AN MD80 ON APCH AT 5000 FT DSNDED BELOW ASSIGNED ALT IN A TURN DUE TO AUTOPLT PITCH CHANNEL MALFUNCTION.

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WE LOST ALT WHEN AUTOPLT WOULD NOT HOLD ALT. AUTOPLT TRIM WAS MEL'ED INOP. IT WAS MY FIRST TIME FLYING WITH THIS MEL. I SELECTED 30 DEGS BANK 'SELECTOR SWITCH.' THE AUTOPLT COMMANDED A 30 DEG BANK AND THEN LET THE NOSE DROP. AUTOPLT DID HAVE ALT CAPTURE ANNUNCIATED. ATC GAVE US A LARGE INTERCEPT ANGLE TO THE APCH AND THE ACFT ROLLED TO THE 30 DEG BANK IMMEDIATELY. WITH AUTOPLT TRIM INOP IT COULD NOT MAINTAIN ALT. WE HAD BEEN MANUALLY TRIMMING ACFT FOR CLBS/DSCNTS (EVERY TIME WE WOULD DISCONNECT AUTOPLT). INITIALLY WE LOST 300 FT THEN 300 FT TO RECOVER. IT CAUGHT ME BY SURPRISE. I WAS BUSY REVIEWING APCH PLATE CONFIGN; ETC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE ONLY HAD 50 YRS EXPERIENCE ON THIS AIRPLANE WHEN THIS INCIDENT OCCURRED. THE RPTR SAID THE AUTOPLT TRIM COUPLER WAS DEFERRED AS INOP AND MANUAL TRIM WAS USED THE ENTIRE FLT WHEN NEEDED AND THE AUTOPLT WAS RE-ENGAGED AFTER TRIMMING. THE RPTR STATED WHEN APCH CTL REQUESTED HOLD 5000 FT AND A R TURN TO 270 DEGS A TURN WITH 30 DEG BANK ANGLE WAS STARTED WITH THE AUTOPLT. THE RPTR SAID WHILE IN THE TURN HE WAS DISTR LOOKING FOR TFC WHEN THE NOSE DROPPED AND 300 FT WAS LOST AND ANOTHER 300 FT TO REGAIN CTL. THE RPTR SAID THE AUTOPLT MADE NO NOSE UP INPUT WHILE IN THE TURN.