A B737-300 WAS DISPATCHED WITH THE INCORRECT CAPT'S AUDIO PANEL INSTALLED.

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PLT RPTED A BLEEDOVER HUM IN AUDIO SYS. I CHKED SYS (A COMPUTER SYS TO VERIFY PART NUMBER EFFECTIVE FOR SPECIFIC ACFT). I TOOK PART TO PLANE AND INSTALLED AND CHKED SYS -- OP WAS NORMAL. THIS HAPPENED ON FEB/WED/05. ON FEB/MON/05 CORRECT PART WAS INSTALLED AFTER A RECORDS/LOGBOOK CHK. I HAD INSTALLED THE WRONG DASH NUMBER PART ON ACFT (MPN X-X; THE CORRECT PART NUMBER WAS X-Y). ACR HAS A SYS THAT MECH MUST CHK BEFORE INSTALLING ANY PART ON AN ACFT. THE ACFT INVOLVED WAS ACFT X. I WAS ASSIGNED TO WORK ON 2 ACFT THAT NIGHT AND HELPED OTHERS WITH AN ADDITIONAL 2 ACFT. I CAN'T REMEMBER HOW I MADE THIS MISTAKE; BUT I REMEMBER I CHKED SYS SO I WOULD KNOW WHAT PART TO TAKE TO THE ACFT. I MUST HAVE TYPED IN THE WRONG ACFT NUMBER EITHER BY TRANSPOSING NUMBERS OR TYPING IN ONE OF THE OTHER ACFT I WORKED ON THAT NIGHT BY MISTAKE. I HAVE NO EXCUSES; I JUST MESSED UP. WORK FACTORS WERE REALLY NOT AN ISSUE; THE WORKLOAD WAS FAIRLY LIGHT AND SINCE IT WAS A REMAIN OVERNIGHT CHK; THERE WAS NO TIME FACTOR INVOLVED.