As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SHORTLY AFTER TAKEOFF, THE PILOT NOTICED THE LT PROP RPM DROPPING. THE OTHER ENGINE INSTRUMENTS DID NOT SEEM TO BE AFFECTED, BUT THE PILOT THOUGHT THE ENGINE WAS LOOSING POWER. THE AIRCRAFT RETURNED AND LANDED AT THE SAME AIRPORT AS THE TAKEOFF. THE AIRCRAFT LANDED WITHOUT INCIDENT. THE ENGINE WAS OPERATED ON THE GROUND FOR TROUBLESHOOTING AND THE PROPELLER WAS INDICATING 9100 RPM AT IDLE. MAINTENANCE TROUBLESHOOTING AND FOUND THE PROP TACH GENERATOR TO BE DEFECTIVE.