[Tom9589]

Finally found the real gremlin. I had narrowed it down to the vertical output transformer. A member on the Antique Radio Forum came up with a way to simulate peak DC current loading on the vertical output transformer to see if it was the cause of the instability. Nope. It was rock steady.

I reconnected the transformer and still had the instability. I go back and tap and wiggle parts on the deflection board again and notice that wiggling the horizontal oscillator tube makes the instability worse. That doesn't make sense. I get my magnifiers out and start looking at every solder joint and PC run. Then I find it. Hidden under a component mounted on a nearby terminal strip is what looks like a break in a PC run. I didn't have continuity, but I suspect that there was some high resistance connection. I scraped away the solder mask and bridged the break. That cured the problem. The PC run connected the Boost B+ to the 390K vertical charging resistor.

Glad I didn't need a vertical output transformer after all.