Another AP Autohelm 5000 issue

[Express]

Hi everybody,
Another Autohelm 5000 (analogue) issue. A brief history of the symptoms:
I’ve had the unit for 10 seasons (came with the boat) and liked it a lot, until it stopped working last year; With the AP ON, but disengaged at the remote (aux) unit in the cockpit, the control system supposed to track the course, but when engaged at some point (again from the cockpit) it would steer the boat violently in the direction of some earlier course. I had to resort to resetting AP every time I intended to use it, but after a while even that didn’t work. Guessing that the internal compass seized up, I opened the main control unit (needed Cap Dan’s hint to finally do it – no word in the user manual …). After disengaging the belt I turned the compass cylinder by hand; the Pilot Lights were responding and at one point both went OFF. I turned the unit 90 deg, turned the compass neg. 90 and the lights went OFF again. However, the compass cylinder would not move (by itself) … L
One more thing; after opening the unit I didn’t noticed any rust, dirt or oil film anywhere …
Any idea? (short of “it’s time for a new one” ;-)
Thanks,
Peter