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A quick easy way to isolate it is to disconnect the normal CRT heater path and run a loop or two of hookup wire around a convenient part of the FBT ferrite core, keeping it well away from the HV overwind, and connect this to the CRT filament. You can judge how many turns around the FBT by looking at the color of the CRT filament glow. If the fault doesn't return then you can safely assume it's an intermittent heater to cathode short on the blue gun. I've used this method many times to revive otherwise uneconomical set repairs. If the tube is faulty you've got nothing to lose. Sony Trinitrons were very prone to CRT H/C shorts. If the fault is still present then you've isolated the CRT.
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