NotTaR of Television Sets : Comments on bright spot at power-off    
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Comments on bright spot at power-off

With really old TVs, it was almost expected that when shut off, the raster would collapse to a spot which would then gradually fade away. If severe enough, the result after awhile would be a permanent dark spot in the center of the screen. Modern sets usually avoid this by forcing the CRT to be blanked for a few seconds after power-off while the high voltage decays or unblanked as the raster collapses to discharge the high voltage quickly with a white flash. However, claiming they all do this by design may be optimistic! :)

The following was in response to questions about a Sony TV but should apply in general.

(From: JURB6006 (jurb6006@aol.com).)

I think in this case the filter for the supply to the final video outs has gotten to a point where is discharges faster than the tube. Sony designs usually use fast blanking at power down, but it can only blank while the +220 VDC line is alive. There might also be a cap between the 220V line and the G1 biasing circuit. Also Sony's almost always incorporate some sort of HV bleeder but if it opens up usually there is a convergence problem. If it's not any of the above, the plot thickens. Look for a bad diode, or even an active spot killer circuit that has failed.

You might want to look into this, as I tell customers, when it collapses to a line it is 480 times as bright, it won't look 480 times as bright because the phosphor simply cant put out that much light. I've also made comments in here on how fast a CRT will burn if the yoke is unplugged. It's then the original 480X multiplied by 640. If that beam is in focus that means the "drive" to the phosphor is over 300,000 X what it should be. Even if the spot is defocused to 100X it's normal size, that is still 3,000X the current on that part of the screen. Even if yours doesn't burn the screen, in time you might expect a minor purity problem in that area on a fine pitch color CRT. This is an effect known as "doming", and it happened even on some low resolution NTSC TVs! It usually happened more at the sides, and if it became permanent you could fix it up if you used enough stick-on magnets, but you can't do that in the middle.

I bet the thing is a real pain to work on too, so I might consider just never shutting it off, or a simple modification. If G1 is grounded, it is easy. One resistor, a cap and a diode, ba-da-bing ba-da-boom. Two caps if you like to be fastidious. There are some designs in which this will not work. It does work 99% though, if the vidouts are driven they will discharge the +220 VDC supply, what I do is to AC couple the supply to the G1. It can be done with three component, but there are enhancements I recommend to protect the CRT from excessive K-G1 voltage. Don't want to cause a short there. Five or six components will do it on almost anything.

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