[dsergeant]

Symptom of a worn tube. Most sets monitor the beam current during the turn on period and wait until it has reached a certain value before unblanking the video. This data is also used for black-level correction on the three guns.
As the tube ages and you have a weak gun the beam current on that gun will never reach the turn-on point. If you monitor the cathode voltages with a scope you will see the weak gun sat at a much lower voltage than the others - the correction has to set it at a lower level to get the same current.
Adjusting the screen voltage allows you to increase the levels up to a point where it will come on. Up to a point the black-level compensation will result in a reasonable colour balance until the weak gun can be compensated no more.

Only cure I am afraid is a new tube (or you can try rejuvenating it with mixed and unpredictable results.)

Dave