ViewSonic N2751W dead power board

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  • WHaThEFLuX
    Badcaps Veteran
    • Feb 2015
    • 277
    • USA
    #1

    ViewSonic N2751W dead power board

    Hello All, I have a ViewSonic Monitor with a dead power board and was wondering if anyone could tell me where to begin with this thing.

    I've diagnosed and repaired just about every other board on various TVs(all thanks to this amazing forum)but this power board is driving me crazy:



    The transformer on the right hums when plugged in, but other than that sign of life I can't seem to find any useful reading that might uncover where the issue lies. All I know for sure is there is absolutely no power getting to the main board whatsoever.

    I know my way around a meter, but please treat me like I'm naive. I've attached the service manual, it may or may not be useful. If anyone could point me in the right direction or explain how I should start to troubleshoot this board, I'd be forever grateful!
    Attached Files
    Last edited by WHaThEFLuX; 09-19-2016, 11:37 PM.
    "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

    -retiredcaps
    Tags: None
    • budm
      Badcaps Legend
      • Feb 2010
      • 40746
      • USA
      #2
      Re: ViewSonic N2751W dead power board

      That top right transformer is the Standby power supply, so that will be the section to get it working first.
      So did you check the rectifier diode on the cold side of that standby power supply, the MOSFET, and the SMPS IC yet?
      The bottom left the inductor for the PFC Voltage booster.
      The top left is the 12V/24V power supply.
      BTW, the schematic for the power supply does not have the designators or the value of the parts, and I am not sure if it matches your power supply board or not since this TV model uses many versions and manufacturers of the power supply.
      Last edited by budm; 09-19-2016, 11:46 PM.
      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

        • WHaThEFLuX
          Badcaps Veteran
          • Feb 2015
          • 277
          • USA
          #3
          Re: ViewSonic N2751W dead power board

          budm, thanks for replying so quickly!

          The diode and the FET are alive and well, not sure how to check SMPS IC or where it's located?

          Before I drive you crazy with a million questions, is there a way to test the connection on the top right for output? I only ask because I fear that the board may not be as dead as I originally thought, in other words I'm most likely testing it incorrectly!
          "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

          -retiredcaps

            Comment

            • WHaThEFLuX
              Badcaps Veteran
              • Feb 2015
              • 277
              • USA
              #4
              Re: ViewSonic N2751W dead power board

              bump
              Last edited by WHaThEFLuX; 09-20-2016, 07:37 AM.
              "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

              -retiredcaps

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA
                  #5
                  Re: ViewSonic N2751W dead power board

                  I need good straight shot pictures of the whole bottom side of the board that is clear enough for me to trace the copper traces on the board, I also need straight shot of the whole top board but a little closer because I have a hard time reading the designator of parts.
                  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

                    • WHaThEFLuX
                      Badcaps Veteran
                      • Feb 2015
                      • 277
                      • USA
                      #6
                      Re: ViewSonic N2751W dead power board

                      no prob, here you go:





                      "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

                      -retiredcaps

                        Comment

                        • budm
                          Badcaps Legend
                          • Feb 2010
                          • 40746
                          • USA
                          #7
                          Re: ViewSonic N2751W dead power board

                          Great, now I need the bottom side pictures.
                          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

                            • WHaThEFLuX
                              Badcaps Veteran
                              • Feb 2015
                              • 277
                              • USA
                              #8
                              Re: ViewSonic N2751W dead power board

                              My apologies, I guess I put the bottom side picture in as a link, should be fixed now.
                              "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

                              -retiredcaps

                                Comment

                                • WHaThEFLuX
                                  Badcaps Veteran
                                  • Feb 2015
                                  • 277
                                  • USA
                                  #9
                                  Re: ViewSonic N2751W dead power board

                                  bump
                                  "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

                                  -retiredcaps

                                    Comment

                                    • budm
                                      Badcaps Legend
                                      • Feb 2010
                                      • 40746
                                      • USA
                                      #10
                                      Re: ViewSonic N2751W dead power board

                                      Sorry, I have been VERY BUSY and do not have the time yet to trace out 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

                                        • WHaThEFLuX
                                          Badcaps Veteran
                                          • Feb 2015
                                          • 277
                                          • USA
                                          #11
                                          Re: ViewSonic N2751W dead power board

