What was found
PHL - FLT 673 - SHORTLY AFTER T/O, THE NR 1 AND NR 2 COMPASS SYS SHIFTED WITH NO FLAGS. THE NR 1 COMPASS HAD A 25 DEGREE ERROR AND THE NR 2 COMPASS HAD A 55 DEGREE ERROR. F/A CHECKED CABIN FOR ELECTRONIC INTERFERENCE AND FOUND NONE. FLT RETURNED TO FIELD AT PHL. NO EMERGENCY WAS DECLARED AND THE FLT LANDED W/O FURTHER INCIDENT. AFTER LANDING, ERRORS WERE 30 DEGREES AND 60 DEGREES RESPECTFULLY. MX ACCOMP COMPASS SYS TEST IN ASSOCIATION WITH MM 34-21-0. COMPASS SYS NR 1 AND NR 2 OP CHECK WAS NORM. NO FAULTS WERE DUP. FOUND STANDBY COMPASS HEADING ERROR EXCESSIVE. ACCOMP CALIBRATION PROCEDURE IN ASSOCIATION WITH SPM 34-20-01. ALSO TESTED COMPARATOR SYSTEM COMPASS WARNING IN ASSOCIATION WITH MM 34-29-0.