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I think all you really needed to do to build a spot killer was to bypass to B+ the brightness line feeding the CRT grid 1. This in sets that fed the video into the CRT cathode. SO when the set was turned off, the B+ would collapse, and the bypassed grid would be pushed very negative, cutting off the cathode. and the cathode cools off by the time the time constant of the cap and resistances around it bleeds off charge.
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