What was found
AIR TURNBACK, AFTER TAKEOFF THE AFT CARGO COMPARTMENT FIRE WARNING LIGHT ILLUMINATED WITH ASSOCIATED AURAL WARNING. THE CREW DISCHARGED THE NR 1 FIRE EXTINGUISHER AFTERWARDS THE FIRE INDICATION WAS STILL PRESENT. THE ACFT WAS 15 MIN OUT AND RETURNED TO PHX LANDING WITHOUT INCIDENT. THE FIRE DEPT MET THE ACFT AND DETERMINED THERE WAS NO FIRE OR SMOKE. DUE TO THE SERIOUS NATURE OF THIS INCIDENT THE FOLLOWING COMPONENTS WERE REPLACED NR 1 THRU NR 7 AFT SMOKE DETECTOR UNITS, CENTRAL CONTROL UNIT, DIVERTER VALVE, METERING VALVE AND BOTH FIRE BOTTLES (DUE TO DISCHARGE). TWO SMOKE DETECTOR UNITS HAD BAD SENSORS. SMOKE DETECTOR UNIT - MFG: SECURAPLANE - P/N: 100-0649-01 - S/N 9190, TT 169 - TC 114.