AN AVIONICS TECHNICIAN IS INFORMED OF INSTALLING A B737-400 PRE-RECORDED ANNOUNCEMENT MODULE (PRAMS) ON A NEXT GENERATION B737-800 ACFT.

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I WAS WORKING ON ACFT X; A B737-400. I REPLACED A PRE-RECORDED ANNOUNCEMENT MODULE. THIS PART WAS INCORRECT FOR THE INSTALLATION. SAFETY OF THE ACFT WAS NOT JEOPARDIZED AS BOTH PART NUMBERS ARE SCHEMATICALLY THE SAME. PARTS PLACARDING COULD HAVE PREVENTED THE INCORRECT INSTALLATION; OR CERTIFICATION FOR USE ON BOTH ACFT OF THE SAME PART. FURTHER; CONNECTORS ON BOTH PRE-RECORDED ANNOUNCEMENT MODULES HAVE THE SAME KEYWAYS ALLOWING FOR THE INADVERTENT INSTALLATION. THIS; TOO; COULD HAVE PREVENTED THE INSTALLATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED ORIGINAL PROBLEM STARTED WITH A CABIN VOLUME STATIC ISSUE. THEY ENDED UP CHANGING THE CABIN PRE-RECORDED ANNOUNCEMENT MODULE (PRAMS). PART OF THE PRAMS MODULE IS THE EMERGENCY ANNOUNCEMENT NOTIFYING PASSENGERS VIA THE PA SYSTEM THE OXYGEN MASKS WILL BE DEPLOYED. THIS MESSAGE WILL OVERRIDE ANY ONGOING PA COMMUNICATION AT THAT MOMENT. THE B737-400 AND THE B737-800 NEXT GENERATION PRAM MODULES ARE SCHEMATICALLY AND PHYSICALLY INTERCHANGEABLE. THERE WERE NOT ANY PLACARDS OR NOTICES STATING THESE PRAMS WERE NOT COMPATIBLE. NOW THERE IS. THEIR BIGGEST WEAKNESS IS THE ON/OFF SWITCHING IN THE CIRCUITRY THAT FAILS AND SENDS AN UNCOMMANDED CABIN ANNOUNCEMENT OVER THE PA SYSTEM STATING THE OXYGEN MASKS ARE ABOUT TO DROP. THIS CARRIER'S MAINT MANUALS AND GENERAL MANUAL ARE NOT UPDATED FREQUENTLY.