crt red gun

[DaveWM]

before assuming the CRT is the issue, you should prob check to make sure everthing else is ok. I would start by monitoring the voltages at the CRT pins on the red gun and see what happens when it goes away. I am assuming the filament is staying lit.

after that I would put a scope on the pins that control the signal, again watching to see what happens. Or just hook it up to a known working chassis if you have one.