Horizontal Drive waveforms on my 1950 Zenith porthole

[Mike Stamper]

I'm confused as to why the high voltage fades out as I increase the horizontal drive setting. I took pictures of the waveforms at three different settings: minimum, halfway, and maximum rotation of the horizontal drive control. Here is the picture at minimum:

http://img02.picoodle.com/img/img02/...dm_600e144.jpg

Here it is at midpoint:

http://img01.picoodle.com/img/img01/...dm_ba3d48d.jpg

At at maximum:

http://img03.picoodle.com/img/img03/...dm_26c47b0.jpg

Notice that the sawtooth wave form becomes more and more distorted as the control is advanced. The voltage (at pin 5 of the 6BG6G where these pictures were taken) changes from 13VAC to 21VAC as the control is advanced to maximum. I don't think this is right. Can anyone tell me what SHOULD happen as the horizontal drive is advanced? Should the voltage go up? Should the wave form change? And finally, if this indicates a problem, where should I look? Thanks!