Problems with Samsung 740N monitors

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  • Behemot
    Badcaps Legend
    • Dec 2009
    • 4845
    • CZ

    #481
    Re: Problems with Samsung 740N monitors

    Guess its 7.5 V either single with two pads connected to one end, or two in parallel. Should not matter anyway.
    Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

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    Comment

    • budm
      Badcaps Legend
      • Feb 2010
      • 40746
      • USA

      #482
      Re: Problems with Samsung 740N monitors

      Can we see the straight shot pictures of the top and the bottom side 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

      • jondoe
        Badcaps Veteran
        • Nov 2016
        • 547
        • UK

        #483
        Re: Problems with Samsung 740N monitors

        I've lifted a number of the large resistors to check them out of circuit, and most of the transistors/large mosfets, they all appear to be OK.

        Pictures attached

        Edit: I borrowed some cheap calipers, the chip is approx 2.9mm wide at it's longest point, so I suspect this is a SOT23 package.
        Attached Files
        Last edited by jondoe; 11-16-2016, 04:04 AM.

        Comment

        • jondoe
          Badcaps Veteran
          • Nov 2016
          • 547
          • UK

          #484
          Re: Problems with Samsung 740N monitors

          In the meantime I've ordered two IRF7319PBF and a BZX84C7V5LT1G. I'm fairly sure those are suitable replacement parts, I'll find out next week!

          Comment

          • budm
            Badcaps Legend
            • Feb 2010
            • 40746
            • USA

            #485
            Re: Problems with Samsung 740N monitors

            It looks like R217 is connected in parallel (need verification) with ZD201. Did you test the ZD201 off the board?
            The Cathode (PIN 3) of ZD201 is connected directly to the B+ for running the inverter circuit.
            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

            • jondoe
              Badcaps Veteran
              • Nov 2016
              • 547
              • UK

              #486
              Re: Problems with Samsung 740N monitors

              I didn't test it off the board, but I did compare readings with a working, identical board and the two were wildly different, I guess that's not a great way to test it. I'll remove it and test regardless at some point tomorrow.

              Thanks!

              Comment

              • budm
                Badcaps Legend
                • Feb 2010
                • 40746
                • USA

                #487
                Re: Problems with Samsung 740N monitors

                The reading error may be due to the bad 8-PIN SMD Power MOSFETs.
                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

                • jondoe
                  Badcaps Veteran
                  • Nov 2016
                  • 547
                  • UK

                  #488
                  Re: Problems with Samsung 740N monitors

                  A very valid point, that did cross my mind. I could always change the mosfets first before the zener and test

                  Comment

                  • jondoe
                    Badcaps Veteran
                    • Nov 2016
                    • 547
                    • UK

                    #489
                    Re: Problems with Samsung 740N monitors

                    Well, that was a little more difficult to remove than I had hoped, she was glued and didn't want play.

                    I've tested it off the board with my DMM

                    Code:
                      |
                    [ZD201]
                    |   |
                    0.7   OL
                    One leg doesn't appear to work at all, which according to the datasheet I think is correct (at least it is with the replacement ordered). So I assume this was working fine.

                    I'll replace it with the new one as the old one looks a little worse for being pulled out with my ham fists
                    Attached Files
                    Last edited by jondoe; 11-17-2016, 03:07 AM.

                    Comment

                    • Behemot
                      Badcaps Legend
                      • Dec 2009
                      • 4845
                      • CZ

                      #490
                      Re: Problems with Samsung 740N monitors

                      Yeah they usually glue them to hold in place during wave soldering.

                      Nice work with the magnifying glass BTW, I am surprised you managed to focus it
                      Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

                      Exclusive caps, meters and more!
                      Hardware Insights - power supply reviews and more!

