How can I fix my Singer 774?

I have a Singer 6233 stitches do not form properly. The problem is either with the dial on the front of the machine not changing to the proper cam to make the stitch or the cams themselves that are worn or slipping. Straight stitch will work but the decorative stitches don't. Is this something I can fix or is it doomed? I have another machine so I would not want to spend money to have this one repaired but it would be nice as a backup.