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    LG 42LN5400 no standby.

    My boss has asked me to look at this TV. There is no standby light and I can't find the standby voltage. I do measure 338v across the main cap.

    I do find a fluctuating 11-13v on the VCC of the sc1s311 on the rear of the board which looking at the data sheet it suggests the turn on voltage is 13.8v.

    Is this voltage being held low as there is a fault elsewhere (led, mainboard) or is it because there is a fault with the PSU ? I'd still expect to find a standby voltage though ?

    I've tested the diodes, caps and MOSFETs and find nothing out of order.

    Can someone give me a clue on where to check please ? Many thanks.
    Attached Files
    Last edited by diif; 06-05-2016, 06:28 AM.

    #2
    Re: LG 42LN5400 no standby.

    Here's a link to an example supply using that chip. I think VCC => 20V to start up (supplied via ST pin) and then it's maintained with the supply from the transformer as long as the voltage => 13.8V so it looks like you are not getting the supply needed to start the IC running. Check the resistors leading to the ST pin. Then you need to check all the components in the area linking to VCC. Also check the mosfet the chip is driving.

    http://www.daonna.com/sc1s311-%EB%8D...B%9C%ED%8A%B8/i

    Edit
    Check R102, R103, R114, ZD105
    Last edited by dick_barton; 06-05-2016, 06:23 AM.
    Willing to help but I'm no expert.

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      #3
      Re: LG 42LN5400 no standby.

      Thanks, here's the data sheet, i forgot to attach it.

      I pulled the FET, tested ok. 337v on the Drain.
      I shall pull the resistors and measure them.
      Attached Files

      Comment


        #4
        Re: LG 42LN5400 no standby.

        Originally posted by diif View Post
        I do find a fluctuating 11-13v on the VCC of the sc1s311 on the rear of the board which looking at the data sheet it suggests the turn on voltage is 13.8v.
        13.8 is the minimum and 15.1 is typical. You should getting around 15.1. If your VCC is fluctuating, check the electrolytic cap (startup cap). Chances are it might be bad.

        I've tested the diodes, caps and MOSFETs and find nothing out of order.
        How did you test the caps, in particular the startup cap? This startup cap is likely to be 22uF 50V or thereabouts.
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          #5
          Re: LG 42LN5400 no standby.

          Common issue is the main board on these sets. Have you tried jumping Pson and Led drive on to see if the voltages come up ?
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            #6
            Re: LG 42LN5400 no standby.

            Originally posted by ReeceyBurger123 View Post
            Common issue is the main board on these sets.
            If the main board is bad, then just disconnect the main board and see if standby voltage is present as it may be dragging down the voltage due to a short or something.
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              #7
              Re: LG 42LN5400 no standby.

              Originally posted by retiredcaps View Post
              If the main board is bad, then just disconnect the main board and see if standby voltage is present as it may be dragging down the voltage due to a short or something.
              If you see the connector this does not have a standby rail. The motherboard uses the 3.5v for the Ir and standby Led so if they are present and the others are the Psu is likely ok. Diif you need to bridge the Ps to ground and see if you can get the other rails to become stable.
              Last edited by ReeceyBurger123; 06-05-2016, 07:34 AM.
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                #8
                Re: LG 42LN5400 no standby.

                I can't make out the rails in the photo, but I will take your word for it since you work on this more than I have. I haven't repaired a TV in almost 1 year now. I haven't been looking that much anymore.
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                  #9
                  Re: LG 42LN5400 no standby.

                  Originally posted by retiredcaps View Post
                  How did you test the caps, in particular the startup cap? This startup cap is likely to be 22uF 50V or thereabouts.
                  With my ESR meter. I perhaps hadn't checked them all (done now)
                  I find a couple showing a higher capacitance than they should

                  c201 1500uf - 1756uf
                  c203 470 uf - 505uf
                  c208 100uf -101uf
                  c209 470uf - 477uf
                  c222 220uf - 242uf

                  They all have low ESR 0.03-0.06 ohms.

