What was found
WHILE PERFORMING PRE-FLIGHT, COCKPIT FILLED WITH SMOKE. POWER SHUT DOWN, PROBLEM FOUND TO BE, SHORTED BUSS PROTECTION PANEL AND ASSOCIATED WIRING. REMOVED AND REPLACED BUSS PROTECTION PANEL AND BURNT WIRING SYSTEM; TESTED IAW MM 24-20-7. FOUND NOTE: SWISPORT GPU NR 350 IN USE AT THE TIME OF PROBLEM FOUND OK, TO HAVE FLUCTUATING VOLTS AND FREQUENCY.