What was found
AFTER TAKEOFF,THERE WAS A SMELL OF SMOKE IN THE COCKPIT. THE SMELL WAS COMING FROM THE AREA OF AVIONICS MASTER BREAKER/SWITCH. DISCOVERED THE POTTER & BRUMFIELD BREAKER/SWITCH, PN W31-X1000-50, HAD OVERHEATED AND THE BREAKER POINTS HAD WELDED SHUT. THIS RENDERED THE SWITCH PORTION OF THE BREAKER/SWITCH INOPERATIVE. THIS BREAKER/SWITCH HAD FAILED BEFORE ON THER AIRCRAFT, BUT NEVER IN FLIGHT. THE FAILURES USUALLY OCCURRED ON THE GROUND DURING GROUND OPERATION OR PRE-FLIGHT. THE AVIONICS SYSTEM IS PROTECTED BY A SECOND CIRCUIT BREAKER, PN 7815-15-50, LOCATED IN THE LEFT WING ROOT FWD OF THE BATTERY BOX WHICH WILL TRIP IF THERE IS AN OVERLOAD. (X)