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    #1

    42LG50 Won't Turn On

    SUMMARY
    ======
    An LG 42LG50 that won't turn on (though the standby light does turn on).

    WHAT I HAVE DONE
    ============
    I'm totally new to this so I'm sorta fumbling around as to where to go. I ordered a replacement power board, but wanted to see what else that I could do while I awaited it. In research, I found that often times a faulty main board is to blame, so I wanted to see if I could figure out if that were the case with mine.

    I went through the instructions located in this post and tested the voltage regulators on the main board. I have attached photos of this main board.

    QUESTIONS
    =======
    1. I'm hoping that someone can look over the results that I found, as well as the attached photos and datasheets, and let me know if I am correct in saying that IC704 is bad but that IC703 and IC707 are good.

    2. I had a hard time finding the correct datasheet for IC709 and was hoping someone might be able to assist me in determining if that is bad or good.

    3. Finally, I'm also wondering if I should have tested any other regulators that I did not test.

    VOLTAGE REGULATOR TEST RESULTS
    =======================

    PCB Designation: IC703
    Line 1 of IC: AZ1085S
    Line 2 of IC: 3.3E1
    Pin 1: 00.1
    Pin 2: 3.29
    Pin 3: 5.10

    PCB Designation: IC704
    Line 1 of IC: AZ1085S
    Line 2 of IC: ADJE1
    Pin 1: 1.384
    Pin 2: 2.62
    Pin 3: 5.11

    PCB Designation: IC707
    Line 1 of IC: AZ1085S
    Line 2 of IC: ADJE1
    Pin 1: 00.1
    Pin 2: 1.245
    Pin 3: 3.29

    PCB Designation: IC709
    Line 1 of IC: SC1565I5M
    Line 2 of IC: 0816
    Pin 1: 5.05
    Pin 2: 5.05
    Pin 3: 00.2
    Pin 4: 3.37
    Pin 5: 1.195
    Attached Files
    Last edited by craig34; 09-05-2012, 11:29 AM.
    Tags: None
    #2
    Re: LG 42LG50 Won't Turn On

    Originally posted by craig34 View Post
    QUESTIONS
    =======
    1. I'm hoping that someone can look over the results that I found, as well as the attached photos and datasheets, and let me know if I am correct in saying that IC704 is bad but that IC703 and IC707 are good.
    Thanks for doing some research and homework first. It helps us when you use my format to post things.

    For IC704, look at line 2. It says ADJ. That tells you it is an adjustable regulator.

    For an adjustable regulator, the formula is pin 1 + 1.25V = output of pin 2. 1.384 + 1.25 = 2.634 (close enough to 2.62 => I use a +/- 1% tolerance).

    2. I had a hard time finding the correct datasheet for IC709 and was hoping someone might be able to assist me in determining if that is bad or good.
    http://pdf1.alldatasheet.com/datashe...I5M-X.XTR.html

    Hint, your package is a TO-263-5 (page 4). Pin 5 should tell you if the output voltage on pin 4 is correct or not. In addition, I circled two resistors R116 and R117 (in yellow). Their values will help you determine with is the correct output voltage as the forumula presented on page 5 in the ADJ column.

    IC709 is left for you to check.

    3. Finally, I'm also wondering if I should have tested any other regulators that I did not test.
    Yes. Always. It is easy to check and only takes a few minutes. I count 11 on the picture (circled in red) with the possibly more on the backside of the board.
    Attached Files
    Last edited by retiredcaps; 09-05-2012, 11:52 PM.
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    Comment

      #3
      Re: 42LG50 Won't Turn On

      Thanks for your help and the clarification about the adjustable regulators, that makes sense.

      I'm going get the readings on the rest of the regulators this morning, but was wondering if you could tell me if the TV symptoms (won't turn on, but standby light will) are even consistent with a bad regulator in general?

      Comment

        #4
        Re: LG 42LG50 Won't Turn On

        Originally posted by retiredcaps View Post
        Hint, your package is a TO-263-5 (page 4). Pin 5 should tell you if the output voltage on pin 4 is correct or not. In addition, I circled two resistors R116 and R117 (in yellow). Their values will help you determine with is the correct output voltage as the forumula presented on page 5 in the ADJ column.
        I'm a quick study but I fear that my lack of (really any) knowledge in this area is catching up to me. I can see that the formula to determine the output voltage of the 5th pin is 1.200(R1 + R2) / R2, but I can't figure out how to determine the values of R1 and R2.

        I understand that those values correlate to the resistors that you circled (they are actually R716 & R717, not what you had written) but I'm not sure how to determine those values. I searched the datasheet that you provided as well as the internet in general but I'm having a hard time finding anything that makes sense to me.

