As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHEN VHF COM WAS KEYED, THE MODE C ALTITUDE REPORTING WOULD JUMP BY 200 TO 500 FEET. REPLACED SSD120 ALTITUDE ENCODER WITH NEW SSD120 ENCODER CORRECTED DEFECT. PERFORMED TRANSPONDER AND DATA INTEGRATION TESTS IAW CARS STD 571 APPENDIX F. STATIC LEAK CHECK PERFORMED IAW CARS STD 571 APPENDIX B.