Portacolor got the wiggles

[AlanInSitges]

This is an HE chassis that's exhibiting behavior I have never seen before and I'm not even sure where to go about starting to look. Video

The lower-left corner of the screen is completely stable. As you move further to the right or up toward the top, it starts getting jittery, kinda, in that it's the height and width of the picture that vary, not the position. In the upper right-hand corner it's very pronounced.

So far I've determined that B+ voltages are correct and there's no variation of them. If it were only happening on either H or V I might at least start looking at voltages in those sections but since it's affecting both, and by the same amount, I think there is a risk of going down a rabbit hole and chasing the wrong things.

Another bit of information that's probably important: This is a US-market set, NTSC, designed for 120V 60Hz. I'm feeding it a standard NTSC-M signal from a Fluke PM5515 signal generator, and have tried with the output from a HLLY transmitter set for NTSC-M and being fed an NTSC signal from a DVD player, and the behavior is identical. So I don't think this is signal-related. The set is running on 120V, 50Hz AC. I've tried both a variac and a normal autoformer to step down my domestic 230V 650Hz current.

One thought I had was that there is some interaction between the 60Hz vertical scan frequency and the 50Hz power, but the jiggle is around 2-3Hz, so it wouldn't be an artefact of the two beating together or the difference, and it's affecting the horizontal too, so 🤷. I also haven't found any evidence of other 60Hz sets having problems like this on 50Hz current, looking around the various fora.

Can anyone shed some light on what's going on here?