What was found
PDX - DURING PRE-DEPARTURE ON THE GROUND AT PDX, ELECTRICAL SMELL NOTED IN AFT LEFT LAVATORY. MAINTENANCE REMOVED FLUORESCENT LIGHT COVER AND FOUND ARCING, FLAME AND SMOKE WITH LIGHT SOCKET MELTED. LAVATORY BLOCKED AND AIRCRAFT RELEASED PER MEL 25-10. MAINTENANCE SUBSEQUENTLY, REMOVED AND REPLACED SOCKET. (W)