        I'm also unsure as to how to determine which is R1 and which is R2, though based on the Typical Application Circuits chart - I suspect that R716 is R1 and R717 is R2.

        Comment

          #5
          Re: LG 42LG50 Won't Turn On

          Originally posted by craig34 View Post
          but I can't figure out how to determine the values of R1 and R2.
          There should be a code on each resistor telling you the value. For example, 202 means 2000 ohms.

          You should also measure each resistor with your multimeter to verify.

          http://www.learnabout-electronics.org/resistors_07.php

          I understand that those values correlate to the resistors that you circled (they are actually R716 & R717, not what you had written) but I'm not sure how to determine those values.
          On my monitor they looked like R116 and R117.

          I'm also unsure as to how to determine which is R1 and which is R2, though based on the Typical Application Circuits chart - I suspect that R716 is R1 and R717 is R2.
          Put your multimeter on continuity test mode and you can ohm or buzz it out. R2 will have one leg on the ground rail.
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          Comment

            #6
            Re: 42LG50 Won't Turn On

            Originally posted by craig34 View Post
            but was wondering if you could tell me if the TV symptoms (won't turn on, but standby light will) are even consistent with a bad regulator in general?
            Yes, a bad regulator could prevent the TV from powering on.

            When you plug the TV into the AC outlet, 5V SB is automatically on. That should mean the standby light is on. When you press the power button, the main board is responsible for turning that power LED to green. If any component on the main board is bad, then the power LED will stay in standby mode.

            Vizios seem to be plagued with bad voltage regulators and "won't turn on" is one of those symptoms. I know your TV is not a Vizio.
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            Comment

              #7
              Re: 42LG50 Won't Turn On

              Originally posted by retiredcaps View Post
              If any component on the main board is bad, then the power LED will stay in standby mode.
              WARNING: Likely obvious newbie question coming up!

              Is there a way to, at a high level, determine whether the main board is good by measuring something on the power board (which appears to be next in line)?

              Comment

                #8
                Re: 42LG50 Won't Turn On

                ...like the easy button from Staples?

                Comment

                  #9
                  Re: 42LG50 Won't Turn On

                  Originally posted by tibimakai View Post
                  ...like the easy button from Staples?
                  Haha, something like that. I was more thinking that maybe there was a way to disconnect something and test the power on the main board, etc. I'm a programmer and I'm approaching this like that - trying to be able to identify roughly where the problem is, so I can focus my testing.

                  Comment

                    #10
                    Re: 42LG50 Won't Turn On

                    Originally posted by craig34 View Post
                    Is there a way to, at a high level, determine whether the main board is good by measuring something on the power board (which appears to be next in line)?
                    Not really, the main board does a lot of things and it is hard to test them all in a standalone environment. The main board does send the commands PS_ON (power on) and BL_ON (backlight on) to the power board and you can check for its presence on the connector between the two.
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                      #11
                      Re: 42LG50 Won't Turn On

                      Originally posted by retiredcaps View Post
                      The main board does send the commands PS_ON (power on) and BL_ON (backlight on) to the power board and you can check for its presence on the connector between the two.
                      How would I do that? Do I need anything more than a multimeter?

                      If I were to test for those commands and find that they are being sent from the main board successfully - would that prove that the problem is with the power board?

                      Comment

                        #12
                        Re: 42LG50 Won't Turn On

                        Originally posted by craig34 View Post
                        How would I do that? Do I need anything more than a multimeter?
                        Using those pins go "hgh". That is from 0V to something > 3V when you press the on button. Some boards will have a legend right by the connector with PS_ON, BL_ON. I don't know if yours does or not since you haven't provided a picture of the power supply (hint).

                        If I were to test for those commands and find that they are being sent from the main board successfully - would that prove that the problem is with the power board?
                        Not really. The main board might be sending PS_ON to the power board and the power board is trying to start, but something on the main board is dragging it down. I experienced that first hand with a shorted ceramic capacitor on the main board. Details at

                        https://www.badcaps.net/forum/showthread.php?p=114994
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                        Comment

                          #13
                          Re: 42LG50 Won't Turn On

                          Originally posted by retiredcaps View Post
                          I don't know if yours does or not since you haven't provided a picture of the power supply (hint).
                          Here are photos of the power board.
                          Attached Files

                          Comment

                            #14
                            Re: 42LG50 Won't Turn On

                            The two connectors in the top left hand corner appear to have a legend, but are obscured by the cabling.

                            That is where you may find PS_ON, BL_ON, etc.
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                            Comment

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