I met Shane at the airport and tinkered. First off, I checked that I could get the rotor RPM gauge to work by running a drill by it - found out it needs to be the motor end of the drill by the sensor - on Randy's plane (because it's next to Shane's) it worked a treat.
Next, I checked that with the rotor brake on, the RRPM gauge worked on Shane's plane. It did. Taking the brake off stopped it working.
We swapped the switch, and now it worked when the rotor brake was off.
Because Shane's as pessimistic as I am, we pulled his gyro out of the hangar and actually span the rotor up to make sure the gauge worked, then put it away again.