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Took apart the transmission on a 1992 Yamaha TDM 850 (MK1) due to the 3rd gear "slipping" and popping down to second gear. No noticeable issues when the trans was taken apart (broken teeth, extremely worn dogs, etc...).
This video is to help diagnose the issue as well as provide others information on what gears/dogs/cogs are connected when you are in each gear.