As if it wasn't bad enough that the worm gear can fail in Sharp MiniDisc player/recorders, there are different sizes!
Update 8/28/2023 - no surprise, the Kenwood DMC-J7R uses the same gear/gearshaft as the Sharp MD-MS702
Update 7/30/2023 - Fixed a Sharp MD-MS702 and it is a Large size but see this photo - you can see that the gearshaft has a different design as well with a large, flat section and a smaller plastic gear...
Update 7/17/2023 - Opened up a MD-DS55 to clean and saw it had the normal Large gear/shaft
Update 7/8/2023 - Looks like there's yet another size of gear/shaft! As I was taking apart an MD-DR7, I found yet another type of worm gear and shaft. See picture below along with the Large one referenced in the original post. I will call this one the "Large Recorder" for now, because I'm betting that they're the same for the other Sharp Auvi recorders (MD-DR77, for example).
If you've been looking for Sharp MiniDisc devices on Buyee, you may see a few selling as "junk" with a UTOC or other TOC ERROR. Often, it's the worm gear that cracks so the laser sled can't move back and forth smoothly.
I'll call them Small/Medium/Large for now based on the length and gear diameter in the picture below (Small is closest to the ruler).
It looks like the Small and Medium are both 29mm in length, but the Small has a 2mm diameter gear while the Medium has a 3mm diameter gear.
The Large is 36mm long, has a 2mm diameter gear, but has a longer length gear as well. I believe that's the gear in most devices/newer devices.
Here's what I've noticed so far (info from MiniDisc.wiki):
Small gear - 29mm gearshaft:
- Sharp MD-ST70 - released in 2/2001
- Sharp MD-ST55 - released in 2/2000
- Sharp MD-ST77 - released in 12/2000
Medium gear - 29mm gearshaft, larger diameter gear:
- Sharp MD-SS323 - released in 4/1999 - but spec looks like 10 T, Mod 0.3, Outside diameter: 3mm
Large gear - 36-37mm gearshaft:
- Sharp MD-ST700 - released in 2/2002
- Sharp MD-DS5 (Auvi) - released in 2/2003
- Sharp MD-ST600 - released in 3/2003
- Sharp MD-DS33 (Auvi) - released in 1/2005
- Sharp MD-DS55 (Auvi) - released in 7/2004
- Sharp MD-MS701/702 - released in 8/1997 (note different shaft design, plastic replacement gear worked fine)
- (Planned test of a brass gear - spec seems to be: 10 Teeth, Modulus: 0.2, Outside diameter: 2.3-2.4mm, Inside diameter: 1mm/0.98mm for friction fit <-- this was too big, need smaller, thickness: 2.5mm)
- Kenwood DMC-J7R - released in 9/1997 (note different shaft design, plastic replacement gear worked fine)
- Sharp MD-DR7 - released in 12/2002
- Sharp MD-MT770 - released in 8/2001