Zenith porthole poor width question....

[DavGoodlin]

I see "jailbars" toward the left side which mean the flyback, as a tuned tank circuit, may be out of tune and ringing in a non-linear mode.

What is the chassis or model number? the TelAid might have something on this.
It could be as simple as a bypass cap off the boost supply, or even a cap in the yoke, if used.