As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) THE HELICOPTER WAS IN CRUISE WHEN ALL OF A SUDDEN THE ROTOR TACH GENERATOR INDICATOR WENT TO ZERO.BACK AT THE BASE AN ENGINEER TROUBLE SHOOTED THE PROBLEM AND FOUND THE TACH. GEN DRIVE WAS DISCONNECTED. THE ROTOR TACH GENERATOR DRIVE IS LOCATED IN THE #1 HYDRAULIC QUILL ASSEMBLY OF THE MAIN ROTOR TRANSMISSION. SUSPECT THE CHAIN HAS FAILED INSIDE THE HYDRAULIC/TACH GEN DRIVE QUILL.