As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RRPM GAUGE WOULD DROP TO ZERO INTERMITTENTLY IN FLIGHT WITH LOW RRPM AUDIO. ALL OTHER ENGINE INDICATORS WERE NORMAL. HAPPENED SEVERAL TIMES IN FLIGHT. A TEST FLIGHT EVAL FOUND ROTOR RPM WENT TO 0 WHEN INCREASING COLLECTIVE PRIOR TO TAKEOFF TWO TIMES. RPM ON TRIPLE TACH AND DIGITAL INDICATION WAS 0. REMOVED AND REPLACED RPM SPEED PICK-UP SENSOR IAW AMM 62-41-00, 4-1.