A F27 HAS 3 CONSECUTIVE RPTS OF SMOKE AND SMOKE ALARM IN THE AFT CABIN LAVATORY BEFORE POSITIVE FIX ACCOMPLISHED.

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ON MAY/WED/00; THE ACFT WE WERE TO CREW ON MAY/THU/00; EXPERIENCED A SMOKE/FIRE WARNING IN THE LAVATORY (LOCATED IN THE AFT CABIN). IT WAS LATER DETERMINED THAT THE CAUSE WAS USE OF WRONG BULBS IN THE 'RETURN TO SEAT SIGN.' SUBSEQUENT TO ALARM; FLT ATTENDANT OBSERVED SMOKE AND POSSIBLE OPEN FLAME FROM PLASTIC COVER OF SIGN. HE NOTIFIED CREW AND DISCHARGED EXTINGUISHER. THE CREW FAILED TO NOTIFY ATC; CALL FOR EQUIP STANDBY ON LNDG; AND DID NOT INDICATE OR WRITE UP EVEN IN ACFT MAINT LOG. IN ADDITION; THE FIRE EXTINGUISHER WAS RESAFETIED AND PUT BACK IN ACFT BY COMPANY MECH; WITHOUT RECHARGE/RECERTIFICATION. THE FLC CONSISTED OF A NEW CAPT UPGRADE; AND AN FO WHOSE PERFORMANCE HAS BEEN QUESTIONED SINCE HE CAME ON LINE. I HAD WARNED OUR CHIEF PLT NOT TO PAIR THESE TWO. WE WERE ASSIGNED TO THE ACFT THE FOLLOWING DAY WITHOUT KNOWLEDGE OF THE PREVIOUS DAY'S EVENTS. LATE IN THE CLBOUT; OUR FLT ATTENDANT RPTED A SMOKE ALARM IN LAVATORY. I XFERRED CTL OF ACFT TO FO AND IMMEDIATELY WENT AFT. NEITHER I NOR THE FLT ATTENDANT COULD FIND EVIDENCE OF SMOKE OR FIRE ANYWHERE (TRASH RECEPTACLE; ETC). WE THEN SUSPECTED A FAULTY SMOKE ALARM. I ORDERED THE FLT ATTENDANT TO KEEP LAVATORY DOOR OPEN; AND REMAIN AT HIS STATION TO MONITOR; THE SEAT BELT SIGN WAS NOW OFF. ON THE NEXT LEG; THE SAME THING HAPPENED ON THE SUBSEQUENT LEG; AGAIN I WENT AFT; AND AGAIN NO EVIDENCE OF SMOKE OR FIRE. THE PREVIOUS PRECAUTIONS WERE ALREADY IN EFFECT. ON LNDG; WE NOTIFIED MAINT. THEY REPLACED SMOKE ALARM; AND WE ALL DID A THOROUGH CHK OF AFT CABIN AND TAIL. NOTHING IN EVIDENCE TO INDICATE A SOURCE OF SMOKE. AGAIN WE DEPARTED. THIS TIME WITH A NEW FLT ATTENDANT. HE HAD BEEN THE FLT ATTENDANT ON PREVIOUS DAY'S FLT. I WAS NOT COMPLETELY INFORMED OF THAT DAY'S EVENTS. WE DEPARTED FOR ZZZ; AND AGAIN THE SMOKE ALARM WENT OFF. I WENT AFT; AND WAS NOW INFORMED OF WHAT HAD TRANSPIRED PREVIOUS DAY. BY NOW; SEAT BELT SIGN WAS OFF; AND INSPECTION REVEALED A SOFTENING OF PLASTIC 'RETURN TO SEAT' SIGN. WE LEFT SIGN OFF; RADIOED DISPATCH OF EVENT; AND INFORMED OF OUR INTENT TO GND ACFT AT ZZZ UNTIL PROB WAS RESOLVED. AT ZZZ; THE LAVATORY WAS TAKEN OTS; AN MEL ISSUED; AND FLT CONTINUED WITHOUT FURTHER INCIDENT. THE RETURN TO SEAT SIGN WAS DEACTIVATED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE FIRST CREW DID IN FACT WRITE UP THE OVERHEATED PLASTIC SIGN BY ENTERING IN THE LOGBOOK THE RPT 'LAVATORY RETURN TO SEAT SIGN INOP.' THE RPTR SAID THE TECHNICIAN WHO PLACED THE SAME FIRE BOTTLE BACK ON THE ACFT AFTER BEING USED HAD IN FACT WEIGHED THE BOTTLE AND FOUND IT WAS SERVICEABLE. THE RPTR SAID THE TECHNICIAN WAS TOLD VERBALLY THAT ONLY THE ARMING PIN WAS PULLED OUT. THE RPTR SAID THIS PROB OF THE WRONG LIGHT BULBS BEING INSTALLED IS A FREQUENT PROB AND OCCURS MAINLY ON THE PAX READING LIGHT PLASTIC MODULES. THE RPTR STATED THE COMPANY IS NOW ADDING LABELS ON THE LIGHT FIXTURES WITH THE CORRECT LAMP NUMBER AND WATTAGE.