Vertical Output failure on Zenith Roundie

[Zenith26kc20]

Replaced the vertical output on my 24MC33 this weekend. It appeared not to be the transformer (badly stretched vertical, rolling, popping in sound in sync with dots on the screen) but it was... Transformer 95-1239. The new one was used but was perfect on turn on. Checked voltages and waveforms, all fine. No overheating, set was great. Did notice convergence was out so reconverged. Watched it for a few hours, no problem.
Saturday nite, watched the game, spectacular picture! 5 minutes after the game, same problem!
Winding "JK" shows 500 megohm leakage on my megger to the primary (the one with b+ on it). Removing the two wires to the winding makes the vertical normal again but convergenge is poor. Checking the voltage on the "JK" winding shows B+ and pulse present so it is leaking (shorted) to the primary.
I am sending the original transformer to a transformer manufacturer I have used in the past with excellent results. My question is, do the roundie Zeniths share a transformer chassis to chassis (the replacement had a note showing how to use it in another application)? If so, I could get more than a couple made (if they can reproduce it) with improved insulation to prevent this problem.
Maybe this problem can be cured!