Most machines have adjustments for both travel limits of either the carriage or the type basket as case may be. You want them adjusted so (1) upper and lower case are in line and (2) they are hitting the curvature of the platen like so:  --|( , that is, letter centered on the arc.  That sounds simple except that for each machine you must locate the adjustments, get at them, and move them, in increasing probability of trouble. That said I've received at least one machine with one of the two adjustments bottomed (or topped) out and to no purpose - some other mechanical derangement was interfering with the range of motion so all that was accomplished was to put the limits out of adjustment while not fixing the real problem. Once again a little knowledge is dangerous. I've found at least two machines with such a problem where the ribbon vibrator was also FUBAR and I would think there was a linkage, but I gave up. Both were SM-7’s so they seem to be prone to this kind of dysplasia. That’s all I know.