IBM RT PC 10 Hardware Maintenance And Service page 311

Adjustments
Vertical Size 1
11-12 Service Manual
The Adjustments are on the rear panel of the display. Use the following
descriptions to set the controls.
1.
Set the power switch on the system unit to Off.
2. Set the power switch on the display to On.
3. Turn the Brightness control fully clockwise.
4. Adjust the Vertical Size 1 control counterclockwise until a black area
appears at the top and bottom of the screen.. Then turn the control
clockwise until the black areas disappear.
5. Set the Brightness control to the desired setting.
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Vertical
Size 1