What was found
PILOT REPORTED DESCENDING THROUGH 12,000 FEET, SMOKE OBSERVED COMING FROM UNDER COPILOT'S INSTRUMENT PANEL. DECLARED AN EMERGENCY, LANDED WITHOUT INCIDENT. FOUND DIODE ACROSS THE 'NO SMOKE' AND 'FASTEN SEAT BELT'. SWITCH WAS OVERHEATED AND THE CABIN INFO CIRCUIT BREAKER TRIPPED. REPLACED DIODE, RESET BREAKER SYSTEM, OPERATED NORMAL.