You cannot clean the key switches on an mc4,they are sealed,you can but then u have to cut the bottom of the switch to open it,then use a 2 compount conductive substance that can be applied to the conductive rubber pad,then glue the switch again and put it back in place,
Note its a hell to replace a key from the numeric keypad because they all hold into a metal frame,its probable that u need to desolder all keys switches 2 remove the frame where those switches are clipsed into it.
same thing happens in a cr8000 (a real hell)
with an overflow of evaporating contact cleaner that leaves no residue you can maybe improve the contact,but it will never get as new.
CY sell those switches from time to time on ebay,they are the same as the start/stop of the 808..
this whas my setup in early 90's that i used to made complete tracks with it:
http://matrixsynth.blogspot.com/2007/01 ... -mc-4.html