The Sharp portable MiniDisc devices are prone to a failure of the plastic worm gear that is connected to a rod that moves the laser sled back and forth. A common symptom seems to be a device that is unable to read discs. The usual errors are “TOC ERROR”, “TOC ERRORa”, “UTOC”, etc.
If you can find a spare (harvested from another device), you can replace and repair a cracked gear to restore functionality to a device.
To make matters more difficult, it seems there are at least 3 different gear sizes (and connecting rod) depending on the model of player.
It looks like the Small (bottom, black) and Medium (middle, blue) are both 29mm in length, but the Small has a 2mm diameter gear while the Medium has a 2.5-3mm diameter gear.
The Large (top, white) is 36mm long, has a 2mm diameter gear, but has a longer length gear as well. I believe that's the gear in the newer devices.
Looks like there's yet another size of gear/shaft. Inside a Sharp MD-DR7, I found another type of worm gear and shaft. See picture below along with the Large one referenced in the original post. Let's call this one the “Large Recorder” for now, because they're probably the same in the other Sharp Auvi recorders (MD-DR77, for example).
As you can see, this one is 44mm in length and just a bit larger in diameter than the Large (white gear).
Another size: From the Sharp MD-MS702 - same length (37mm) as the “Large” size but notice the shaft difference in the picture below - it's the one closest to the ruler (the other ones are: “Large” with white gear, “Medium”, and “Large Recorder”):
Another size: From the Sharp MD-MT821 - has a shaft length of 33mm, with a gear diameter of 3.5mm and gear length of 2.95mm.
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
33mm gearshaft -
- Sharp MD-MS721 - released in 9/1998
- Kenwood DMC-K7R - released in 9/1998
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-MS701/702 - released in 8/1997 (note different shaft design, plastic replacement gear worked fine)
- Kenwood DMC-J7R - released in 9/1997 (note different shaft design, plastic replacement gear worked fine)
Large Recorder gear - 44mm gearshaft:
- Sharp MD-DR7 - released in 12/2002
- Sharp MD-DR77 - released in 4/2004
- Sharp MD-MT770 - released in 8/2001
There are sellers providing replacement plastic gears that can be heated and attached to the shaft after removing the old, cracked gear.
There are also reports of a brass/copper/metal gear that is working on some of the MD-MS701/702/722, though the link referenced is a non-helical gear. I've reached out to the poster to confirm before linking.