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Panasonic TH37PA50E 3yrs old In AV1 mode, disconnected from all video sources, scintillating blue horizontal bands slowly scrolling up the screen. Turns red when set to component video. When connected to antenna produces clear undistorted picture, but with horizontal scrolling bands discernable in background of dark scenes (not disturbing picture itself) but making it irritating to watch. Have disconnected the set from all other video devices and replaced it with another set which works fine. Thus source of picture problem is definitely internal to the TV set. Onset of the problem was sudden, (watching it at the time) and has been ever present since start.

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  • harry99 Apr 29, 2008

    Thanks very much for that.

    All the video inputs go via the video processing function on a Yamaha AV system first. The set is 5 metres from the AV amp so a long component cable is used to connect them just to the component sockets.

    Is there a video amplifier board that can be replaced?

  • harry99 Apr 29, 2008

    Yes. It didn't help.

  • harry99 Apr 30, 2008

    Bestdarngood,
    Sorry not to responded.

    Many thanks for your advice. I am inclined to think you are right about this. I have obtained a service manual, and although I am familiar with using a signal generator and scope; it is many years since I undertook this kind of work. Happy to pull a computer apart and fit boards etc but I think this is over my comfort limit.

    The set is obsolete in terms of the way the UK is now rapidly moving, and the cost of a repair is high in relation to aquiring a new one. I didn't pay anything like the retail price on the high street for it in the first place, so I think it is time to replace it.

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  • Master
  • 3,037 Answers

Alright, this problem is called hum or more correctly 60Hz-120Hz Hum it is caused by a partially open or an open filter capacitor in the power supply of the TV. The filter capacitor's job is to filter out the 60Hz - 120Hz ripple voltage that is present on the power supply's output.

When this cap fails the hum bleeds into the various stages and causes the lines that you are seeing on the video screen. To troubleshoot this problem you will need a schematic diagram (to locate the power supply test points and to get pictures of the correct waveforms that should be present), a signal generator (to generate a reference signal), and an Oscilloscope (to view the waveforms at the test points). What you will be looking at are the outputs of the power supply checking to see the level of the ripple voltage present on the outputs.

If you do not have the tools needed or any knowledge of working on electronics, then I would suggest getting a qualified repairman to fix this problem and the cheapest way is to take it in and not have them come out.

Let me know if you have any further questions on this problem

thanks

Posted on Apr 29, 2008

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  • Benjamin Patri
    Benjamin Patri Apr 29, 2008

    its not the amplifier

  • Benjamin Patri
    Benjamin Patri Apr 29, 2008

    have you taken a look at my advice at all?

  • Benjamin Patri
    Benjamin Patri Apr 30, 2008

    no problem, good luck with the replacing

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  • Master
  • 2,267 Answers

It looks like the TV might have retrace lines that are cause by a defective video amplifier IC, couple of fuse resistors burnt.
You might be able to make an adjustment to the blue screen voltage by reducing it slightly to get rid of the lines.
The devices that you connect to the TV have a different output and the TV is very sensitive to the higher video outputs from the DVD or any other device that you connect using the AV inputs.
Let me know if you need more assistance.

Posted on Apr 29, 2008

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  • Jaime Hernandez Apr 29, 2008

    Plasmas are a bit more difficult to repair due to complexity of their boards.

    At this point there might be the need for some adjustments to the voltages powering up all the assemblies on your TV.

    It requires the use of a good multimeter, a 200Mhz oscilloscope and the know how.

  • Jaime Hernandez Apr 29, 2008

    Have you tryied Switch on P-NR in Picture menu to reduce picture noise?

  • Jaime Hernandez Apr 29, 2008

    Try connecting the other video sources directly to the TV with shorter cables, just to eliminate some noise that could be picked up by the long cables.

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