Problem turned out to be the pilot's triple tach.
Should one be surprised that the copilot triple tach went u/s also?
Seems to me I heard a story a long time ago concerning the way the pilot and copilot triple tachs were wired (linked), or something to that effect, although I can no longer remember what the details were. It's been a while so I might be out in space on this one.
Cheers