What was found
FLIGHT CREW REPORTED NR 1 IRS AND TCAS SYS FAILED. FAULT COULD BE DUPLICATED ON GROUND, BUT ONLY AFTER SEVERAL ATTEMPTS. FAULT ISOLATED TO THE NR 1 AVIONICS J BOX LOCATED IN THE NOSE COMPARTMENT OF THE ACFT. NR 1 AND NR 2 J-BOXES SWAPPED AND FAULT MOVED TO NR 2 SYS. UNITS RETURNED TO ORIGINAL POSITION AND NR 1 J-BOX REPLACED WITH REPAIRED UNIT FROM MFG. (TEARDOWN OF UNIT INSTALLED IDENTIFIED AS REPAIRED, MODIFICATION, HOT AND COLD TEST PERFORMED) UNIT INSTALLED , TESTED AND FOUND TO BE SERVICEABLE IAW AMM 34. TCAS SYS WILL REPORT A FAULT ANY TIME 1 OF THE IRS UNITS IS INOPERATIVE. AFTER THE J-BOX WAS REPLACED THE TCAS SYS TESTED SERVICEABLE IAW AMM 34.