Wiggle the CRT connector (carefully)!! A good 33% of the time the
problem is corroded contacts right there.

Tweak the blue screen and blue drive controls -- 18.7% of the time it's
a dirty control.

Carefully use a scope or voltmeter to look at the voltages between the
blue drive transistor emitter, base, and collector. Compare this with
the volatages on the red and green transistors. About 19% of the time
the problem is a bad transistor right there.