LX-200 Declination Motor Assembly
This page is dedicated to the details of the declination drive assembly of the Meade LX-200. All pictures and descriptions apply to my 10-inch LX-200 classic.
In June 2002, I took apart my declination drive assembly in an attempt to reduce retrograde motion in my declination axis. Although I was not entirely successful in my attempt, I did learn a lot about my drive. Below are some photos of my dec drive and descriptions of what I learned. I hope this information will be useful to you.
At the bottom of this page are some links to other useful information on the LX-200 declination drive.
Below is a picture of the declination drive assembly with the plastic cover removed. To remove the cover, you need to remove the three small screws and completely unscrew the dec clutch knob. Then remove the cover. In this picture, I have removed the plastic cover but screwed the clutch knob back on.
There are two 8x32 cap screws that hold the dec drive assembly in the fork arm. They can be removed with an allen wrench. I had to put the long end of the allen wrench in to reach the head of the screws and loosen it with a pair of pliers gripping the short end of the allen wrench. Below we see the fork arm with the dec drive assembly (and the clutch knob) removed. Of course, you must also disconnect the drive cable that is attached to the dec drive assembly. It plugs into a connector on the inside of the fork arm (just opposite where you plug the dec drive cable into the outside of the fork arm). Press down on the plastic tab and disconnect the plug. Then you can pull the cable and plug out with the entire dec drive assembly.
Note in the above photo that the screw holes had Helicoils installed in them (you can't make them out in this photo). This should not be! Helicoils are normally used to repair stripped bolt holes. Helicoils are made of spring steel and look like coil springs. When a bolt hole is stripped, the hole must be drilled out, tapped, and then the Helicoils screwed into the hole using a special insertion tool. After this is done, the bolt then screws into the Helicoil. A bolt hole repaired with a Helicoil should be as strong (or stronger) than the original bolt hole, but mine should not have come from the factory in this condition. Apparently the bolt holes were stripped by Meade and repaired using Helicoils at the factory. The problem is that one of the Helicoils unscrewed out of the bolt hole when I removed the screw. Once a Helicoil comes out, it must be discarded and replaced with a new one. Since I did not have a spare Helicoil on hand (who knew?), I had to order a replacement set (from McMaster-Carr). This delayed reassembly by several days while I waited for the replacements.
The fork arms are made of soft aluminum, so stripping out the screw holes is easy if you put too much torque on the cap screws when tightening them. If you strip the threads, your only option is to put in Helicoils.
The photo below is the declination drive assembly itself (top view). The worm is part of the "carriage" that "floats" on the hinge. The lower part of the assembly is firmly attached to the fork arm (using the two cap screws discussed above). The carriage is free to rotate on the hinge. The reason for this is that the worm gear (the large toothed gear visible in the photos above) is not perfectly round. If the worm's position were solidly fixed, it would tend to bind when the clearance between the worm gear and the worm was too tight, and might slip when the clearance was too great. A small spring under the carriage maintains a steady force that pushes the carriage with the worm up into the worm gear.
It is imperative that the worm and the hinge have no excess clearance, otherwise the carriage and/or worm can flex and cause unwanted motion, especially when reversing directions in declination.
Below is yet another view of the dec drive assembly. In this photo, the the carriage has been rotated up on the hinge to get a better view of the spring and "stop screw". The stop screw threads through the small block of aluminum that the spring fits into. The purpose of the stop screw is to limit how far down the carriage can move. Under some conditions, the spring force might not be sufficient to keep the worm and worm gear engaged. Under these conditions (high loads, for example), the teeth of the worm and worm gear might disengage and slip (not a good sound, I'm sure). The stop screw should be set for a clearance of 0.5 mm from the bottom of the carriage at the point where the worm gear pushes the carriage down the greatest. As far as I can tell, the only way to determine this is to install the dec drive assembly in the fork arm and rotate the worm gear all the way around, carefully observing the carriage to see the point at which it is pushed down the greatest distance. If the carriage contacts the stop screw, it will result in binding between the worm gear and worm. This will cause the motor to overload or stall. After the proper clearance has been set, the stop screw should be Loctited (glued) in place to prevent it from backing out on its own. (It should come Loctited from the factory, but *sigh*, mine was not.)
Below we see an end view of the dec drive assembly. If the hinge end screw is loose, it will allow side to side play in the hinge (toward and away from you in the view in the picture below). This screw presses against a ball bearing, which in turn presses against the hinge. The end screw should be tightened just enough to eliminate any play. The end screw should be Loctited in place, so it may take considerable force to turn it. Make sure not to damage any other components if you must apply a lot of force to this screw to move it.
The worm end screw performs a similar function. It should be tightened just enough to eliminate side to side play in the worm itself. If you tighten this end screw too much, the motor will have difficulty turning the screw. What I did was temporarily connect the dec drive cable to the scope drive and powered it up. You can tell by the sound of the motor and by observing the ammeter LEDs if the motor is straining while slewing in declination. I tightened the worm end screw just enough so that there was no sign of the motor straining, no more.
Below is yet another view of the dec drive assembly (and part of my hand as well).
With everything snug and Loctited, it's time to put the dec drive assembly back in the fork arm. Feed the drive cable through the fork arm first, then carefully put the drive assembly in place. You can turn the dec drive knob back and forth slightly to move the entire assembly from side to side in order to get the bolt holes in the drive assembly lined up with the bolt holes in the fork arm. Then tighten the two cap screws to firmly hold the drive assembly in place. Remember that the bolt holes are in aluminum, so don't over torque, or you will strip the bolt hole threads and end up having to put in Helicoils (like what I received from the factory, boo, hiss).
Below is a picture of the dec drive assembly reinstalled in the fork arm.
The pictures below were taken by Paolo Gramigna, who graciously gave me advice and guidance when I was struggling with my dec drive problems. Paolo made some modifications to his assembly to allow him to make adjustments to it without having to remove it from the fork arms. In fact, he says he is able to make these adjustments in the field in order to get the smoothest motion from his dec drive.
The photo below shows how Paolo drilled a small hole in his fork arm to allow him to insert an allen wrench. He apparently uses the allen wrench to tighten the hinge end screw. This allows him to keep the play on the hinge to a minimum under all temperature conditions.
In Paolo's next photo, he shows how he uses a piece of rubber tubing to adjust a new carriage stop screw (which he added). He drilled and tapped a new hole near the "factory" stop screw, then installed his own stop screw. The end of this new screw is just accessible, and he uses the rubber tubing to tighten it to just contact the carriage when he is taking a photo. This keeps the worm firmly against the worm gear. Since there is very little dec motion while taking a photo, binding is not a problem. However, he must remember to back off his stop screw before slewing in declination.
Paolo's modifications are, of course, completely optional.
I would like to thank the following people for their suggestions, advice, and
guidance with my dec drive assembly problems:
Here are some links to other information on the LX-200 declination drive
Declination Drive Adjustments (without rebuild)
A VERY thorough discussion of dec drive issues by Doc G
Rebuild of the declination bearings (not for the faint of heart)
I certainly hope this information is helpful to you. If you see any errors, or if you have any comments on this, please e-mail me,
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