What was found
HOT/HRO - FLT 1215 - AIRRAFT WAS RELEASED FOR TAKEOFF FORM HOT TO HRO. WHILE EN ROUTE TO HRO, THE LT RPM GAUGE SLOWLY DROPPED TO ZERO WITH NO APPARENT DROP IN ACTUAL ENGINE RPM. THE FLIGHT CREW CONTACTED MAINTENANCE CONTROL WHO INSTRUCTED THEM TO RETURN TO HOT IN ORDER FOR MAINTENANCE TO BE PERFORMED. MAINTENANCE INSPECTED THE AIRCRAFT AND FOUND THE TACH GENERATOR HAD FAILED. THE TACH GENERATOR WAS REPLACED WITH A SERVICEABLE UNIT AND OPERATIONAL CHECK GOOD. THE AIRCRAFT WAS RETURNED TO REVENUE SERVICE.