Samsung 740B plus won´t start

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  • selldoor
    Slow Learner
    • Dec 2010
    • 7870

    #21
    Re: Samsung 740B plus won´t start

    I think its still most likely that c107 - when you said you changed it was that for a previously used one. It needs to be a good cap with low esr.
    Please upload pictures using attachment function when ask for help on the repair
    http://www.badcaps.net/forum/showthread.php?t=39740

    Comment

    • rogfanther
      Badcaps Veteran
      • Jul 2006
      • 458
      • Brazil

      #22
      Re: Samsung 740B plus won´t start

      Hmm, well, to heat the opto with the soldering iron, I was near the C107 also ...

      The original wasn´t that low ESR ( 0.45 ohms as per the datasheet ) so, as I didn´t find a low one around here, just used a general purpose one.

      I´m waiting for the board to cool and giving time for all the caps to discharge, just to repeat the experiment. Will try to put some isolation between the heat and C107, otherwise will try transplanting the C107 from the good psu to the bad one.

      Comment

      • selldoor
        Slow Learner
        • Dec 2010
        • 7870

        #23
        Re: Samsung 740B plus won´t start

        OK - you could also check its not a bad joint on that D103 looks a bit odd in the pic.
        Please upload pictures using attachment function when ask for help on the repair
        http://www.badcaps.net/forum/showthread.php?t=39740

        Comment

        • rogfanther
          Badcaps Veteran
          • Jul 2006
          • 458
          • Brazil

          #24
          Re: Samsung 740B plus won´t start

          Ok, redid joint. I think it is a little odd because I had already replaced that diode.
          Well : used the C107 from the "good" psu in the "bad" one, and it didn´t work . Tried also with another cap, from an ATX psu, but same thing.

          Also, replaced the optocoupler with one from the parts box, still no change. If I heat the diode side of the opto ( near the CN2 connector ) , voltage climbs up and the controller start working.

          Tomorrow I will see if the resistors around the TL431 measure ok, and maybe replace it.

          Comment

          • selldoor
            Slow Learner
            • Dec 2010
            • 7870

            #25
            Re: Samsung 740B plus won´t start

            Not much left to go at in that area - a bunch of caps that might work better when heated? How about heating further out and work in towards the opto perhaps start at the diodes above the caps.
            Please upload pictures using attachment function when ask for help on the repair
            http://www.badcaps.net/forum/showthread.php?t=39740

            Comment

            • Rtech
              Badcaps Legend
              • Jul 2010
              • 1095

              #26
              Re: Samsung 740B plus won´t start

              Disconnect D104,105.106 and 107 from the transformer side,and see what happens to the Vcc then.If the Vcc then works every time, you have isolated the problem.The next thing, without a diagram I am assuming that C107 is fed from the transformer via D103,you have proved the cap ok, but the diode could be 'leaky', easy to test by swapping it with the one from your good board.

              Comment

              • rogfanther
                Badcaps Veteran
                • Jul 2006
                • 458
                • Brazil

                #27
                Re: Samsung 740B plus won´t start

                @selldoor :
                Will try heating the diodes. As of now, for it to work, I´ve been heating with the solder iron directly over the opto.

                @rtech
                I believe that disconnecting the diodes would be the same as not connecting the logic board flat cable ? Without the logic board, the supply seems to come up ok, with Vcc a little low ( around 10.7V ) that I suppose is due to no load. But will try to disconnect the diodes tomorrow.
                I have already replaced the diode by a new one.( 1N4007, as it was ) .

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA

                  #28
                  Re: Samsung 740B plus won´t start

                  Did you swap the ref diode IC102 (TL431)? It is used to regulate the output voltage by turning on the LED inside the OPTO which causes the opto-transistor on the receiving side to conduct more which will clamp down the output voltage if the output voltage rise above the ref point, or conduct less if the output voltage drops down.
                  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

                  • rogfanther
                    Badcaps Veteran
                    • Jul 2006
                    • 458
                    • Brazil

                    #29
                    Re: Samsung 740B plus won´t start

                    Good Morning,

                    Not yet. That will be done today.

                    Comment

                    • rogfanther
                      Badcaps Veteran
                      • Jul 2006
                      • 458
                      • Brazil

                      #30
                      Re: Samsung 740B plus won´t start

                      Hi,
                      Aaaand, ..... it works.

                      The captain´s log :

                      - Revised all solder joints. Defect remains.
                      - substitute the zeners and pwm - defect remains
                      - substitute the (already replaced ) 1000uF x 10V caps, just because they were a different brand from those that worked in the other psu. Still nothing.
                      - before replacing the TL 431, observed that the psu would start after just touching the meter probe to the 431 ref output.
                      - tried reflowing the 431´s solder joints again. Same thing.
                      - Then, replaced the TL431. Finally, the thing works. Left it rest for some hours to coold and discharge the caps. Tested ok. Repeated some times. Still ok.

                      So, I consider it is fixed. Will order good new caps for them today, so I don´t need to open this thing again in the next months. Then test it correctly.


                      Thanks you all for the help.

                      Comment

                      • selldoor
                        Slow Learner
                        • Dec 2010
                        • 7870

                        #31
                        Re: Samsung 740B plus won´t start

                        Great!! not sure how/why heating helped but at least it meant you were able to find the fault.
                        Well done.
                        Please upload pictures using attachment function when ask for help on the repair
                        http://www.badcaps.net/forum/showthread.php?t=39740

                        Comment

                        • rogfanther
                          Badcaps Veteran
                          • Jul 2006
                          • 458
                          • Brazil

                          #32
                          Re: Samsung 740B plus won´t start

                          Thanks. Not let´s hope with the new, good caps it behaves itself.

                          Comment

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