PLT CONCERN ABOUT SAFETY OF POWER SOURCE FOR LAVATORY SMOKE DETECTORS.

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FLT DISPATCH RELEASE LISTED AN MEL 23-4 STATING 'SVC INTERPHONE INOP.' ACFT LOG HAD A CREW WRITE-UP STATING THAT THE PAX AND CREW CALL/LAV SMOKE DETECTOR CB HAD POPPED AND BEEN RESET; THEN POPPED AGAIN. THE MAINT SIGN-OFF WAS 'DO NO USE ATTENDANT CALL BUTTONS.' I QUESTIONED THIS WRITE-UP AND WAS INFORMED BY SAN DIEGO MX PERSONNEL THAT THE SVC INTERPHONE WAS IN FACT OPERATIVE AND THE ONLY PROB WAS WITH THE ATTENDANT CALL BUTTONS. IT WAS EXPLAINED THAT AS LONG AS WE DIDN'T USE THE CALL BUTTONS; THE CB WOULD NOT POP AND THUS WOULD HAVE NO ADVERSE AFFECT ON THE LAV SMOKE DETECTORS. SINCE THE ATTENDANT CALL FUNCTION IS NOT REQUIRED FOR FLT AS PER MEL 23-4; I ACCEPTED THE ACFT AND THE FLT TO LAX WAS COMPLETED W/O INCIDENT. WHILE THE EXPLANATION I RECEIVED FROM MX SEEMED REASONABLE AT THE TIME; FURTHER DISCUSSION OF THIS SITUATION HAS RAISED THE QUESTION OF THE EXACT STATUS OF THE LAV SMOKE DETECTORS. I UNDERSTAND THAT MX LATER CORRECTED THE PROB OF THIS ACFT BY CHANGING A BULB WHICH HAD SHORTED OUT IN THE AFT ATTENDANT PINK CALL LIGHT. MY QUESTION IS; DOES A SHORT CIRCUIT IN ONE PART OF THE SYS RENDER THE ENTIRE SYS INOP? PERHAPS THE SMOKE DETECTION SYS SHOULD HAVE ITS OWN CIRCUITRY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: ACFT WAS LATER FERRIED TO HOME MAINT BASE BECAUSE OF THIS PROB. RPTR SAID WASN'T UNTIL AFTER HE FLEW HIS LEG THAT WHILE LEGAL ON PAPER HE WONDERED IF THE LAV SMOKE DETECTOR WAS ACTIVATED IT WOULD ALSO POP THE CB RENDERING THE SYS INOP. HE WASN'T CERTAIN IF LATER CREW REFUSED THE ACFT OR IF MAINT DECIDED THE SYS WAS QUESTIONABLE. SUSPECT IT REQUIRED MORE THAN JUST CHANGING THE BULB TO FIX THE CIRCUIT.