What was found
BOTH TRANSPONDERS AND BOTH ENCODERS WERE INTERMITTENTLY REPORTING INCORRECT ALTITUDE ABOVE 14,700 FEET DUE TO PIN 1 OF BM-72 BEING INTERMITTENTLY OPEN ON BUSS MODULE. BOTH GRAY CODE INFORMATION IS PARALLELED AT BUSS MODULES WHICH, IF ANY ONE PIN WOULD OPEN, IT WOULD HAMPER BOTH TRANSPONDERS MODE 'C' READ-OUT FROM EITHER ENCODER. REPLACED BM-72 BUSS MODULE TO REPAIR PROBLEM. RECOMMEND GRAY CODE LINES BE ISOLATED WITH RELAY SWITCHING TO FURTHER PROVIDE DUAL ISOLATION.