I think it is a 7800, bought it in '04. What happens is when I up-shift (go from large cog to smaller cog) it does not shift. This only seems to happen when I am in the largest cog, but I'm not sure of that. I'll press the lever and -nothing, no click, no movement. I do it a few times and it works and keeps working (I often move several cogs at once. I would take it apart and see how it works, and what piece is failing and then either buy or make the part (if I can - I have a machine shop) but... I don't know it it is possible or advisable to do that (take it apart). Any words of wisdom out there?