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LG L246WH No Backlight

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    #1

    LG L246WH No Backlight

    Hello everyone I have a LG monitor that I picked up for free because it had no power. Opened it up did some testing and found that it had a blown MOSFET, diode, and fuse. I replaced all and it worked great. I took it on an RV trip for use as a tv (since it was HD). On the ride it must have got bumped because when I plug it in I get no backlight. The screen image is ok but just no backlight. At first if i would bump the back of the monitor it would come on until bumped again. Now I can't get any backlight (I'm assuming it's still the same problem. I have tested the voltages on the connectors on the power supply board and the connector to the logic board is fine but there is no power going to the backlight inverter board. I would go along testing components but this is odd because it worked when it was bumped. It is obviously the power board cuz thee is no power coming out. Any help getting pointed in the right direction would be appreciated.

    canadaboy25
    canadaboy25

    -Sometimes the light at the end of a tunnel is an on-coming train
    Tags: None
    #2
    Re: LG L246WH No Backlight

    Originally posted by canadaboy25 View Post
    Hello everyone I have a LG monitor that I picked up for free because it had no power. Opened it up did some testing and found that it had a blown MOSFET, diode, and fuse. I replaced all and it worked great. I took it on an RV trip for use as a tv (since it was HD). On the ride it must have got bumped because when I plug it in I get no backlight. The screen image is ok but just no backlight. At first if i would bump the back of the monitor it would come on until bumped again. Now I can't get any backlight (I'm assuming it's still the same problem. I have tested the voltages on the connectors on the power supply board and the connector to the logic board is fine but there is no power going to the backlight inverter board. I would go along testing components but this is odd because it worked when it was bumped. It is obviously the power board cuz thee is no power coming out. Any help getting pointed in the right direction would be appreciated.
    Look for cold/dry solder joints. Also possibly loose connection at the backlight connectors on both sides. As for the power issue, check the fuse and the rectifier diodes for the inverter section.
    Last edited by lexwalker; 07-12-2013, 02:22 AM.

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      #3
      Re: LG L246WH No Backlight

      check on/off, brightness and vin voltages of inverter circuit.

      Comment

        #4
        Re: LG L246WH No Backlight

        The inverter is a separate board with a fuse but its still fine. There are no diodes on the board so what am I to be looking for?

        canadaboy25
        canadaboy25

        -Sometimes the light at the end of a tunnel is an on-coming train

        Comment

          #5
          Re: LG L246WH No Backlight

          any body out their can help me with a ps 50c96hd samsung as their will not let me use threads for some reason

          Comment

            #6
            Re: LG L246WH No Backlight

            I just reflow soldered all the joints on the power supply board and still no luck I think it is a component.

            canadaboy25
            canadaboy25

            -Sometimes the light at the end of a tunnel is an on-coming train

            Comment

              #7
              Re: LG L246WH No Backlight

              Originally posted by gary19610 View Post
              any body out their can help me with a ps 50c96hd samsung as their will not let me use threads for some reason
              Put it in the correct Forum and you may get some help !!

              Comment

                #8
                Re: LG L246WH No Backlight

                Originally posted by gary19610 View Post
                any body out their can help me with a ps 50c96hd samsung as their will not let me use threads for some reason
                After about 5 posts they will let you.

                canadaboy25
                canadaboy25

                -Sometimes the light at the end of a tunnel is an on-coming train

                Comment

                  #9
                  Re: LG L246WH No Backlight

                  Originally posted by canadaboy25 View Post
                  The inverter is a separate board with a fuse but its still fine. There are no diodes on the board so what am I to be looking for?
                  The rectifiers/diodes located right at the SMPS transformer (at the power supply side), usually higher voltage for for inverter power supply. Check the rectifiers/diodes to see if they're faulty (either open or short).

