I'm not exactly sure, but I believe that the modulation button (lfo 2) on the Jupiter 8 is the same switch as the Jupiter 6 which is the same as the MC-4. If so, then you can repair the switch 2 ways.
1. Buy another from chipforbrains on ebay or track one down (alps-1).
2. Assuming no damage to the plastic or rubber stop, repair the switch with a rubber keypad repair kit. (I.E. Chemtronics Circuitworks Rubber Keypad Repair Kit, http://www.partstore.com/Part/Chemtroni ... 5/New.aspx
To repair: remove the switch, take it apart, take out the rubber stop, mix the solution, paint it on the contact circle, let dry 6 hours and reassemble.
It would be cheaper to just buy a replacement switch than the kit, but if you have an MC-4 that needs new switches or something then you can kill multiple things at once. The solution is only good for about 24 hours (even though it says 72) so you pretty much have to do everything that you are going to do in one or two gos so it is not really worth it to buy the repair kit unless you have more than one switch to repair since you can't save the solution, but it is an option if you can not track down a replacement switch. I also need 2 replacement switches for an MC-4, because I can not repair them (the plastic is broken) and I am trying to get chipforbrains to sell me some.
Keep in mind that the switch may not be what I am thinking it is, but I am fairly sure about it. You will know I'm wrong if it is not an ALPS-1.
EDIT: I just checked the old auction that chipforbrains had up and according to him, I am right about the switch being the same.
( http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0156941755