .75A/76V fuse F1 color black not white.
        Attached Files
        Last edited by dpal; 08-24-2012, 08:54 PM. Reason: Named the wrong board

        Comment

        • budm
          Badcaps Legend
          • Feb 2010
          • 40746
          • USA

          #84
          Re: 42LG50 no backlight

          Yep, a lot of people overlook at this type of fuse thinking it is surface mounted resistor. This fuse is used a lot in Chi-Mei (CMO) inverter 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

          • dpal
            New Member
            • Aug 2012
            • 3
            • USA

            #85
            Re: 42LG50 no backlight

            Having no schematics for inverter boards tends to flat-line my learning curve on LCD TVs. This forum helps so much to keep me enthusiastic while trouble shooting.

            Comment

            • budm
              Badcaps Legend
              • Feb 2010
              • 40746
              • USA

              #86
              Re: 42LG50 no backlight

              And if you want to see some pictures of what usually go wrong with TV's, Monitors, electronics, you can see them here for example:
              http://s807.photobucket.com/albums/yy352/budm/SAMSUNG/
              Main albums with sub albums on the right:
              http://s807.photobucket.com/albums/yy352/budm/
              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

              • Solipsist
                New Member
                • Nov 2012
                • 5
                • USA

                #87
                Re: 42LG50 no backlight

                I'm having the same issue as Torpex505. Plug the TV in, I get the red standby light. I press the power button and the blue light comes on, but no backlight on the screen. The flashlight test does show the menu on the screen, so it seems like just a backlight issue. Caps all look visibly ok. I also checked to make sure the 4 white fuses on the master invertor aren't open, but just noticed dpal's mention of the black fuse, which I'll test in the morning (near my bedtime here).
                Torpex, did you ever find a resolution?

                Comment

                • dpal
                  New Member
                  • Aug 2012
                  • 3
                  • USA

                  #88
                  Re: 42LG50 no backlight

                  The black fuse was the issue. I studied this TV for a couple of weeks off and on overlooking that fuse. The voltage measurements going in and out of the master inverter was not enough for me to diagnose the problem area. I did replace the inverter caps as well and ended up using a pico axial fuse soldered carefully. It is still working great.

                  Comment

                  • Dgtech
                    E. Technician
                    • Apr 2009
                    • 1462
                    • Steeler

                    #89
                    Re: 42LG50 no backlight

                    SOME SIMPLE BASIC ELECTRONICS ADVICE:

                    A fuse blows whenever it's max current rating is met by the circuit it is trying to protect. If a fuse blows in a circuit, it is because something in that circuit is drawing more current that it is supposed to from the power source. A lot of times this comes from a shorted component, or shorting component, that has a decreased resistance from it's original state causing a path for more current to flow to ground.

                    If you change this fuse out and find that the set works, the problem may still be there waiting for it's chance to take out another fuse. There are some instances where a fuse may just be weak. I usually see this in much, much higher voltage situations.
                    The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

                    Comment

                    • Dgtech
                      E. Technician
                      • Apr 2009
                      • 1462
                      • Steeler

                      #90
                      Re: 42LG50 no backlight

                      By the way, depending on what inverter board you have, there are some replacements on ebay that will save you a whole lot of time. You will find these by putting in the board numbers or the TV set model number into ebay search. I seen these boards range between $10 and $50. To be real with you - I only replace boards now for the most part. If I cannot find the bad component within 15 minutes, I replace the board. These boards are getting cheaper and cheaper.
                      The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

                      Comment

                      • tom66
                        EVs Rule
                        • Apr 2011
                        • 32560
                        • UK

                        #91
                        Re: 42LG50 no backlight

                        Dgtech, not necessarily true about a fuse.

                        Fuses are thermal devices. They usually consist of wire that breaks when a certain temperature is reached. Now, they do not blow at their rated current -- in fact, often it takes two or three times that current to fail within one second. Fuses are rated as being time delay (they take around 30 seconds to one hour to blow), or fast blow (they'll blow within a few seconds at the rated current), as well as other categories such as anti-surge or very fast blow. BUT, if your fuse blows at, say 1 amp in 1 hour, it can also blow at 0.5 amps in 4 hours! It's an inverse square rule. (Note -- The characteristic tends to be inaccurate at low currents -- a 1 amp fuse will essentially indefinitely withstand half an amp or so. But it gives you a good idea about how the fuse behaves.)

                        In many cases, these fuses simply fail because they have run at a continuous current for too long and thermal fatigue has caused them to fail.

                        It is very rare for components to fail partially leaky, such that the fuse would blow now and again, and if they did, it would usually result in a lot of heat very quickly. For example, say the device did draw 1.5A at 24V continuously, enough not to blow the fuse. It would dissipate 36W of heat. For comparison, a typical small IC on that inverter can safely dissipate about 1W of heat before failure. So, if a fault were to cause 36W of heat to be dissipated, it would likely result in flames.

