Because the adjustment potentiometers on the main logic board of the Apple
High-Res RGB monitor can accidentally be bumped during installation, the
monitor screen's vertical convergence can sometimes be off by as much as 1/8".
Adjusting the V-twist control will allow only a small section of the screen to
show the correct vertical adjustment, while the top and bottom borders remain
ill-adjusted. While the center of the screen may appear to be adjusted
correctly, there will be distinct red-, green- and blue-shadowed images at the
top and bottom of the screen. These shadowed images will appear in different
sizes (i.e. the green image is larger than the red image, and the red image is
larger than the blue image.)
The top and bottom sections of the screen can be adjusted correctly by
adjusting the V-TOP and V-BOTTOM controls on the main logic board.