A B757-200 ETOPS FLIGHT CREW WAS INFORMED BY MAINT CONTROLLER THAT THE VOLTAGE VALUES LISTED IN THEIR MEL'S CONTRADICTED THEIR MAINT MANUALS.

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CTLR DISCOVERED THAT BOEING MAINT MANUAL HAD DIFFERENT VOLTAGE VALUES LISTED. THE VALUES IN THE MAINT MANUAL WERE CONSIDERED LOGICAL TO MAINT CTLR AND FLT CREW. AUTH TO DEVIATE FROM THE INCORRECT MEL VALUES WAS FAXED TO FLT CREW IN ORDER TO DISPATCH ACFT BACK TO ZZZ. DISCREPANCY IN MEL BOOK ON AC. HYD MOTOR GENERATOR CHK PER REF AS APPLIED TO MEL. VALUES FOR AC VOLTAGE LISTED IN MEL BOOK ARE INCORRECT. ACCORDING TO THE PROVISIONS OF THE MEL; DISPATCH WAS ILLEGAL. MGMNT AND FAA OVERSIGHT FOR MEL MANUAL CHANGES RESULTING FROM EQUIP INTEGRATION WERE LACKING OR NON-EXISTENT. THE IMPLICATIONS THAT THIS EVENT HAS ON SAFETY MGMNT AND OVERSIGHT SHOULD NOT GO UNNOTICED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE ETOPS REQUIREMENTS FOR AN APU DEFERRAL NEEDED TO VERIFY THE VOLTAGE VALUES OF THE HYD MOTOR GENERATOR. THE VALUES LISTED IN THE MEL CONTRADICTED THE MAINT MANUAL. BECAUSE OF EQUIPMENT INTEGRATION; NEW MAINT COMPUTER SYSTEM AND LOG BOOK CHANGES THE MM REFERENCE DATA WAS INCORRECTLY TRANSCRIBED TO THE MEL. REPORTER ADDED HE WAS APPRECIATIVE THE MAINT CONTROLLER FOUND THE DISCREPANCY BUT WONDERS HOW MANY OTHER MISTAKES MAY HAVE BEEN MADE DURING THEIR INTERNAL CHANGES.