What was found
DURING FLIGHT AND ON APPROACH, AN UNUSUAL VIBRATION WAS ENCOUNTERED WHICH SEEMED TO BE COMING FROM BELOW OR FORWARD OF COCKPIT. AFTER LANDING AND PASSENGER DEPLANEMENT, BLACK SMOKE FILLED THE COCKPIT. EXTINGUISHED ALL POWER TO AIRCRAFT AND PULLED ALL CIRCUIT BREAKERS OVERHEAD. SMOKE CEASED, FOUND AC BUS CONTROL PANEL HAD FAILED. REPLACED PANEL, OPERATIONAL CHECK OK. AIRCRAFT RETURNED TO SERVICE.