                  Originally posted by ReeceyBurger123 View Post
                  If you see the connector this does not have a standby rail. The motherboard uses the 3.5v for the Ir and standby Led so if they are present and the others are the Psu is likely ok. Diif you need to bridge the Ps to ground and see if you can get the other rails to become stable.
                  I understand the concept Reece, but am unsure where to get the Pson voltage from, where do i find it ?

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                    #10
                    Re: LG 42LN5400 no standby.

                    Wait there a sec, can you do you get the 3.5v 12v and 24v on the motherboard connector ?
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                      #11
                      Re: LG 42LN5400 no standby.

                      Originally posted by ReeceyBurger123 View Post
                      Wait there a sec, can you do you get the 3.5v 12v and 24v on the motherboard connector ?
                      No Reece, I get none of the voltages shown on the legend in Pic 2.

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                        #12
                        Re: LG 42LN5400 no standby.

                        Originally posted by diif View Post
                        c201 1500uf - 1756uf
                        c203 470 uf - 505uf
                        c208 100uf -101uf
                        c209 470uf - 477uf
                        c222 220uf - 242uf
                        If SC1S311 is your SMPS chip, it is designated IC101 on the backside. As such, its startup cap would have a C1xx designator (hot side). C2xx looks like it is all on the cold side.

                        Your photo is too blurry to see, but there is a cap just right of Q101 and below IC102. That might be the startup cap.

                        If you look at the SC1S311 datasheet, page 9 shows C2 connected to pin 7 (VCC) of the IC. You need to find the equivalent of C2. As I suggested, it might be a 22uF 50V cap or some small value < 47uF.
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                          #13
                          Re: LG 42LN5400 no standby.

                          Originally posted by retiredcaps View Post
                          If SC1S311 is your SMPS chip, it is designated IC101 on the backside. As such, its startup cap would have a C1xx designator (hot side). C2xx looks like it is all on the cold side.

                          Your photo is too blurry to see, but there is a cap just right of Q101 and below IC102. That might be the startup cap.

                          If you look at the SC1S311 datasheet, page 9 shows C2 connected to pin 7 (VCC) of the IC. You need to find the equivalent of C2. As I suggested, it might be a 22uF 50V cap or some small value < 47uF.
                          The cap on the right is C103. 100uf 50v, it measures 99uf 0.07ohms.

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                            #14
                            Re: LG 42LN5400 no standby.

                            Did you try any of dick_barton's suggestions?
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                              #15
                              Re: LG 42LN5400 no standby.

                              R102, R103, R114, ZD105 all measure ok.
                              I have just found R111 showing as open.

                              Maybe not, i replaced it anyway. Still the same, with a fluctuating voltage not getting over 13.5v.
                              Attached Files
                              Last edited by diif; 06-05-2016, 02:34 PM.

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                                #16
                                Re: LG 42LN5400 no standby.

                                Originally posted by diif View Post
                                I have just found R111 showing as open.
                                Without a schematic, I can't tell if R111 is crucial to the SMPS start or not.

                                103 = 10k ohm

                                From your picture it looks like pin 5 (DRV) of the SMPS IC goes to R108 and then to R111. You may want to check R108 as well.
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                                  #17
                                  Re: LG 42LN5400 no standby.

                                  Another possible reason for the SMPS IC not starting is that there is something shorted. Possible candidates are diodes and transistors.
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                                    #18
                                    Re: LG 42LN5400 no standby.

                                    I've tested all of the SMD resistors and the diode on the bottom of the board on the high voltage side.

                                    I think I've tested every component on the top apart from the blue caps. Might as well do those now before bed.

                                    Found it, the broken side was facing the heatsink.

                                    It's a 331K 1KV
                                    Attached Files
                                    Last edited by diif; 06-05-2016, 04:41 PM.

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                                      #19
                                      Re: LG 42LN5400 no standby.

                                      Ahh nice find Non of the fets are shorted then bud ?
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                                        #20
                                        Re: LG 42LN5400 no standby.

                                        Cheers Reece and thank you to retiredcaps and dick barton for your assistance.
                                        The fets check fine with my DCA55.
                                        I'll update once it arrives and I've fitted it.

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