If your raster exhibits only modest trapezoidal distortion, you can probably fix it by adjusting one or more of the permanent magnets arrayed around the yoke. Rotate only the ones that correspond to the region of the raster that needs correction. I recommend noting the initial positions before tweakage, just in case you discover that you only make things worse. [There's a picture of these magnets in the classicmac2.pdf article.]

Before doing any of that, though, make sure that you don't have any magnetic sources nearby. Also try moving the Mac around and see if there are noticeable orientation-dependent effects.