IBM RT PC Manual To Operations page 75

6. Turn the Mode 1 Vertical Size Control clockwise until the black areas at
the top and bottom of the screen disappear. If one of the black areas
disappears before the other, continue to turn the Mode 1 Vertical Size
Control until the second black area is gone.
7. Adjust the Brightness and Contrast Controls for eye comfort.
8. Repeat steps 1-7 using the Mode 2 Vertical Size Control.
3-10 mM RT PC Guide To Operations