[NSRCA-discussion] Servo question

chris moon cjm767driver at hotmail.com
Sat Jul 5 12:26:57 AKDT 2008



Jon:



Thanks for the great info.  Never knew that one.  I looked in there and
saw the slot you mentioned.  I'll leave it up to Horizon to figure out
why it changed all of a sudden.



Chris



Jon Lowe wrote:

   3421's have the pot
shaft driven directly by the output shaft. If you reach down thru the
hole for the servo arm screw with a jewelers screwdriver, there is a
slot that can be used to adjust centering of the pot which is a press
fit into the gear. The pot shaft may have slipped in the output gear.
Pot shaft could be binding, or the output gear may be cracked
internally allowing the shaft to slip. You could just need a new output
gear. I'd try adjusting it to see how easy it slips. A VERY small
movement of the pot shaft will affect centering in a BIG way, so be
careful. Perhaps, the pot may also be coming apart or wearing badly
internally, affecting centering. Definitely determine the cause or send
it back to have at least the pot replaced, along with the gears.

  

Years ago, many servo pots were driven like this, and didn't caouse a
problem except for servo pot wear. Replacing pots after 25 flights was
not all that uncommon. Most (all) larger servo pots anymore are
indirect drive to help prevent pot wear. 3421 is probably just too
small to do an indirect drive.

  
   

  
  Jon Lowe
   

  
   

  
-----Original Message-----

From: chris moon 

To: nsrca-discussion at lists.nsrca.org

Sent: Sat, 5 Jul 2008 12:47 pm

Subject: [NSRCA-discussion] Servo question

  

  
  
  Ok,
here is a brain teaser for you experts.

  

On Sunday 2 weeks ago I went out flying maybe 4-5 flights. All was 

fine. When I got to the flying site on Friday for the Lake City 

contest, I made a practice flight and the plane was climbing like mad. 

I got it down and found that 1 of the elevator halves was about 3/8" up
  

from the other. Before the flight, I checked the operation of the 

elevators and they were moving correctly but I don't remember 

specifically if I paid any attention to if they were centered 

perfectly. Now, nothing was changed between the the last time I flew 

and this flight. The transmitter was on the right model. The linkage 

had not slipped, nor had the control horn bent. The control surface 

showed no sign of trauma from a severe bump possibly causing a stripped
  

gear. The servo (JR 3421sa) was functioning 100% normally it was just 

the center had changed on it's own. I removed the servo arm (alum - not
  

stripped either) and moved it one spline and adjusted the sub trim 

slightly and flew a few test flights and the contest with no problems. 

Now at home, I removed the servo to send it back and inspected it and 

found nothing abnormal. The gears all look fine. Here is what I have 

eliminated:

  

servo arm - not stripped

servo gears - not stripped

pushrod and linkage - no play and not slipped on the clevis threads

transmitter - correct model selected and could not be elevator pot in 

transmitter since it did not affect the other elevator servo's centering

  

I am sending it back for a check up but just was wondering if anyone
has 

had any similar experience ever or if they have any possible causes.

  

Thanks

  

Chris

  

  

  

  

  

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