[old_tv_nut]

Phosphor burn would be dark.

I know nothing about this set and was hoping to learn, but since no definite answer yet, I'm willing to display my ignorance with some questions.

This is an electrostatic deflection tube, correct? And the centering is a pot, not a magnet on the neck? If so, could this effect be caused by one of the vertical deflection plates being disconnected and having a floating voltage?

When you have a picture, is there a fold or distortion at the bright region?

Can you link to a schematic?