Do you mean the red wire leading from the CRT to the plug that inserts into the analog board? Or do you mean the red wire from the video board that is directly soldered to the analog board?

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Generally speaking, any wire that conveys the red signal is a logical candidate for investigation. However, I'd not recommend playing around with wires that go straight into the crt (aside from a careful visual inspection, and with the power cord unplugged) unless and until you've ruled out problems in the lower-voltage parts of the signal chain.