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DPRG: Servo Modification Problem

Subject: DPRG: Servo Modification Problem
From: john.r.strohm at bix.com john.r.strohm at bix.com
Date: Sun Feb 21 11:46:51 CST 1999

>	I just modified two Cirrus CS-70 servos, and have run into what seems
>to be a strange problem.  I am testing it using a 555 timer circuit with
>a pot to change the pulse-width.  Everything works as it should, except
>for at one setting of the pulse circuit the servo takes in much more
>current then it did before (from 20 mA normally to about 50 mA at this
>one spot). The servo does this at about half throttle and only going in
>one direction. It also make much more noise at this one setting.  And
>right below and above that spot everything is good.  I know the problem
>doesn't have anything to do with the test circuit because it looks fine
>on a scope.  Do you have any idea what could be wrong?  What can I do to
>fix the problem?

Bet you a dollar you have a spot of dirt on the servo pot at that spot.

Servos use a potentiometer to measure shaft position.  They compare the pot
reading against the commanded position, get an error signal, and amplify
that to drive the motor.  If the pot isn't clean, you will not get a good
error signal, and the servo will "hunt", trying to find the commanded
position, overshooting, going back the other way, overshooting again, and
so on..  This is called "buzzing" in the R/C hobby, for obvious reasons.

Try hitting the servo pot with spray tuner cleaner.

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