                      Comment

                      • jondoe
                        Badcaps Veteran
                        • Nov 2016
                        • 547
                        • UK

                        #491
                        Re: Problems with Samsung 740N monitors

                        I used a dirt cheap plastic jewelers loupe and my camera phone for the picture, they come out surprisingly well

                        Comment

                        • jondoe
                          Badcaps Veteran
                          • Nov 2016
                          • 547
                          • UK

                          #492
                          Re: Problems with Samsung 740N monitors

                          That was a little harder than I imagined, but the MOSFETs are in place, the new zener and a fresh fuse. The heat sink on the back of the MOSFETs common ground makes soldering a pain in the butt, removal before hand might have been easier.

                          I destroyed one pad while removing one of them, but managed to bend the leg a little and scrape some trace so it could take a tiny bit of solder make the connection.

                          I won't post a picture of the work, I'm too embarrassed

                          It fired up first time, back lights working, etc, so I'm leaving it on the bench to see if it pops a fuse again.

                          Comment

                          • jondoe
                            Badcaps Veteran
                            • Nov 2016
                            • 547
                            • UK

                            #493
                            Re: Problems with Samsung 740N monitors

                            So far, so good. I left it turned on all day yesterday and last night and she's still running. Thanks to everyone who contributed in this thread, past and present

                            Comment

                            • Venomus28
                              Member
                              • Oct 2016
                              • 38
                              • Albania

                              #494
                              Re: Problems with Samsung 740N monitors

                              Did you changed the 2 mosfets too ?

                              Comment

                              • jondoe
                                Badcaps Veteran
                                • Nov 2016
                                • 547
                                • UK

                                #495
                                Re: Problems with Samsung 740N monitors

                                In all, I changed 3 of the large capacitors, 1 pico fuse, 2 x MOSFETs (replaced with IRF7319PBF) and a zener diode (BZX84C7V5LT1G) although I didn't need to change the Zener, I just destroyed when removing it to test.

                                It's still working fine today, i turned it off over night to cool then powered it back up this morning.
                                Last edited by jondoe; 11-24-2016, 02:58 AM.

                                Comment

                                • Venomus28
                                  Member
                                  • Oct 2016
                                  • 38
                                  • Albania

                                  #496
                                  Re: Problems with Samsung 740N monitors

                                  Was it hard to solder the new mosfets ?
                                  I have had 2 ,Samsung 940T and on the 1st one (im using now) i changed 3 caps and one pico fuse
                                  and at the other one i have changed 3 caps and the pico fuse and i am waiting for the mosfets to get delivered so i can change them too

                                  Comment

                                  • jondoe
                                    Badcaps Veteran
                                    • Nov 2016
                                    • 547
                                    • UK

                                    #497
                                    Re: Problems with Samsung 740N monitors

                                    How easy it is will depend on your own experience and the tools you have. Removing them was probably harder than replacing them, the glue holding them in place didn't want to let go. The heat sink also made it harder work than it needed to be, removal of that before hand might have made it easier.

                                    I used a little solder wick to clean the pads up before placing the new ones on the board, they aren't perfectly straight, but they are all working. I do have a cheap hot air station, it's quality is questionable so I managed without it!

                                    Comment

                                    • Venomus28
                                      Member
                                      • Oct 2016
                                      • 38
                                      • Albania

                                      #498
                                      Re: Problems with Samsung 740N monitors

                                      Yes i know, can you put up a pic ?

                                      Comment

                                      • jondoe
                                        Badcaps Veteran
                                        • Nov 2016
                                        • 547
                                        • UK

                                        #499
                                        Re: Problems with Samsung 740N monitors

                                        The monitor is reassembled now being tested, so I can't take a picture, sorry.

                                        Comment

                                        • Venomus28
                                          Member
                                          • Oct 2016
                                          • 38
                                          • Albania

                                          #500
                                          Re: Problems with Samsung 740N monitors

                                          After changing both mosfets the monitor turned on and worked right for about 10 mins then it the back light turned off again
                                          What could it be ?

                                          Comment

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