What was found
COCKPIT FILLED WITH SMOKE AFTER MAIN BATTERY SELECTED ON AND AVIONICS TURNED ON. A GPU WAS ALREADY CONNECTED BEFORE POWER-UP. POWERED UP ACFT WITH GROUND POWER CART. NO SIGHT OR SMELL OF SMOKE FOUND. RAN ENGINES, AND CHECKED ALL SYS IN PRE-FLIGHT CHECKLIST. REMOVED NR 1 AND NR 3 PFD'S. REMOVED NR 1 AND NR 2 CDU'S. REMOVED OVERHEAD SWITCH PANEL. GAINED ACCESS TO BACKS OF CIRCUIT BREAKER PANEL. INSPECTED ALL AREAS WITH NO DEFECTS NOTED. INSTALLED ALL COMPONENTS LISTED ABOVE. RAN ACFT OPS CHECKED GOOD IAW PILOTS CHECKLIST AND MM CH 20-20-01.