Model Forum / Radio Controlled / Helicopters / June 2004

I only wish I had pictures to show you ...

After noticing that some of my servos exhibited jittery behavior today
(usually around center, but sometimes jittery around end of travel), I
disassembled all my slightly misbehaving servos and placed them under a
microscope.  This was very educational (it's a whole different world under
the microscope):

These servos have been used for only a couple years, but not very often.  I
expected them to be in pretty good shape.  I thought maybe I could just
clean them with contact cleaner spray.  When I look at the feedback
potentiometers with my naked eye, they look basically OK.  A little bit of
wear, but nothing significant.

HOWEVER, under the microscope, I saw huge rivers of petroleum (I thought it
was oil collected from exhaust fumes penetrating the rubber seals, but Beav
tells me it is normal to use vaseline) running across the surface of the
feedback potentiometer and carrying huge boulders of debris across the worn
paths where the wipers "rub".  Only, under the microscope, the wipers don't
just "rub" (instead, they SCRAPE huge rocks off the resistive layer).  In
the servos I looked at, the wipers had scraped ALL of the painted layer of
resistive material off the surface of the feedback potentiometer (near
center)!!  The bottoms of the wipers were
rubbing on bare board with only slight intermittent contact with the edges
of the remaining painted resistive layer.  At this stage, the jittering
begins and rapidly wears off additional resistive material.  If the wiper
reaches an area where there is no contact with any resistive material, the
servo hunts both sides of the gap in attempt to drive the error signal back
to zero, but never finds a happy spot (unable to eliminate the error between
demanded position and actual position).  One of my servos (where the
intermittent portion of the movement was at the end of travel) was unable to
return the wiper back to center, after the aileron demand was returned to
center.   Unfortunately, I learned the hard way because this caused my last
crash a few weeks ago (this aileron servo locked to max left position).

The microscope made it crystal clear to me that, unless the jittery servos
are almost new (cleanable), I just dispose of them.  As some pilots have
told me recently, servos should be considered "consumables".  Continuing to
use a jittery servo that has been used for a couple years is only asking for
expensive damage or possible injuries.  To me, this is an unacceptable risk.

Fly safe!

Dan