What was found
PDX - WHILE EN ROUTE FROM SEA AT 13000 FT AND 220 KTS, EXPERIENCED ST ELMOS FIRE ON FIRST OFFICERS WINDSCREEN WITH SIMULTANEOUS ILLUMINATION OF 3 RED GEAR UNSAFE LIGHTS. THE LEFT GEAR DOOR AMBER LIGHT AND THE LANDING GEAR INOP CAUTION LIGHT. ACCOMPLISHED ALTERNATE EXTENSION. NO FURTHER PROBLEMS EXPERIENCED. PERFORMED A BITE DIAGNOSTIC TEST OF THE PSEU SYSTEM FOUND FAULT 53. REPLACED PSEU IAW TASK 32-61-00-000-801. PERFORMED A BITE TEST, NO DEFECTS NOTED. PERFORMED A GEAR SWING OPERATIONAL CHECK GOOD. AIRCRAFT RETURNED TO SERVICE.