those tubes are known to be schizz..
id start by the obious, wiggle conectors... if the red comes and goes while wigging the video innector check for cracked solder joint on the header pin. if no issue foud also swap the red video input wire with the green, just to make sure the red input is good....if the screen gets a green pic with the red input wire, boardset and wiring is good.....
from there try swapping transistors etc...worst case youll need somebody with a rejuve to test the tube for ya...