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magnets and tvs

Discussion in 'TVs' started by Chris_D, Oct 8, 2003.

  1. Chris_D

    Chris_D
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    Hiya,

    Could anyone tell me why moving magnets (unshielded speakers) near a TV leaves a coloured smudge on it and how to get rid of them once their on there? I had a similar problem a while ago but resolved it by moving some older speakers out the room. I now have a Mission centre speaker which is says its sheilded but having got back from holiday theres coloured marks in the top corners of the tv. Anyone have any advice on what to do? TV is a Philips 32PW9308 if its any help.

    Thanks,
    Chris
     
  2. LV426

    LV426
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    On a regular CRT TV there are three 'guns' at the back of the tube that fire a stream of electrons at the front. When this stream lands on the front, a phosphor coating reacts by glowing. This is what illuminates the tube.

    On a colour CRT, there are stripes or a grid of phosphors that each glow with one of the primary colours, red, green and blue.

    The stream of electrons is 'steered' to cover the full front of the tube by an array of powerful electromagnets around the neck of the tube.

    The arrangement of the phosphor grid (in three colours) and the arrangement of the guns is such that, working properly, only the green electron stream hits the green phosphor, ditto for red and blue.

    This steering is further controlled by a shadow mask (a metal grid) just behind the front of the tube. Imagine your eye is positioned exactly where, say, the green electron gun is. Looking through the shadow mask, you can only see the green phosphors. The rest are hidden by the mask. Move your eye to where the blue gun is, and you can only see the blue phosphors. Ditto for red.

    Now, if an external magnetic field exists near the tube, OR if the shadow mask itself has become magnetised, the beams of electrons can get diverted from their path after they have passed through the shadow mask, and end up taking a bent path which makes them strike the wrong phosphor, causing it to illuminate.

    This causes the colour staining effect.

    It can only be cured by

    1: ensuring there are no external magnetic fields
    2: ensuring the shadow mask is not magnetised. All colour CRTs have a built in demagnetiser (degausser) which works automatically when the TV is switched on from cold.

    Finally, if the effect is permanent, it can be cured by a well-trained engineer adding small permanent magnets to the neck of the tube, so as to re-steer the electron beams to their correct phosphors. If needed, this technique is best done in situ as it will account for any local magnetic fields, such as that of the earth, and/or any magnetised objects in your room.
     

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