My 201 has a Century IIB Auto-pilot along with a Sys 60 PSS. Works OK intermittently so I can't depend on it for serious IFR flying.

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It performs OK sometimes, but it may suddenly lose its bearings and begin an oscillatory behaviour and deviate significantly from assigned Nav or Hdg and I have to disengage it. Same with the PSS, it may work fine for 15 minutes, then suddenly it will send the plane climbing or descending and I have to overrde it. Is it a matter of servicing or calibration? Any suggestions on shops in (Northern) California who might be experts on these systems?

They may be related. We have a similar setup with a CIIB and a STEC-30ALT. The 30ALT has no trim feature, so when the plane turns in HDG mode it banks 22 degrees and the nose drops slightly before the 30ALT recognizes an altitude deviation and corrects. In ROLL mode (turning the roll knob) it banks 30 degrees, which the 30ALT doesnt like. It has a larger (like 50ft) deviation followed by the trim beeper. It can't correct for all the loss of lift in that much of a bank. Your CIIB may need adjustment, and the 60PSS is just tryign to correct for the altitude excursions because it is playing catchup.

There are 4 adjust ent screws behind the Century panel. The left and right bank adjustments should be adjusted ot 30 degree bank in roll mode. The centering adjustment ot to center the ehading bug, turn the screw the opposite direction of the heading bug displacement.

The roll sensitivity needs adjustment in our plane once a year. Back it out until a lazy wandering develops. Then screw it in until it has a fast oscillation.

Then split the differnece. They are 20-turn trimpots.

Do it in flight. If it looses its bearings, there is most likely a loose connection in the radio coupler. The CIIB autopilot is an analog autopilot that sums up to three error signals to get the final error signal to correct the roll angle of the plane.

The lowest level error comes from AI, and with the heading switch off the error is the difference between the AI output and the roll knob. The first thing to check is performance of the autopilot with the heading switch off. This will eliminate the radio coupler and DG. With the heading switch on, the error signal is the sum of the bank angle from the AI and the error signal from the radio coupler. The radio coupler has two levels of operation, the most basic is heading mode. In heading mode the error from the radio coupler is the error signal from the DG which is the difference between the heading bug position and straight up. Testing the the autopilot in heading mode will eliminate any problems caused by your radios (GPS, LOC, NAV).

The highest level of operation is when the autopilot is coupled to a radio navigation source. The radio coupler will sum the error from the DG and the radio to form the error signal sent to the amplifier (the box with the roll knob).

By testing these three modes seperatly you can find where the intermittent connection is that is causing your auropilot to loose its way. I got mine repaired at Airtronics at San Andreas, CA. The servos needed cleaning, no parts were replaced. One of the vacuum hoses was disconnected for the Sys 60 PSS.

The AP problem was not reliably fixed the last time. I started to recently see the AP lose its way and turn in the wrong direction etc.

I took it in again, and they found a loose capacitor in the left/right roll amplifier. After resoldering, the AP works fine now. Whew, rather a simple issue! Also the AP board was reshimmed properly so there is no play in the connector and won't vibrate now. They may be related.

We have a similar setup with a CIIB and a STEC-30ALT. The 30ALT has no trim feature, so when the plane turns in HDG mode it banks 22 degrees and the nose drops slightly before the 30ALT recognizes an altitude deviation and corrects. In ROLL mode (turning the roll knob) it banks 30 degrees, which the 30ALT doesnt like.