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    LG L1953S doesn't power on

    Hello
    I have this LG monitor that doesn't power on.
    Optically everything seems fine and I have check all mosfets,diodes, capacitors but couldn't find anything wrong..
    Voltages on connector are absent with one strange expection:The moment I plug the board in I have 12volts but they are descending at a slow rate..
    I mean at the begging I have 12volts,ten seconds after I have 11.5, another ten after I have 11, and so on..
    I have found service manual but I had no luck with it..I'm starting to think it has a faulty PWM controller but not really sure..
    Attached Files

    #2
    Re: LG L1953S doesn't power on

    The power supply is always on even you do not turn on the monitor, so please check the the DC voltages without turning on the monitor, see if the voltages stay on, if you turn on uinsing the switch and then you see the voltages go down, then the power supply may have gone into shutdown mode due to over current draw due to short circuits.
    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


      #3
      Re: LG L1953S doesn't power on

      Check zener diode (located on bottom of board) zd301 for shorts. It looks a little discolored.

      Comment


        #4
        Re: LG L1953S doesn't power on

        Thank you for your answer..
        The voltages I'm talking about are without connecting anything on power supply(just like you see it on pictures )I have soldered a plug and taking measurements but the voltages on connector are missing..
        ZD301 is OK, it just has some kind of varnish on it
        Last edited by Joun; 04-11-2012, 11:43 AM.

        Comment


          #5
          Re: LG L1953S doesn't power on

          What about the 5V output? Does it do the same thing as the 12V output?

          Comment


            #6
            Re: LG L1953S doesn't power on

            No, I've got no 5V at all.

            Comment


              #7
              Re: LG L1953S doesn't power on

              Since 5v and 12v are generated by the same SMPS, it sounds like it turns on then it goes off. Do you hear any fainted ticking sound from the yellow transformer? And What DC voltage do you have on that large Main DC cap?
              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


                #8
                Re: LG L1953S doesn't power on

                The ceramic cap next to D107 seems burned to me.

                Check also the transformer below the main cap, next to C102.

                Comment


                  #9
                  Re: LG L1953S doesn't power on

                  Originally posted by budm View Post
                  Since 5v and 12v are generated by the same SMPS, it sounds like it turns on then it goes off. Do you hear any fainted ticking sound from the yellow transformer? And What DC voltage do you have on that large Main DC cap?
                  Maybe it turns on for half a second or something..
                  I don't hear any sound from the transformer.Main cap has its voltage OK about 330V DC(230 VAC in Greece )I have measure its ESR and capacitance, both are OK.
                  Well the thing is a have voltages everywhere on primary but nowhere on secondary if thats any help..That's why I'm thinking maybe the PWM controller is faulty( but my knowledge on SMPS is very limited so I'm possibly wrong).
                  Thank you all for your answers.

                  Comment


                    #10
                    Re: LG L1953S doesn't power on

                    Pics could have been a bit sharper - try and get them as near 2mb as possible ,should look llike this one:https://www.badcaps.net/forum/attach...1&d=1290283049
                    Did you replace the caps- what make/series are they - have any not been replaced,what make series are they?
                    Can you check on the bottom of the board the bottom pin on d201 looks cold? (gap in solder)
                    Near what looks like C02 there is a solder splash can you check it hasnt burnt through to the trace - clean it off.
                    C203 the leg looks like it is just sticking through the solder (cold?)
                    What is the triangular bit of soldering near D105?
                    I am only asking as I cant see enough in the pics

                    Can you test the optocoupler http://www.datasheetarchive.com/PC82...tml#datasheets
                    Last edited by selldoor; 04-11-2012, 01:47 PM.
                    Please upload pictures using attachment function when ask for help on the repair
                    http://www.badcaps.net/forum/showthread.php?t=39740

                    Comment


                      #11
                      Re: LG L1953S doesn't power on

                      Please try this first: heat up these two caps with hair dryer to see if the PS will stay on (you can monitor the 12v line), remove and check the values of these two caps or replace them, these two caps are part of the kick start and self powered circuits for the SMPS IC.
                      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


                        #12
                        Re: LG L1953S doesn't power on

                        Well the bigger cap is OK( I don't remember its value and I'm away from it now)
                        The smaller has nominal value 0.47 uF /50V.I check it with my ESR meter and it says 0 uF( which I quess is normal, since it can check caps above 1 uF) but it has ESR 20 Ohm!
                        I looked at an ESR table and a 1uF/50V cap must have an ESR of 4.5Ohm..
                        So my cap has gone bad? Or it has too low value and meter can't check it properly?

                        P.S My other cap meter this one says 390nF so it has normal capacitance but what about high ESR?
                        Last edited by Joun; 04-11-2012, 11:14 PM.

                        Comment


                          #13
                          Re: LG L1953S doesn't power on

                          Well, I would replace those caps as I redlined. If you heat up the bad caps, the ESR will go down. Did you heat up the two caps to see if the voltages come back?
                          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


                            #14
                            Re: LG L1953S doesn't power on

                            No,since I have my lab at home and didn't want to wake up everyone by using hair dryer..
                            I also don't have another .47uF capacitor.I guess its value is quite critical so I can use only an exact replacement ,right?

                            Comment


                              #15
                              Re: LG L1953S doesn't power on

                              You can use 1uF for testing. You can also use the soldering iron to quickly heat up the cap (out off the board) to see if the ESR goes 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


                                #16
                                Re: LG L1953S doesn't power on

                                OK, I will use hot air gun and post results..Thank you

                                Comment


                                  #17
                                  Re: LG L1953S doesn't power on

                                  Well I heat 0.47uF cap but nothing changed..The moment I plug it in I have up to 15volts at 12V point of connector and it is descending at a rate of 0.05V/sec(meaning in 10 seconds it has dropped about 0.5Volts)

                                  Comment


                                    #18
                                    Re: LG L1953S doesn't power on

                                    So you did replace the caps and still get the same problem?
                                    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


                                      #19
                                      Re: LG L1953S doesn't power on

                                      No,didn't have that value in my drawer..Hopefully a friend will bring it to me tommorow.
                                      Thank you for your interest..
                                      Last edited by Joun; 04-12-2012, 12:38 PM.

                                      Comment


                                        #20
                                        Re: LG L1953S doesn't power on

                                        I have read somewhere that the PWM controller ic needs Vcc of about 15-20 volts..Mine has 9.8volts.
                                        Does that indicates a problem or is it normal ?

                                        Comment

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