                  Comment

                    #10
                    Re: LG L246WH No Backlight

                    Originally posted by lexwalker View Post
                    The rectifiers/diodes located right at the SMPS transformer (at the power supply side), usually higher voltage for for inverter power supply. Check the rectifiers/diodes to see if they're faulty (either open or short).
                    Ok so I have 3 of these transformers onthe power supply board so i dont know what to test. So basically you are telling me to test the diodes. When i was working on myfirst problem (no power) i already tested all transistors, MOSFET's, zener diodes, rectifier diodes, and capacitors. Is it very likely that another diode would have blown in that short of time (a month)? I am sill not quite sure how a blown coponent like a diode would make it only work when bumped. As stated in my first post it used to work when bumped and there was no power to the inverter so I know it is a power supply problem but what would cause it to only work when bumped other than a connector or solder. Please correct me if I am wrong but do components work like that too, work when bumped sometimes? This bump thing has really got me stumped.

                    canadaboy25
                    Last edited by canadaboy25; 07-14-2013, 12:54 AM. Reason: Typo
                    canadaboy25

                    -Sometimes the light at the end of a tunnel is an on-coming train

                    Comment

                      #11
                      Re: LG L246WH No Backlight

                      Originally posted by canadaboy25 View Post
                      Ok so I have 3 of these transformers onthe power supply board so i dont know what to test. So basically you are telling me to test the diodes. When i was working on myfirst problem (no power) i already tested all transistors, MOSFET's, zener diodes, rectifier diodes, and capacitors. Is it very likely that another diode would have blown in that short of time (a month)? I am sill not quite sure how a blown coponent like a diode would make it only work when bumped. As stated in my first post it used to work when bumped and there was no power to the inverter so I know it is a power supply problem but what would cause it to only work when bumped other than a connector or solder. Please correct me if I am wrong but do components work like that too, work when bumped sometimes? This bump thing has really got me stumped.
                      When you mentioned earlier that there was no power going to the backlight inverter board, the first suspicions would be the rectifiers/diodes at the SMPS transformer. You can connect the board to the mains to power it up, then check the voltages at the rectifiers/diodes on the secondary side. If your power supply board uses multiple SMPS transformers, its possible that the power to the inverter section is controlled by a transistor which is controlled by a standby circuit. This is similar to some of the LCD/LED TV boards I've repaired before (e.g. recently a Sony 42", one of the schottky diodes at the +3.3V standby side was found shorted thus the standby circuit could not operate normally). As for the bump problem, I've already mentioned earlier about looking out for dry/cold solder joints (ie. cracked solder joints also under that category).

                      Comment

                        #12
                        Re: LG L246WH No Backlight

                        Originally posted by lexwalker View Post
                        When you mentioned earlier that there was no power going to the backlight inverter board, the first suspicions would be the rectifiers/diodes at the SMPS transformer. You can connect the board to the mains to power it up, then check the voltages at the rectifiers/diodes on the secondary side. If your power supply board uses multiple SMPS transformers, its possible that the power to the inverter section is controlled by a transistor which is controlled by a standby circuit. This is similar to some of the LCD/LED TV boards I've repaired before (e.g. recently a Sony 42", one of the schottky diodes at the +3.3V standby side was found shorted thus the standby circuit could not operate normally). As for the bump problem, I've already mentioned earlier about looking out for dry/cold solder joints (ie. cracked solder joints also under that category).
                        Ok thanks for more info but how would i go along testing the voltages? Would I just put my multimeter on each side of the diode and how would I tell if it was bad on the multimeter screen?

                        canadaboy25
                        canadaboy25

                        -Sometimes the light at the end of a tunnel is an on-coming train

                        Comment

                          #13
                          Re: LG L246WH No Backlight

                          Anyone?
                          canadaboy25

                          -Sometimes the light at the end of a tunnel is an on-coming train

                          Comment

                            #14
                            Re: LG L246WH No Backlight

                            Nobody knows?
                            canadaboy25

                            -Sometimes the light at the end of a tunnel is an on-coming train

                            Comment

                              #15
                              Re: LG L246WH No Backlight

                              on psu/main there will be pin outs printed in white where cables come an go on boards! test those points for what is listed! you will be looking for psu/on , 3,5,12 ect! you should download service manual, will have trouble shooting section.

                              Comment

                                #16
                                Re: LG L246WH No Backlight

                                Originally posted by t.j. View Post
                                on psu/main there will be pin outs printed in white where cables come an go on boards! test those points for what is listed! you will be looking for psu/on , 3,5,12 ect! you should download service manual, will have trouble shooting section.
                                I already tested and there is power going to the main board but no power going to the inverter board.

                                canadaboy25
                                canadaboy25

                                -Sometimes the light at the end of a tunnel is an on-coming train

                                Comment

                                  #17
                                  Re: LG L246WH No Backlight

                                  Can I get to see good clear pictures of the board, both side as whole, and as in small sections?
                                  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

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