                                          No worries, just let me know when you get a chance to check it out!
                                          "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

                                          -retiredcaps

                                            Comment

                                            • budm
                                              Badcaps Legend
                                              • Feb 2010
                                              • 40746
                                              • USA
                                              #12
                                              Re: ViewSonic N2751W dead power board

                                              Check those rectifiers in that standby section.
                                              Check or replace those two startup/running caps and also the other 2 caps by the PFC section.
                                              BTW, check the output +5V STBY without having the board connected to other boards to see if +5V STBY is present and steady or not.
                                              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

                                                • WHaThEFLuX
                                                  Badcaps Veteran
                                                  • Feb 2015
                                                  • 277
                                                  • USA
                                                  #13
                                                  Re: ViewSonic N2751W dead power board

                                                  Bud, thank you so much for taking the time to trace the board!

                                                  Before I start checking everything I wanted you to see that two of the rectifiers' traces have actually lifted off the board on all 3 pins although nothing looks burnt. I'm really hoping they turn out to be shorted when I take them out of circuit, when I get a chance to test everything I'll let you know what I find.




                                                  "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

                                                  -retiredcaps

                                                    Comment

                                                    • budm
                                                      Badcaps Legend
                                                      • Feb 2010
                                                      • 40746
                                                      • USA
                                                      #14
                                                      Re: ViewSonic N2751W dead power board

                                                      Those two rectifiers are for the switched output supply (it will have output at the connector when the PS-ON is present).
                                                      So at this point do you have the +5V Standby output Voltage on the pin I marked as +5VSTBY?
                                                      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

                                                        • WHaThEFLuX
                                                          Badcaps Veteran
                                                          • Feb 2015
                                                          • 277
                                                          • USA
                                                          #15
                                                          Re: ViewSonic N2751W dead power board

                                                          Just checked that out, getting absolutely nothing at the +5V Standby......
                                                          "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

                                                          -retiredcaps

                                                            Comment

                                                            • budm
                                                              Badcaps Legend
                                                              • Feb 2010
                                                              • 40746
                                                              • USA
                                                              #16
                                                              Re: ViewSonic N2751W dead power board

                                                              OK, then we need to get the P/N of the STBY SMPS IC so we can the spec sheet to check and see what DCV we have on each pin and what each pin is for.
                                                              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

                                                                • WHaThEFLuX
                                                                  Badcaps Veteran
                                                                  • Feb 2015
                                                                  • 277
                                                                  • USA
                                                                  #17
                                                                  Re: ViewSonic N2751W dead power board

                                                                  The IC marking was extremely faded but I believe this is it:

                                                                  http://www.leadtrend.com.tw/archive/...52B-DS-01a.pdf
                                                                  "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

                                                                  -retiredcaps

                                                                    Comment

                                                                    • budm
                                                                      Badcaps Legend
                                                                      • Feb 2010
                                                                      • 40746
                                                                      • USA
                                                                      #18
                                                                      Re: ViewSonic N2751W dead power board

                                                                      If it is the correct IC (LD7552B), then you need to see if it has VCC Voltage at pin 7 with ref. to the GND pin 1.
                                                                      BTW, you may also try heating up those 4 small caps I marked in the picture with hair dryer then apply the AC to see if the +5VSTBY will be present.
                                                                      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

                                                                        • WHaThEFLuX
                                                                          Badcaps Veteran
                                                                          • Feb 2015
                                                                          • 277
                                                                          • USA
                                                                          #19
                                                                          Re: ViewSonic N2751W dead power board

                                                                          Will try the hair dryer now, GND to VCC gives me .327
                                                                          "You can only learn so much doing simple recapping and fixes. Ideally you want a really hard one to fix where you end up with a lot of dead ends and frustration. Then, and only then, do you learn how things really work."

                                                                          -retiredcaps

                                                                            Comment

                                                                            • budm
                                                                              Badcaps Legend
                                                                              • Feb 2010
                                                                              • 40746
                                                                              • USA
                                                                              #20
                                                                              Re: ViewSonic N2751W dead power board

                                                                              Originally posted by WHaThEFLuX
                                                                              Will try the hair dryer now, GND to VCC gives me .327
                                                                              Is that reading after the caps are heated up? If it is, then check the resistance between VCC and Gnd next. I hope that is the correct P/N of the IC.
                                                                              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

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