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LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

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    #1

    LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

    Make and Model: LG 60PB6650-UA
    Issue: Turns on, no backlight, can hear static (no channel)

    I've been repairing TVs for a couple years now. I know how to use a multimeter for the most part and know how to test most components for shorts.

    The power board connects to the ZSUS and then connects to the YSUS. The ZSUS is where I believe the failure is because the 16v line is only 3.8V, even when I disconnect the ZSUS from the YSUS.

    This particular model seems to have a common failure with its YSUS. My goal earlier was to prove the YSUS was bad so I could send it away for repair. Using CoppellTV's ebay auction repair kit as a guide*, I tested most of the components it lists in its kit (except for IC402, IC403, and IC100) and all were OK. For the 3 components mentioned, I contacted CoppellTV via ebay to get their advice about those components and they said that if Q401,Q402,Q404,Q405 and Q406 were good, IC402 and IC403 should be too. At this point I took a break.

    Today, I took some Zsus measurements and found that good VS, VA, and 5V power is getting to the board, but on the connection from the board to the YSUS, only 3.8v of 16v was leaving it.

    I've read the other posts on badcaps concerning this TV and recall someone else having this exact issue but no resolution (unless I missed something).

    I prefer not to randomly send boards off for repair. Any suggestions?

    * https://www.ebay.com/itm/REPAIR-KIT-...ss!26582!US!-1)
    Attached Files
    Last edited by JDC; 09-13-2018, 02:20 PM.
    Tags: None
    #2
    Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

    What voltage do you get on R232 (white ceramic 5watt) resistor on the Y-sus? this voltage comes from the same yellow transformer.
    I believe the 16 volts comes from that transformer, rectified by D203 (back of board) and goes through ic201

    I wonder if a bad buffer board might cause this.

    I see there is also 17 volts on the power supply board, but that feeds something else
    Last edited by R_J; 09-13-2018, 05:12 PM.

    Comment

      #3
      Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

      You need to identify if this is one of the LG sets that generates the 16V on the Y-sus, usually from the yellow transformer.
      Does the power board have a 16V output?
      Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
      For service manual, schematic, boardview (board view), datasheet, cad - use our search.

      Comment

        #4
        Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

        CORRECTION: Its 18V. I read it wrong.


        .02.32 V for the resistor I'm not sure I did that right as the meter added the 0.

        The power board does not have a 16v output.

        Comment

          #5
          Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

          That resistor is fed by the same transformer via diode D20? (next to D207) This circuit also supplies the VY voltage.
          Have you tried disconnecting the buffer boards from the Y-sus to see if the 16 volts comes up?

          BTW pin 1 & 2 of the power supply has +17 volts but goes to the main board, likely for Audio circuit.

          Comment

            #6
            Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

            Originally posted by R_J View Post
            That resistor is fed by the same transformer via diode D20? (next to D207) This circuit also supplies the VY voltage.
            Have you tried disconnecting the buffer boards from the Y-sus to see if the 16 volts comes up?

            BTW pin 1 & 2 of the power supply has +17 volts but goes to the main board, likely for Audio circuit.
            Ill try disconnecting the boards one at a time.

            Comment

              #7
              Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

              No change in voltage from removing just the lower, then both, and then just the upper. 3.9v remains.

              Something else I noticed: if there is supposed to be a lit-up LED on the center board (Main logic board?), it's not there.

              Comment

                #8
                Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                Its hard to say about the led, when you first turn on the tv does it come on then go off? or does it never come on?
                It could be that the control board (main logic) is not working. If it is not working the Y-sus will not be geting drive and that could be whats happening.

                I checked the other threads on this model and it seems like the led should be blinking. I see reports of C873 blowing on the upper buffer board. and C975 on the lower buffer
                Last edited by R_J; 09-13-2018, 06:30 PM.

                Comment

                  #9
                  Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                  No lighting up, nothing. Can hear static like it's not tuned to a channel. C873 and C975 aren't shorted so I'm stumped.

                  Comment

                    #10
                    Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                    Well search your model "60PB6650" and there are other threads, and from what I can tell the control board led should come on and flash on and off,

                    Comment

                      #11
                      Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                      Originally posted by R_J View Post
                      Well search your model "60PB6650" and there are other threads, and from what I can tell the control board led should come on and flash on and off,
                      It's not, though. Hmmm....

                      Comment

                        #12
                        Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                        Any other ideas, folks?

                        Comment

                          #13
                          Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                          Have you checked to see if you have any voltages on the control board? I see a couple regulator ic's IC264 (2.5v) and IC253 (can't see the number)
                          if you short the two pads labeled EXT_AUTO_GEN and plug in the tv does that change anything

                          Comment

                            #14
                            Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                            Done. No change.

                            Comment

                              #15
                              Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                              You have to unplug the ldvs cable from the mainboard also to test.

                              Comment

                                #16
                                Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                                You can try disconnecting the cable from the main and do the autogen test, sometimes you can leave it connected, I was'nt sure if the cable from the main also supplied the supply voltage for the ctrl board or it came from the ysus, A lot of the control boards have a seperate cable for the supply voltage.

                                Comment

                                  #17
                                  Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                                  Originally posted by tvtimmy View Post
                                  You have to unplug the ldvs cable from the mainboard also to test.
                                  I'm assuming you mean the flat ribbon cable running from the main board to the logic board. I unplugged each cable of the 5 cables from the logic board one at a time and other than the 3.9V going up to 3.99 eventually, nothing has changed. Still hear static.

                                  Comment

                                    #18
                                    Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                                    It is starting to look like it could be the control board, like I said earlier, other people reported the led would pulse when the tv was on, even if there was a fault elsewere

                                    Comment

                                      #19
                                      Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                                      Originally posted by R_J View Post
                                      It is starting to look like it could be the control board, like I said earlier, other people reported the led would pulse when the tv was on, even if there was a fault elsewere
                                      Thanks for your help so far. I'll keep bumping this thread in the coming days.

                                      Comment

                                        #20
                                        Re: LG 60PB6650 - Bad ZSUS (3.8v to YSUS instead of 16v)?

                                        Bump. Any help is appreciated.

                                        Comment

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