Something that has happened a couple of times now, so take note, on color sets that use a shunt tube, some have a backup system that will hold HV somewhat in check in case of a failure to restrain HV due to shunt tube failure.

The horz out uses a neg bias from the blanker tube (think this 1st started with the CTC-15, so its in a lot of RCA clones as well). IF a resistor from the blanker tube grid (the source of the neg bias) to the ground opens it will allow the blanker grid voltage to go too negative, this in turn is coupled to the horz out grid. the result is a neg bias that is too large (more so than the normal grid rectified voltage from the horz osc) which will result in a HOT that is not operating at full power. So if you are fighting low HV and voltage checks so an overly negative grid on the HOT, look to that resistor on the grid to the ground of the horz blanker.

This same fault popped up on a slyvania D0-1 chassis..