                        I do sometime board swap, but for something like a fuse, I'll replace that first and not worry about it. I might increase the current rating slightly; there is no harm in doing this, as the fuse is simply designed to prevent short circuits, not leaky current draw.
                        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

                        • Solipsist
                          New Member
                          • Nov 2012
                          • 5
                          • USA

                          #92
                          Re: 42LG50 no backlight

                          Thanks for the info. I somehow missed that the black fuse was the OP's issue.

                          I can simply put the leads of an ohmmeter on each side of the fuse while there is no power to test if the fuse is open or not, correct? I've been a bit hesitant to work near the inverter boards with the TV energized.

                          Comment

                          • tom66
                            EVs Rule
                            • Apr 2011
                            • 32560
                            • UK

                            #93
                            Re: 42LG50 no backlight

                            You should test the resistance of the fuse when the TV is off.

                            A good fuse will read less than 2 ohms on most multimeters (the 2 ohms account for the resistance of test leads.)

                            A bad fuse will read "OL" or a single "1" on the 200 ohm range. It may read a few hundred kohm or a few meg ohm but it is still blown.
                            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

                            • Solipsist
                              New Member
                              • Nov 2012
                              • 5
                              • USA

                              #94
                              Re: 42LG50 no backlight

                              So, the 4 white ones measure between .3 to 2 ohms, so they are good. That one black one, when I put the leads across it, starts around 15 M ohms and slowly goes down until it settles around 9 M ohms. Is this what you were saying would happen if it's bad?

                              Comment

                              • selldoor
                                Slow Learner
                                • Dec 2010
                                • 7870

                                #95
                                Re: 42LG50 no backlight

                                Originally posted by Solipsist
                                So, the 4 white ones measure between .3 to 2 ohms, so they are good. That one black one, when I put the leads across it, starts around 15 M ohms and slowly goes down until it settles around 9 M ohms. Is this what you were saying would happen if it's bad?
                                Yes it is bad.
                                Please upload pictures using attachment function when ask for help on the repair
                                http://www.badcaps.net/forum/showthread.php?t=39740

                                Comment

                                • Solipsist
                                  New Member
                                  • Nov 2012
                                  • 5
                                  • USA

                                  #96
                                  Re: 42LG50 no backlight

                                  Thank you. You guys have been very helpful.
                                  The bad news is that I can't seem to source this board (1420H1-20D-Master) on ebay (or anywhere else through a simple google search). I'm only seeing the slave inverter board, not the master. I suppose I can try to source the fuse itself and perhaps get lucky that nothing else is shot.
                                  DPAL - do you recall where you found that same fuse?

                                  Comment

                                  • tom66
                                    EVs Rule
                                    • Apr 2011
                                    • 32560
                                    • UK

                                    #97
                                    Re: 42LG50 no backlight

                                    It is bad... but that is good . Replace the fuse. Only if the replacement blows should you look for a new board.
                                    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

                                    • Dgtech
                                      E. Technician
                                      • Apr 2009
                                      • 1462
                                      • Steeler

                                      #98
                                      Re: 42LG50 no backlight

                                      Originally posted by tom66
                                      It is bad... but that is good . Replace the fuse. Only if the replacement blows should you look for a new board.
                                      LOL, thanks for all of the technical aspects to engineering a fused circuit. I was trying to explain it in simple terms for the described situation to someone who may not have known. All that and we came to the same conclusion, replace the board if it blows again.




                                      The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

                                      Comment

                                      • Solipsist
                                        New Member
                                        • Nov 2012
                                        • 5
                                        • USA

                                        #99
                                        Re: 42LG50 no backlight

                                        Since there are no real local stores that carry electronics supply/parts anymore and I don't really want to wait a week or more to order something with the chance it won't work (and not have a TV in the interim), I ran out to Radio Shack to see what I could find. Picked up an in-line fuse holder and a standard fast-blow glass fuse rated 3/4 amp. Since the fuse holder wires were a bit beefy for the surface mount soldering, I also purchased some 22 awg wire to transition from the lands to the fuse holder. Put it all in place and did a quick test before taping the holder inside the cabinet. I now have a functional TV again! And just in time for Dexter and The Walking Dead tonight. I'm considering ordering a few SMT replacement fuses for a permanent fix. I see Mouser has some that seem to fit spec in stock.
                                        Thanks for everyone's input on this problem - it's much appreciated.

                                        Oh, and pics of the jury-rig:



                                        Last edited by Solipsist; 11-25-2012, 05:52 PM.

                                        Comment

                                        • tom66
                                          EVs Rule
                                          • Apr 2011
                                          • 32560
                                          • UK

                                          #100
                                          Re: 42LG50 no backlight

                                          Well, since you know the holder works... you can switch to using a proper fuse. Nice rig!
                                          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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