My S-Tec 50 autopilot all of a sudden won't hold altitude.  When engaged, the
servo engages and it seems to hunt a bit until, somewhere between a few
seconds, and as long as a minute, it loses control and diverges either up or
down.  Sometimes it will catch itself in such an excursion and pitch the other
way, but it won't subsequently stabilize until I disconnect it.  No trim
lights indicate a problem and lateral control is fine.  It's been fine for 3
years, otherwise, although it has occasionally exhibited a sort of hunting in
pitch - noticeable pitch changes, but no drift in altitude.

The S-Tec guy suggested checking the cable attachment to the elevator push
tube for tension.  He also said the servos can build up carbon on the brushes
that might explain this.  Have any of you experienced this sort of problem
with your S-Tec units?  What was the cause and how did you fix it?

Ed de Chazal
N361DC