Oh hang on maybe you mean the speedo drive itself . Well that's what you're after and eBay is the way to go ... Yamaha want £145 for a new one . Funny that as the DTX one which is near identical costs £66 new
i had this same problem and i got speedo cable and it fixed it for a tena not saying this is your problem but id say do this be4 you go paying 145 quid when you dont need to
too the unit off, gave it a really good clean and found what had happened is something had jammed and it had bent the 3 little tabs that stick out and hit the tabs on the wheel tuning the drive.
So took it all apart, straightening out the tabs. in doing this it had also worn down the washer so fitted a new thicker washer.
Ended up messing about with it for a couple of hours and have just finished and got it back together and running, its 90% freer than it was before I started, however it does tighten up slightly, cant understand why I think it just worn slightly.
Hopefully it should be free enough to continue spinning without jamming up again, in the mean time I will keep an eye out on ebay for one coming up that hopefully spins nice and freely.
Also discovered my spindle was bent which could have been the cause so ordered a new one of those from ebay!