A FAIRCHILD SA227 METRO ACFT HAD ONE 'RETURN' TO FIELD DUE TO CAPT HSI NOT FUNCTIONING PROPERLY AND ONE COMPLETED FLIGHT WITH ALL NAV #2 INDICATIONS LOST.

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ACFT X HAD THE #2 HSI BACKLIGHTING ON MEL. I HAD WORKED THE ACFT IN JUNE TROUBLESHOOTING A VOR SPLIT OF 5 DEGS BTWN THE #1 AND #2 SYS. DURING THAT EVENT I HAD REMOVED THE RADIOS FROM THE RACK TO RECORD THE SERIAL NUMBER AND PART NUMBER DATA FOR RESEARCH PURPOSES. THE RADIOS WERE KX165'S NAV/COM'S WITH A FRONT MOUNTED HEX SCREW THAT IS USED TO SECURE THE RADIO IN THE RACK. I HAD CLEANED THE CONNECTORS AND INSTALLED THE RADIOS BACK IN THE RACK. I STOPPED BACK AT THE HANGAR ON MY WAY HOME FROM MY OTHER JOB ON THE MORNING 3 DAYS LATER TO WORK ANOTHER ACFT. I HAD WORKED MY NORMAL SHIFT AT ANOTHER AVIATION COMPANY (13 HRS) AND STOPPED ON MY WAY HOME TO SEE WHAT THE STATUS OF THE FLEET WAS. I CLOCKED IN BEFORE XA00. ACFT X HAD LAUNCHED EARLIER IN THE DAY AND I INITIALLY WORKED ON ANOTHER TYPE OF ACFT IN THE FLEET WITH A DIFFERENT NAV PROB. I LEFT AFTER 2 HRS (XC00) TO GO HOME AND SLEEP. ON MY WAY HOME I GOT A CALL THAT ACFT X HAD RETURNED AND THE CAPT'S HSI WAS NOT FUNCTIONING PROPERLY. I WAS TOO TIRED TO RETURN TO THE HANGAR SO I SAID I WOULD RETURN AT XT30 TO TROUBLESHOOT THE SYS. I GOT HOME AT XE00 AFTER HAVING TO RUN SEVERAL OTHER SMALL ERRANDS. I WOKE BACK UP AT XI00 AND RETURNED TO THE HANGAR. I TESTED THE SYS AND FOUND THAT THE CAPT'S HSI WAS DEFECTIVE AND NEEDED REPLACED DUE TO AN INTERNAL FAILURE. WE HAD THE PART IN STOCK TO FIX THE BACKLIGHTING ON THE OTHER SIDE SO I SIMPLY INSTALLED THAT PART TO FIX THE CAPT'S SIDE. I HAD TO RESEARCH THROUGH SEVERAL INSTALLATION MANUALS TO FIND OUT EXACTLY WHICH NUMBERS WERE EFFECTIVE FOR THIS ACFT. ACFT X USES A 5 VOLT LIGHT SYS THAT IS DIFFERENT THAN THE OTHER METROS THAT WE HAVE HERE. THE OTHER METROS USE THE 28 VOLT LIGHTING IN THE INST AND USE A DIFFERENT DASH NUMBER DISPLAY. I QUICKLY CHKED THE SYS OUT WITH OUR TEST EQUIP AND ESTABLISHED THAT THE HSI WAS OPERATIONAL ON THE PLT'S SIDE. I HAD SWAPPED THE POS OF THE RADIOS SOME TIME DURING THIS VISIT TO MAKE SURE THAT THE DATA WAS GOOD FROM BOTH OF THEM TO BOTH SIDES. I CHKED VOR/LOC/GS OP AND ALL THE PLT'S SIDE INSTS WERE RESPONDING. DURING THESE CHKS I HAD TO STOP SEVERAL TIMES TO LOOK UP INFO FROM DIFFERENT SOURCES AND TO SEE HOW LONG I HAD DUE TO THE ACFT WAS NEEDED TO BE BACK IN SVC ASAP AND HAD ANOTHER SQUAWK IN WORK. I WAS NOT USING A WRITTEN WORK IN PROGRESS CHKLIST; JUST THE COMPANY CHKLIST PROVIDED FOR THE RAMP CHK (AVIONICS RAMP CHKLIST). I SIGNED THE ACFT BACK INTO SVC AND QUICKLY HAD TO LEAVE IN ORDER TO GET TO MY OTHER JOB. I LEFT THE HANGAR AFTER XK30 AND THE ACFT WAS BEING RUN TO CHK OUT ANOTHER PROB UNRELATED TO THE NAV SYS. THE ACFT WAS ASSIGNED A FLT ON THE NEXT DAY AND THE PLT RPTED INFLT THAT ALL THE NAV #2 INDICATIONS WERE LOST ON HIS INST PANEL. HE COMPLETED THE FLT AND MADE A LOGBOOK ENTRY. A LCL REPAIR SHOP WAS CALLED OUT TO EVALUATE THE ACFT. THE #2 RADIO WAS FOUND TO BE LOOSE OR IMPROPERLY SECURED IN THE RACK. THE CONTRACT MAINT COMPANY VERIFIED THE PROB AND CORRECTLY INSTALLED THE RADIO AND OP WAS RETURNED TO NORMAL. I WAS TIRED FROM WORKING SPLIT SHIFTS AND HAD NOT PLANNED ON WORKING THE ACFT ON THE DAY THAT IT RETURNED. I HAD NOT TAKEN THE TIME TO RESEARCH AND DOCUMENT THE SPECIFIC STATUS OF THIS ACFT AND WAS USING TRIBAL KNOWLEDGE OF THE OTHER FLEET ACFT TO GUIDE MY TROUBLESHOOTING. I FAILED TO PERFORM AN AFTER MAINT SECURITY CHK OF ALL THE COMPONENTS I HAD WORKED WITH DURING THAT VISIT AND THE PREVIOUS VISIT. I FAILED TO VERIFY THE SECURITY OF THE RADIO IN THE RACK BECAUSE I WAS DISTR AT SEVERAL KEY POINTS DURING MY CHKS BY RELATED AND UNRELATED TASKS AND REQUESTS. I ALSO WAS NOT USING A MEMORY AID OR WRITTEN CHKLIST TO MAKE SURE I DIDN'T SKIP ANY TASKS. I ALSO DID NOT ATTACH A STREAMER AS A VISUAL REMINDER THAT THE RADIO WAS LOOSE.