I had the same problem with my '91. I hit it with a hammer, as many suggest, with no luck. I decided to rebuild it with the guidance of Dr. Bob's site. When I got it apart I found one of the magnets in the electric motor was broken. I assume it was a result of my hammering.
From what I have read, and now seen in my motor, there seems to be 2 common causes for them to freeze up. 1. There is a rubber 'stop' which is a piece of tubing which slides over a steel post on the actuator wheel. This dry rots and breaks up into small pieces, hindering the movement of the gears. 2. The electrical contacts on the armature get too dirty for the signal to pass. These are fairly simple problems to resolve, however, with the broken magnet I may have to bite the bullet and buy a new unit. I epoxied the magnet back in last night. I am going to try it out today.
