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  #1  
Old 09-11-2003, 02:43 PM
tremorviolet tremorviolet is offline
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green spot on TV screen

Last night, I noticed a green spot in the lower right corner of my television screen. It's only noticable when something red is in that area. I have stereo speakers adjacent to the TV and I know speakers have magnets and I vaguely remember that magnets can affect video screens so I'm guessing this is the problem.
My question is how do magnets cause this? I only have a vague idea how television screens work - isn't there something about photons being shot at the backside of the screen in three different wave lengths? Do the magnets only affect the red wave length? Do magnets affect the new flat screen TVs which I assume work in a different method?
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  #2  
Old 09-11-2003, 02:50 PM
Larry Mudd Larry Mudd is offline
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What's happened is the screen's shadow mask has become magnetized.

(The shadowmask is a perforated grid that seperates the red, blue, and green phospors.)

It should eventually degauss on its own, if your TV doesn't have a built-in degaussing feature.
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Old 09-11-2003, 03:07 PM
Larry Mudd Larry Mudd is offline
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This link will fill in the blanks: (In fact I see I'm spreading bad info above-- the screen doesn't separate red, green, and blue-- it separates groups of red, green, and blue phosphors. Sorry!)

How Colour TV Works
Quote:
When a color TV needs to create a red dot, it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously -- the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a TV screen are combinations of red, green and blue.
A color TV signal starts off looking just like a black-and-white signal. An extra chrominance signal is added by superimposing a 3.579545 MHz sine wave onto the standard black-and-white signal. Right after the horizontal sync pulse, eight cycles of a 3.579545 MHz sine wave are added as a color burst. Following these eight cycles, a phase shift in the chrominance signal indicates the color to display. The amplitude of the signal determines the saturation.
Depending on the polarity of the affected area's magnetic field, the electron stream is pulled off its intended target. Say the green channel is always the bottom pixel of each group of three, and the stream is being forced down, so that even if the colour guns are aiming for the red or blue, they hit the green.
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Old 09-11-2003, 03:17 PM
sturmhauke sturmhauke is offline
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I had a Zenith TV with a big green spot on one side and a big red spot on the other. It got worse over time, not better. Add in a blurry image, and I got a new Samsung, with component jacks for a very nice picture when watching DVDs.
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Old 09-11-2003, 03:21 PM
tremorviolet tremorviolet is offline
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Larry Mudd, cool, thank you for the link! That's exactly what I wanted to know. (wow, that site also has a great explanation of how a VCR tape works which I've always wondered about).
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  #6  
Old 09-11-2003, 03:22 PM
Joe Random Joe Random is offline
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Quote:
Originally posted by tremorviolet
I only have a vague idea how television screens work - isn't there something about photons being shot at the backside of the screen in three different wave lengths?
Not photons. Electrons. The electron beam excites the phosphors in the screen, and the phosphors glow in response.
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Old 09-11-2003, 04:16 PM
handy handy is offline
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All my tvs once had a green spot in the same location. Since it was on all of them, I figured it must be the cable tv companies doing.
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