B727 CREW FOUND BOTH CHANNELS OF THE AUTOPLT WERE MALFUNCTIONING.

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ON APR/WED/00; I WAS CAPT ON ACR FLT ABC FROM SFB TO GYY THAT DEPARTED APPROX XA00. I RECEIVED THE ACFT FROM THE INBOUND CREW WITH THE 'B' CHANNEL OF THE AUTOPLT WRITTEN UP AS INOP AND PLACED ON A DEFERRED MAINT STATUS DUE TO A TRIM AND PORPOISING PROB. THIS LEFT THE 'A' CHANNEL AVAILABLE FOR MY USE. CLBING THROUGH 10000 FT; I ENGAGED THE AUTOPLT IN THE 'A' CHANNEL AND NOTICED VERY QUICKLY THAT THE AUTOPLT WAS NOT TRIMMING THE AIRPLANE AS IT SHOULD. WHEN THE AUTOPLT WAS SELECTED TO ALT HOLD MODE; THE YOKE BEGAN TRAVELING FORWARD; FOLLOWED BY OPPOSITE DIRECTION TRIMMING. I DISENGAGED THE AUTOPLT; MANUALLY TRIMMED THE AIRPLANE AND ONCE AGAIN ENGAGED THE AUTOPLT. THE RESULT WAS THE SAME AS BEFORE. I STARTED THINKING THAT THIS IS EXACTLY WHAT THE LOGBOOK SAID THE 'B' CHANNEL WAS DOING AND THAT MAYBE THEY GOT THE 'A' AND 'B' CHANNEL MIXED UP IN THE DEFERRAL; SO I TRIED THE 'B' CHANNEL WITH EXACTLY THE SAME RESULTS AS THE 'A' CHANNEL. I CONTACTED DISPATCH AND THEY RECOMMENDED CONTINUING TO GYY; ADDING THAT THEY WOULD HAVE MAINT MEET THE AIRPLANE. AFTER LNDG IN GYY; I MADE AN ENTRY IN THE MAINT LOGBOOK AND DESCRIBED THE AUTOPLT DISCREPANCY AS I HAVE JUST STATED. IN RETROSPECT; I SHOULD NOT HAVE TRIED THE 'B' CHANNEL OF THE AUTOPLT AS IT WAS ON A DEFERRED MAINT STATUS.