What was found
CREW DECLARED EMERGENCY EN ROUTE ACK/BOS WHEN SMOKE CAME FROM AFT SIDE PANEL AFTER RE-SETTING AVIONICS MASTER SWITCH, FOR AVIONICS FAILURE. USING EMERGENCY AVIONIC SWITCH, RESUMED COMMUNICATIONS WITH BOS. APPROACH, LANDED WITHOUT INCIDENT. MAINTENANCE REPLACED AVIONICS MASTER SWITCH. SYSTEM CHECKED OK.