NotTaR of Television Sets : Occasional brightness flashes           
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Occasional brightness flashes

These may last only a fraction of a scan line or much much longer.

This could mean an intermittent fault in a variety of places including the video circuitry and SCREEN power supply:

If you get it from all sources, then tuner/IF is ruled out.

Suppose you just have no signal to a direct video input. What do you get? If you still get flashes, it should be real easy to monitor either the video outputs or SCREEN supply (with a HV divider on your scope) for noise. Then trace back to power or noise source.