What was found
(CAN) IN CRUISE AT 3,500 FT THE RT ENGINE PROP RPM REDUCED TO 40 PERCENT WITH TORQUE ON RT ENGINE INCREASING. WE REDUCED POWER. PROP RPM CONTINUED TO REDUCE TO 20 PERCENT. POWER WAS REDUCED FURTHER TO IDLE. NO BETA INDICATION. I CALLED FOR THE PROPELLER FEATHERING CHECKLIST. CARRIED OUT THE CHECKS AND SECURED THE ENGINE. BRIEFED THE PASSENGERS. CARRIED OUT A SINGLE ENGINE APPROACH AND LANDING. TAXIED IN AND SECURED AIRCRAFT.DURING GROUND RUN IN PHC, FOUND RT CONSTANT SPEED UNIT FAILED. UNIT REPLACED AND ALL NECESSARY ENGINE CHECKS REQUIRED C/OUT. AIRCRAFT RETURNED TO SERVICE.