This is on the Setchell Carlson that I just restored. First powerup without the variac, and I get this.

It locks in comfortably slightly out of phase, when I tweak it a little, I can get the lower half of the image to stabilize but the top is torn as illustrated. What typically causes this?

I should note that I'm waiting for a couple caps to come in, so not every single one is fresh....

That is pretty extreme, but if I had to guess I would think its something to do with an anti hunt network used to filter out strong vert sync pulses. look to a RC network near the horz AFC tube grid.

I would also check to see if there is a setup procedure for setting the horz osc.

could also be an issue with decoupling the vert out pulse from the power supply

I would be scoping around the horz circuits looking for a vert rate pulse.

Fixed. Phew. Thanks for the pointer there. It was an easy one fortunately. Looked at the schematic and there was a hidden horizontal frequency control. We're good now :)