What was found
DURING CLIMB, AT 5000 FT IN IMC CONDITIONS, CREW NOTICED RED 'AC FAIL' LIGHT & A FLASHING "MASTER CAUTION" LIGHT. CREW NOTICED AUTOPILOT DISENGAGE, ALTITUDE PRE-SELECT WENT BLANK, FLIGHT DIRECTOR "DISAPPEARED", & NR 1 ADI "TUMBLED". "NR 1 INVERTER FAIL" AMBER LIGHT ILLUMINATED. WHILE REVIEWING ABNORMAL PROCEDURES CHECKLIST, NR 1 ADI ERECTED FOR APPROX 15 SECONDS. CREW CLEARED "MASTER CAUTION" & LEVELED OFF AT ASSIGNED ALTITUDE OF 5000 FT. CREW GIVEN SECOND CLIMB TO 16000 FT WHEN NR 1 ADI TUMBLED AGAIN. PILOT ELECTED TO RETURN TO DEPARTURE. DID NOT DECLARE AN EMERGENCY, ATC ELECTED TO DECLARE AN EMERGENCY. ACFT LANDED & TAXIED WITHOUT INCIDENT. MX TROUBLESHOT SYS, INCLUDING A BENCH TEST OF NR 1 INVERTER. NO OBVIOUS DEFECTS NOTED, MECHS DID CLEAN OXIDATION FROM THE INVERTER CONNECTORS & SWAPPED POSITIONS ON NR 1 AND 2 INVERTERS. ACFT RELEASED FOR SERVICE.