What was found
AIRTURN BACK DUE UPPER DECK FLIGHT LOCK LIGHT ILLUMINATED AFTER TAKE OFF. CABIN WOULD NOT PRESSURIZE. EXTINGUISHED ON LANDING. ATC COULD NOT RECEIVE SQUAK CODE DURING FLIGHT, MULTIPLE RESETS NO HELP. TRANSPONDERS INOPERATIVE. SIMULATED "AIR" MODE IAW MM. THE UPPER DECK DOOR FLT LOCK, LOW SPEED BRAKE REL, AND THE PRIMAREY AIR/GROUND MODE LIGHT ILLUMINATE. CYCLED PRIMARY LG DIS & AMP; CONT CIRCUIT BREAKER, ALL BACK TO NORMAL. REPLACED PRIM LDG LOGIC BOX IAW MM AND ALL OPERATES SATISFACTORY.