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You might take a look at the focus voltage, if you haven't already. This should be approximately 5KV, found on a thick lead going to the CRT socket. That's enough voltage fart and flash, and the loss of this voltage can black out the raster. It is developed in the flyback section, and if I recall correctly you'll find a 1V2 focus rectifier tube involved in the circuitry.
Just a thought - Good Luck!
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