All done! I managed it without recourse to a soldering iron and without following any instructions, just from what I remember from earlier tasks. Not having to get the flash capacitors out of the way was the main difference between this and the MZ3/MZ5. Here's the procedure (sorry, without photos)
- Remove all outer housing parts
- Remove baseplate and loose gear wheels
- Remove the screw holding the DOF preview button bracket and push the bracket to one side
- Remove lens mount, remove lens release button bracket, refit lens mount (more on this later)
- Remove the three accessible screws on the gear box
- You will then be able to open box at the bottom far enough to remove the lower grey gear wheel to access and remove the split motor gear wheel.
- Getting the new wheel on is a bit of a faff, but doable with some patience. I put a small amount of silicone lube on the end of the spindle with a cocktail stick. Once it's on and positioned, return the grey wheel to the box, replace the box screws and reverse the dismantling process.
Regarding the lens mount, the lens release bracket blocks the gearbox from opening sufficiently, which is why it needs to come off. You refit the mount to prevent the ring that controls the aperture from dropping out while you are working on the body. I forgot about this and spent more time getting that back in than I did fixing the motor gear. There are two long springs that sit behind it and connect to the ring and the body. It's very fiddly getting these connected at the four locations. In the end, I actually glued the springs to the ring with a tiny bit of superglue applied to the stud with the end of a cocktail stick.
I was hoping to be able to make a small upgrade to the M, by swapping in an adjustable diopter eyepiece from another MZ series body, but neither of those in my parts kit (MZ-5N and MZ-6) would fit. It might be possible, but the task has "repair to death" written all over it!