AN AIRBUS 320 WAS DISPATCHED IN NON COMPLIANCE WITH THE SKIN AIR EXTRACT VALVE DEFERRED PER THE MEL BUT VALVE POSITIONED INCORRECTLY. ACFT RETURNED UNABLE TO PRESSURIZE.

Narrative

ON OCT/SAT/03; I WAS TO WORK AN AIRBUS A320. 5 MINS BEFORE DEP I WAS CALLED TO THE FLT DECK FOR AN AVIONICS SYS VENT FAULT. I PERFORMED THE BITE TEST AND FOUND THE SKIN EXTRACT VALVE TO BE AT FAULT. I PROCEEDED TO DEFER THE VALVE PER MEL 21-26-04. WHILE PERFORMING THE REQUIREMENTS OF THE MEL; I MISTAKENLY LOCKED OPEN THE MAIN FLAP OF THE EXTRACT VALVE INSTEAD OF THE INTERNAL FLAP OF THE MAIN FLAP. AS A RESULT THE ACFT GAINED ALT BUT COULD NOT PRESSURIZE RESULTING IN A LNDG AND RETURN TO GATE. HUMAN FACTORS INVOLVED: BESIDES THE MAINT ACTION ON THE VALVE I ALSO NEEDED TO DO A DEP WALKAROUND AND ASSIST WITH AIR AND AIR START (INOP APU). I WASN'T BEING RUSHED BUT WAS A LITTLE OVERWHELMED. I WAS NOT FAMILIAR WITH THE MEL PROC. THE OPENING OF THE INTERNAL FLAP IS EXPLAINED CLRLY ON A NOTE ON PAGE 3 OF THE MEL RIGHT BEFORE SECTION B. I OVERLOOKED THIS IMPORTANT NOTE BECAUSE SECTION B IS THE PROC FOR THE OPENING OF THE SKIN EXCHANGE ISOLATION VALVE WHICH IS ALSO PART OF THE MEL. FEELING THAT I HAD COMPLETED THE PROC FOR THE MAIN FLAP I PREMATURELY MOVED ON TO THE ISOLATION VALVE.