First, you determine if it's a WG 4600, 4900, 7000, etc.
Then you try cranking up both the red bias and red cutoff pots.
Then you check your red drive transistor on the neckboard for cold solder, possibly swapping it with the green drive transistor if it looks okay.
Then you check the connector from the PCB to the monitor to make sure the red wire didn't break loose or something.
Then you rebuild the entire monitor, checking the entire thing for cold solder joints.
Then you check the tube with a rejuvenator.
Then you test it with a different game.
Then you buy a new monitor....