Epson SMD-400 diskette drive

I finally gave in and got an Atari 1040ST, I used to have one but foolishly got rid of it a long time ago. However, it has a problem reading diskettes. It can read the root directory and at least one subdirectory and display a file from a diskette with text files on it, but what it prints is completely garbled. I have taken it apart and taken the shield plate off the drive and cleaned the heads with alcohol, but it still won't work. I suspect the problem is with the stepper motor drive. It makes a rather nasty noise when trying to seek; I remember it was fairly loud but this one makes a sharper noise than I remember. Looking at it trying to read a disk, it looks as if the heads don't move at all. As far as I can see, the stepper motor has a cogwheel on the end of the axle, which moves the heads via a rack which meshes with the cogwheel. It looks to my ageing eyes as if the rack has chewed a slot round the middle of the cogwheel, which would explain why the heads don't move. Somehow I don't think the cogwheel should look like that. If the cogwheel is broken, I shall have to get a replacement drive, fortunately they seem fairly common on eBay. Making a new cogwheel and probably a new rack is completely out of the question for me, and replacing the motor and rack doesn't seem like a particularly viable or economic option for me (it probably would be for Tony, but then I don't have a metalworking workshop with a lathe available to me :-) ). Any thoughts? Am I right in supposing that the cogwheel shouldn't look as if it had a slot round the middle? /Jonas