A PVT KING AIR 200 PLT HAD AN ACFT EQUIP PROB AUTOPLT WOULD NOT DISCONNECT; AUTOPLT SERVOS TOO TIGHT; TRIM ROLLING BACK WITH NO PLT INPUT. WX FACTORS 'EXTREMELY HVY ICE.'

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DSNDING INTO PDX WITH AUTOPLT ENGAGED FROM 27000 FT; I BEGAN TO ENCOUNTER FREEZING RAIN AT APPROX 17000 FT. ALL ANTI-ICE SYS WERE IN PLACE PRIOR TO DSCNT PER FLT MANUAL. I CONTINUED DOWN TO 9000 FT AS I CROSSED WYLIE INTXN. XING WYLIE; FREEZING RAIN BECAME MIXED WITH ICE. MY ACFT BEGAN AN ACCRETION OF CLR ICE. AS I WAS BEING VECTORED TO THE NE; THEN TO THE W; I ADVISED APCH CTL I WAS NEARING A TSTM; BUT I WAS DENIED DEV DUE TO TFC. FOLLOWING THE CLOSE ENCOUNTER WITH THE TSTM; FURTHER ICE ACCUMULATION OCCURRED; AND I WAS THEN GIVEN A R HAND TURN TO THE E FOR DOWNWIND TO RWY 28R ILS. SHORTLY THEREAFTER I WAS GIVEN A L TURN/BASE LEG TOWARD LAKER OM. MY AIRSPD WAS REDUCED FROM 170 KTS TO 135 KTS PER ATC REQUEST. AT THIS TIME THE ACFT BEGAN AN UNSCHEDULED CLB. THE AUTOPLT WAS STILL ENGAGED. I ATTEMPTED REPEATEDLY TO DISENGAGE THE AUTOPLT AND THE ELECTRIC TRIM; HOWEVER; THE ACFT DID NOT RESPOND NORMALLY. FLT CTLS; PITCH AND ROLL; DID NOT RESPOND TO YOKE INPUTS. USING DIFFERENTIAL PWR; I WAS ABLE TO CTL PITCH MARGINALLY. MAINTAINING ASSIGNED ALT WAS NOT POSSIBLE; AND I INFORMED TWR THAT I HAD A CTL PROB. THE TWR GAVE ME A 180 DEG R TURN; AND INSTRUCTED ME TO PROCEED AS POSSIBLE. BTWN 600-800 FT ON FINAL; ACFT RESPONDED NORMALLY TO FLT CTL INPUTS AND LNDG WAS NORMAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING HIS OWN KING AIR B-200 WITH HIS NON RATED WIFE IN THE FO SEAT. THE RPTR FOUND HIMSELF IN 'EXTREMELY HVY ICE.' THERE WAS AN 'AUTOPLT GLITCH' IN ADDITION TO THE ICE. HE DID NOT DECLARE AN EMER BUT PDX TREATED THIS AS SUCH. THIS IS A 'FAIRLY NEW ACFT.' THE RPTR HAD 'NO CTL WHATSOEVER' EXCEPT FOR PITCH WITH THE ENGS. ON THE GND; MECHS FOUND THAT THE SERVOS WERE TOO TIGHT. THE AUTOPLT HAD TO BE REPLACED BY COLLINS WHO WOULD NOT GIVE THE RPTR AN EXPLANATION OF THE PROB. THE TRIM WHEEL WAS TRIMMING BACK WITHOUT THE RPTR'S KNOWLEDGE. A FSDO REPRESENTATIVE WAS ON HAND WHEN THE ACFT WAS BEING REPAIRED AND HAD PRAISE FOR THE WAY THE ACFT WAS FLOWN.