Motorcycle Repair: Odometer not working, speedo IS working, yamaha maxim, trip odometer

Question
Hello! I have a 1982 yamaha maxim xj650 that is in really great shape, i've always kept it tuned up with new spark plugs and oil. I bought it with approximately 17k miles on it and now it's at almost 31k. A few days ago the odometer stopped working. the trip reset works, but the mileage on the trip odometer doesn't go up. the Speedo works fine still though. I noticed this because it started sputtering and i had to switch to my reserve tank when the odometer said 26 miles instead of somewhere closer to 120. I'm not sure what to do. I banged on it a little bit while riding it to see if that would get it going again but no such luck. I checked the cable coming out of the speedo and it looks tight and where it connects to the front tire looks tight too. I'm not afraid to get in there and fix it, i just don't know what exactly to look for as i'm not very familiar with speedo/odometer problems. Any help/troubleshooting would greatly be appreciated.
Thank you!

Answer
I believe those had the tripmeter knob that pulled-out and pushed back in, to allow you to reset the tripmeter. See if it is stuck in the out or in position. If thisseems fine then...
This might not explain the tripmeter not working, though, that probably would be the internal workings of the speedo assy. If the speedo works then chances are the tripmeter function has simply failed due to old age. Find a used one on ebay.
You could try to work on it internally but chances are this will not be much of a success, like working on a large watch....and no parts would be available for the insides of the unit.
Thanks.