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Samsung SyncMaster 740N powers on now, very dim screen

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    #1

    Samsung SyncMaster 740N powers on now, very dim screen

    My buddy gave me this monitor for free, it wouldn't turn on. I opened it to find that all 3 CapXon's on the secondary were bulged. The rest of the caps are Samxon KM series, so I left all of those. After replacing the bulged caps, it powers on now but the only way to see the picture is to shine a flashlight on it. The brightness and contrast are set to the highest. Did the lamp go bad in it?
    Attached Files
    Last edited by Pentium4; 10-08-2012, 02:30 PM.
    Tags: None
    #2
    Re: Samsung SyncMaster 740N powers on now, very dim screen

    Originally posted by Pentium4 View Post
    After replacing the bulged caps, it powers on now but the only way to see the picture is to shine a flashlight on it. The brightness and contrast are set to the highest. Did the lamp go bad in it?
    If you get no backlight, not even a quick flicker, you could have one of the following:

    1) open fuse in the inverter section of the board
    2) shorted transistor in the inverter section of the board
    3) main board not sending a backlight on signal (typically > 2V DC)

    A bad lamp would likely cause "2 second to black".
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      #3
      Re: Samsung SyncMaster 740N powers on now, very dim screen

      Originally posted by retiredcaps View Post
      If you get no backlight, not even a quick flicker, you could have one of the following:

      1) open fuse in the inverter section of the board
      2) shorted transistor in the inverter section of the board
      3) main board not sending a backlight on signal (typically > 2V DC)

      A bad lamp would likely cause "2 second to black".
      Do you think bad caps could have caused any of those symptoms or is it just cause they're shitty CapXon so they died anyways? haha

      Comment

        #4
        Re: Samsung SyncMaster 740N powers on now, very dim screen

        Originally posted by Pentium4 View Post
        Do you think bad caps could have caused any of those symptoms or is it just cause they're shitty CapXon so they died anyways? haha
        Possibly. Collateral damaged caused by bad caps is beyond my current level of understanding.
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          #5
          Re: Samsung SyncMaster 740N powers on now, very dim screen

          Originally posted by retiredcaps View Post
          Possibly. Collateral damaged caused by bad caps is beyond my current level of understanding.
          But not budm's. See this post as budm talk about bad caps and how other components suffer.

          https://www.badcaps.net/forum/showthread.php?t=23098
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            #6
            Re: Samsung SyncMaster 740N powers on now, very dim screen

            Okay I verified that the problem is somewhere on the power supply. Had another 740N that was only a month older and replaced each board one at a time. Once I had the power board swapped out, the screen lit up nice and bright. I will bring my multimeter to work tomorrow, what should be the first thing to check? I attached a picture below, I hope it is clear enough.
            Attached Files

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              #7
              Re: Samsung SyncMaster 740N powers on now, very dim screen

              Think these bad joints could be causing issues? The two adjacent ones are a 1000uF 10V cap and the third joint is one on an inductor

              On a side note, is that a mistake of printing on the board?
              Attached Files

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                #8
                Re: Samsung SyncMaster 740N powers on now, very dim screen

                Originally posted by Pentium4 View Post
                I will bring my multimeter to work tomorrow, what should be the first thing to check?
                Same as post #2.
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                  #9
                  Re: Samsung SyncMaster 740N powers on now, very dim screen

                  Originally posted by retiredcaps View Post
                  Same as post #2.
                  Sounds good Do those joints look problematic though?

                  Comment

                    #10
                    Re: Samsung SyncMaster 740N powers on now, very dim screen

                    Lets find out for sure if you are even getting the high voltage pulse from both transformer at the out put side.
                    Unplug all the lamps from the sockets on the board.
                    Solder a piece of solid wire to the base of the lamp, then just touch the pad of the high voltage transformer and turn on the monitor while holding the lamp in your hand (you have to hold the lamp otherwise the lamp will not light up since it needs your body stray capacitance to complete the circuit) you see the lamp lights up for seconds.
                    Attached Files
                    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: Samsung SyncMaster 740N powers on now, very dim screen

                      Originally posted by Pentium4 View Post
                      Sounds good Do those joints look problematic though?
                      Looks like there is left over flux? Other than that, I have seen way way worse joints.
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                        #12
                        Re: Samsung SyncMaster 740N powers on now, very dim screen

                        this works for me after replacing the 3 bad bloated caps and the open pico fuse, now its running smoothly as normal...

                        thanks

                        Comment

                          #13
                          Re: Samsung SyncMaster 740N powers on now, very dim screen

                          can some body tell me where should i check this 2v dc

                          main board not sending a backlight on signal (typically > 2V DC)

                          i have samsung 740n lcd and its screen lights up for a second or two then light goes off but screen is still on. i think its the inverter not getting backlight signal on once after turning the screen on


                          pls help its urgent
                          Attached Files

                          Comment

                            #14
                            Re: Samsung SyncMaster 740N powers on now, very dim screen

                            Originally posted by gautamlondon View Post
                            can some body tell me where should i check this 2v dc

                            main board not sending a backlight on signal (typically > 2V DC)

                            i have samsung 740n lcd and its screen lights up for a second or two then light goes off but screen is still on. i think its the inverter not getting backlight signal on once after turning the screen on


                            pls help its urgent
                            I had one like that and it was bad caps. Samsung liked to use poor caps.

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