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A weak red gun would be weak all the time. Look at the crt filaments while it is happening, make sure one isn't going out. Look in the matrix tubes for the same thing. If it passes that, check the DC voltages at the CRT (avoid the focus pin), see what is changing when it goes bad.
Work your way back through the screen circuitry (if the screen voltage is screwed up, be careful, ~600-900v here). Work back through the bias circuitry if the cathode voltage is off. If it's the grid (most likely) work back through the color demod section.
Anything bad enough to remove red should screw up the DC voltages horribly. Let us know what you find.
John
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