ALS'/DLF' : your daily dose of Arcade Life Style...inside the Dragon's Lair!
Did you try degaussing the metal frame (if any) around the monitor (from the back, can e done while off).If not try that and if it doesnt work you have a problem.
Then its either a bad purity adjustment (which requires moving the yoke) or the tube is dead.....
as long as it doesn't destroy the monitor i can totally live with it (because luckily it's only hte upper right corner). the picture is also slightly distorted in that section, so can it be a cap problem?
i am already looking forward to this pic :
take off that silly board-game box....it completely ruins your brilliant work...doesn't belong there....(Just MHO of course, as always brought in the nicest and must subtle way I can..... )