[old_tv_nut]

I was thinking blue lateral after I posted. It seems a peculiar design to have it attached to the purity rings and not the convergence assembly, but it is what it is.

When you adjust H centering, how are you doing it? Is there a centering control, or are you trying to fiddle with the yoke? Some sets (like my old Magnavox roundy) had no centering control and you just had to take what you got. The H sweep was wide enough to avoid black edges, but you could lose up to 20% or so of the picture.

As yours is not filling the screen, the problem may involve the H output, H drive/efficiency/linearity adjustment, H driver, etc.