Finally I have one Zenith!

[DavGoodlin]

Same 12A10C50 chassis...wow. Then its a heavy beast too. I had a Zenith 1968 15" once, a true luggable with a metal cabinet. My Aunt had a 1970 15" Sylvania color as a bedroom set, of course not as heavy. I don't think I ever had to work on it... I saw a few of those RCA CTC22 15" sets and a Mag or two in the same category. Yours beats them all for true quality.

Thought I saw a transformer in your picture, Zenith did make a few "portable" tube sets without transformers 14CC15 IIRC. A tube Zenith that size is just too cool! For the jitters, the Hor osc tube 6U10?, or I have also seen HV arcing cause this.
My 16" set had an arcing puncture through the HV rectifier tube cup, so I placed it in the dishwasher to clean up, RTV silicone the hole and then put a 1/4" rubber disc under it.

The problem also seems like a bad electrolytic. Sometimes that drops the voltage. The B+ voltage drops as the HV comes up only because that stage draws significant current, making the lack of filtering even worse.

The issue could also be a bad ground on one of the terminal strips. I use a chopstick and poke at the end terminals, ones grounded to the chassis. I have seen this but not often compared to the number of Zeniths I worked on.