[NSRCA-discussion] Servo question

[bill thompson]

Jon
I have some #4721 & #4131 are the pots adjusted the same as the #3421's?
Bill T.
  ----- Original Message ----- 
  From: Jon Lowe 
  To: nsrca-discussion at lists.nsrca.org 
  Sent: Saturday, July 05, 2008 2:55 PM
  Subject: Re: [NSRCA-discussion] Servo question


  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 <cjm767driver at hotmail.com>
  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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