Maybe I've gotten lucky, but I have fixed two of them pretty easily. As Demonic pointed out, they are electro-mechanical. When new, they had some light oil to lubricate them. That oil gets dry and old, and stops the movement. I have had great success with some careful use of carb cleaner and some fresh, light oil. That's it, and bang, they are working again. (They are very easy to bench test too.)
Good luck!