THE AUTOPLT CAME ON WITHOUT BEING TURNED ON BY THE FLC. THE AUTOPLT COULD NOT BE TURNED OFF IN THE NORMAL MANNER BUT ONLY WITH THE CIRCUIT BREAKER.

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ON RAMP IN MZT; MEXICO; FO AND I NOTICED AUTOPLT SWITCH ON FORWARD PANEL CAME 'ON' UNCOMMANDED. WE TURNED SWITCH OFF ONLY TO HAVE IT COME ON AGAIN. CTL WHEEL DID RAPID MOVEMENT AND WAS INDEED LOCKED ONTO AUTOPLT AS WE WERE UNABLE TO MOVE CTL WHEEL. WE ATTEMPTED TO DISCONNECT AUTOPLT WITH CTL WHEEL BUTTON. AUTOPLT WOULD STAY OFF AS LONG AS BUTTON WAS DEPRESSED. ONCE BUTTON WAS RELEASED; AUTOPLT CAME BACK ON. UNABLE TO DISCONNECT AUTOPLT EXCEPT THROUGH CIRCUIT BREAKERS TO AUTOPLT AND FLT GUIDANCE SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THEIR COMPANY TECHNICAL REPRESENTATIVE FROM MCDONNELL DOUGLAS SAID WHAT HE DESCRIBED COULD NOT HAVE HAPPENED. AFTER THE TECHNICAL REPRESENTATIVE WAS BRIEFED BY TECHNICIANS; HE SAID MAYBE IT COULD HAPPEN. HOWEVER; AFTER THEY PLACED THE DEFECTIVE AUTOPLT CTL BOX INTO ANOTHER ACFT; HE FINALLY BECAME A BELIEVER. ANALYSIS REVEALED THAT THE COIL IN THE AUTOPLT'S CTL BOX HAD SHORTED OUT LOCKING THE SYS ON; EVEN THOUGH THE CIRCUIT BREAKER WAS POPPED OPEN. THE RPTR WAS ADVISED BY THE TECHNICAL REPRESENTATIVE THAT IF THEY WOULD HAVE BEEN AIRBORNE; THE CTL WHEEL WOULD NOT HAVE LOCKED UP; BUT COULD HAVE BEEN OVERRIDDEN.