I don't think the 4200 vintage uses a separate computer controller - everything is in the 'control head'. There probably is a master reset function somewhere, do you still have the manuals?
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)