LX50 Magellan II
Encoder Accuracy Improvement


Encoder slop seems to be a large amount of the accuracy problem in some of the Magellan units.  I discovered the large aluminum gear for the RA encoder is not precisely centered on the RA axis and when it rotates the clearance of the RA encoder gear varies from tight enough to flex the RA Encoder bracket to loose enough to show a change of 4-6 boxes on the Magellan display!!  The same problem exists on the DEC axis encoder as well but not nearly as bad.

A simple solution consisting of several springs and some new mounting screws seems to eliminate the play in the gears at all positions and improves the accuracy a bunch.

Quantity Vendor Description
4 Any 4-40 machine screws 3/4" long
4 Any #4 nylon flat washers
1 Serv-A-Lite #104 hook ended spring
2 Serv-A-Lite #71XA 3/16 diameter compression spring cut into 1/3's
1 NAPA #732-1078 GM Fuel filter spring

Both encoders get all mounting hardware replaced.  

Cutting the two  #71XA springs allows you the luxury of loosing 2 in the process of installing the RA encoder hardware.  :o)

The ACE Hardware stores all seem to stock the Serv-A-Lite hardware, everything you need should be available from them.

A small kit of appropriate parts is available for $20US shipped in the lower 48 states. 

Email me for details


Click on images for a larger view of images.

Side shot of the overall installation of the DEC encoder tension adjustment spring and spring loaded encoder hold down washer, spring and screw.   The hole that the long end of the spring is hooked into is a factory hole most likely used when the castings are intended for an LX200.  The strength of this spring is important, you want enough tension to move the encoder but not enough to flex and damage the encoder shaft  I use the Serv-A-Lite #104 ($1.30) spring for this purpose.


Shown here is the DEC axis encoder with the spring loaded encoder hold down screw.  The lower end of the tension adjustment spring is visible hooked over the encoder bracket.  The tension of the hold down spring is adjusted so the encoder can move when pulled by the tensions adjustment spring but not allow the bracket to wiggle in the DEC housing.  


Side shot of the RA encoder hold down spring. the aluminum RA encoder gear is visible in the foreground.  Installing the washers, springs and bolts can be a bit frustrating because the tension spring is installed on the encoder before the assembly in put into the RA housing.  Two people and a pair of tweezers is a good idea!   Like the DEC axis described above the adjustment needs to be set carefully.


The tension adjustment spring for the RA encoder is a GM QuadraJet fuel filter spring (NAPA # 732-1078, cost $1.62 for a pack of two), this particular spring has been tested for the correct tension as not to damage or wear out the encoder shaft bushings.  The spring gets taped to the housing of the RA encoder and the other end rests against the inside of the RA bearing housing.  In this top view only the end of the spring that rides against the RA housing is visible.  The spring gets taped to the housing of the encoder before installation.

  Make sure the gears are meshed when assembling the encoders, if they are not meshed you could damage the encoders or bend or break the gears!!



Copyright 1997-2010 by Bill VanOrden