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Jumping into this discussion a little late, I think it important to consider the variability of HO Tube characteristics which may lead to a drive line. I have jotted down my practical thoughts to see what others think because of the frequency this topic is discussed.
Recall that in the flyback sweep the left hand portion of the sweep after retrace is from the energy recovered from the damper. At initiation of retrace the HO Tube is sharply cut off and the energy recovered from the resonant negative voltage swing powers the first half of the horizontal sweep. When the recovered energy is dispelled, the HO Tube begins to conduct linearly until the tube is cut off at retrace and the cycle begins again.
The white line would be the result of the transition when the HO Tube conducts. If the HO Tube does not begin to conduct before the recovered energy is fully dispelled, the sweep will be momentarily halted resulting in a vertical drive line. Increasing the horizontal drive beyond a certain point will cause the HO Tube to remain cut off longer hence the conduction will not pickup in time resulting in the line.
Changing the HO Tube for one with a characteristic with slightly less transconductance I believe will alleviate the drive line. Reducing slightly the horizontal drive by perhaps replacing the horizontal oscillator tube or else reducing the horizontal oscillator output by some means (resistive divider or shunt capacitance) would do the same thing. Later sets did not have a horizontal drive control to manage the oscillator level: I suppose this was to save cost.
These are just some of my random thoughts. I am glad the problem was resolved by simply swapping a tube.
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