This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Color monitors do vertical and horzontal centering differently than do monochrome monitors. Almost all color monitors have either a jumper arrangement or actual centering controls, sometimes both. Centering adjustments are usually located on the PCB with no access holes, so the back will most likely have to be removed to get to them. If your monitor uses jumpers, there should be 3 tabs that each jumper can be connected to. For vertical adjustment the tabs should be marked as 'up', 'down', and center. If your pix is too high, then you would connect the jumper to the down tab. For horizontal adjustment the three tabs should be marked 'left', 'center', and 'right'. If your monitor has centering controls, then adjust for best centering. Many GS monitors use small tab switches to adjust centering. These are located near the back of the main circuit board. Adjustment of Vertical Size, etc. via shafts on back of GS monitor can affect centering. For small changes, these adjustments may get the job done. Adjustment of Intensity and Focus (see below) can affect centering. By: James Poore ===========================