[old_tv_nut]

Another way to check if the purity trims are not quite right is to rotate the set and cold/hot cycle it to make sure the degaussing coil operates fully. In other words, if the set is facing north/south now, try operating it when facing east/west. If the upper left changes somewhat, it's another indication the purity trims are off a bit, especially if rotating it back brings it back to this same condition.

If the mask is loose, I'd think jostling the set by moving it would produce much worse results.