To see if your 820 mainboard is at fault, try driving
the keyboard
lines with the output of a PC's parallel port (not possible with a
"real" computer like a Mac).
But certainly possible with real computers like PDP8s, PDP11s, PERQs,
HP9000/200s, machines with HPIB or HPIL ports (+ the right interface
converter), Commodores (most had user ports IIRC), BBC micros, etc, etc, etc
Alternatively, can't you see if the existing keyboard is sending anything
using a logic probe?
-tony