I gave the Zenith a very through cleaning using compressed air and brake-kleen (no residue, quick drying). Got rid of a lot of dust & dirt.
I also checked the HV section/cage and all of the power supply areas/tube sockets for any signs of arcing. Couldn't find anything wrong.
I powered it back up again, and it thanked me with the problem you see below.
Basically, the picture is very jittery at the bottom and only fills the top 2/3's of the screen.
I changed every tube in the vertical section (vert output tube is known good, borrowed from my roundie). Then I swapped everything in the Horz section. Finally I tried the Damper and Voltage Reg.
It strikes me that this is might not be a tube problem. Does anyone here have an idea?