What was found
(CAN) TAKEOFF RUNWAY CLIMBING THROUGH 4,500 FT. THE PILOT REPORTED A COMPLETE AVIONICS FAILURE, AND THEN A RETURN TO NORMAL OPERATION. CONDUCTING AN I.F.R. APPROACH TO RUNWAY. THE PILOT REPORTED AN ERRATIC HSI CDI. TOWER GRANTED A CONTACT APPROACH. THE AIRCRAFT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE INSTALLED A SERVICEABLE HSI AND CARRIED OUT EXHAUSTIVE TROUBLE SHOOTING TEST FLIGHT BUT, COULD NOT DUPLICATE THE CONDITIONS REPORTED BY THE PILOT. THE AIRCRAFT WAS RETURNED TO SERVICE.