Samsung 50 inch plasma has analog noise even with no input signal

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  • awilder1015
    Member
    • Sep 2014
    • 15
    • United States

    #1

    Samsung 50 inch plasma has analog noise even with no input signal

    This week I bought a Samsung Plasma TV, model number PN50B450B1D, for pretty cheap in a pawn shop. It was cheap because it had an issue where the image looked a little staticy all the time, even if there was no input source selected. The tag said that it didn't turn on immediately, and would shut off after 6 hours. This tells me the issue is thermal.

    My background is in electrical engineering, and since this digital TV had analog noise all the time, I figured it was a bad capacitor. I jumped at the opportunity to have an easily fixable big tv for cheap.

    I brought it home and took it apart, and to my chagrin, found no bulged caps. I put an o-scope on the power supply, and it looked clean, no noise. Both the x-main and y-main boards look fine too, however there's a buzzing coming from both x-main and y-main boards. The y-main board has buzzing near the transformer, so that might be normal, but the x-main has buzzing near a group of a few polypropylene film caps and a pair of TO-220 package transistors/diodes. The buzzing sounds like 60Hz, but I haven't put my scope on any components yet. As far as the possible thermal issue is concerned, all of the heatsinks on both x and y main boards were quite warm to the touch after the tv was only on for about 5 minutes.

    My next idea is to replace all the caps, diodes, and transistors in the area of buzzing and see if it improves, but I'm having trouble matching the printed part numbers to actual components on digikey. I found one of the diodes I need, but nothing else.

    Does anyone have any suggestions? Are there cap kits available for these boards? would that even fix the issue, or is it a bigger problem? Are there any specific tests I can run? As I mentioned, I already have a multimeter and o-scope (no capacitance meter, that's what I'd check next) I'd rather not buy new boards, since they're about $100 each. My boards' PBA numbers are LJ92-01688A and LJ92-01689A.

    Images of my boards are attached. The picture of the red boxes is the area of buzzing on the x-main board, the red boxes appear to be polypropylene caps, and the diodes/transistors mounted on the heatsink to their left might be culprits too

    Thank you all for your help
    Attached Files
  • mmartell
    Badcaps Legend
    • Oct 2013
    • 3189
    • Canada

    #2
    Re: Samsung 50 inch plasma has analog noise even with no input signal

    What does this noise look like ?

    What you can try is to find the jumper block on the control board (usually under the main board) and jumper pins 3&4. Unplug the connector from main <-> control board then plug it in. You should see test patterns. If the noise is gone you know it was coming from the main board.

    Report back.

    Comment

    • tom66
      EVs Rule
      • Apr 2011
      • 32560
      • UK

      #3
      Re: Samsung 50 inch plasma has analog noise even with no input signal

      X-main is a good possibility on these. Causes dim, underdiffused image.
      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

      • awilder1015
        Member
        • Sep 2014
        • 15
        • United States

        #4
        Re: Samsung 50 inch plasma has analog noise even with no input signal

        I took a picture of the "no signal" screen, to show you the analog noise I'm talking about, as well as a picture of the jumpered pins on the control board. The noise is very slight, the colors are all right, but they're just slightly fuzzy

        The test patterns went across the screen, but the noise was still there. I noticed that the tone of buzzing from the x-main board changed, it got quieter or louder and higher or lower depending on the color that was being displayed.

        I'm almost certain it's a bad transistor, but I think I'd rather replace the whole board.

        Does that sound about right?
        Thanks for your help everyone!
        Attached Files

        Comment

        • tom66
          EVs Rule
          • Apr 2011
          • 32560
          • UK

          #5
          Re: Samsung 50 inch plasma has analog noise even with no input signal

          Hmm, looks like dithering to me. Most plasmas do this. The Samsung panels tend to be obvious up close. Dithering is where pixels in a region will alternate between two nearby colours to render other colours. The low cost plasma panels can only render something like 100 levels per pixel. Dithering makes this close to 1,000 per pixel.

          Generally, if the dithering is not visible at viewing distance, it is not a fault.

          You would still need to find the problem with it shutting down, of course.
          Last edited by tom66; 09-21-2014, 01:53 AM.
          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

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