[old_tv_nut]

1) I don't believe fixing overmodulation will fix focus. Apparently you have been using the word focus to describe something else, or it has to do with too much drive pulling down the high voltage, or something else I can't tell from your description.

2) Does this set have hairpin shaped wire gizmos on the CRT mount? These are edge purity adjustments - turn them this way or that until your blotch is compensated.