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EXT-90
Declination Knob
Fix

This article may be of use to owners of the Meade ETX- series telescopes -- this is the description of how I fixed a broken declination knob on my ETX-90.

The ETX scopes have motors and gear trains that drive the scope in declination (up or down) and in azimuth (right or left).  Each motor drives a gear train that contains a clutch mechanism -- loosen the clutch and you can move the scope by hand, tighten the clutch and the motors drive the scope.

Azimuth Locking Lever

The knobs that lock the clutches are a weak point in the design.  The azimuth clutch is locked by a lever in the base -- see this photo:

 Look at the photo -- see the red device?  It's a dual-bubble level that I use to level the scope.  Look behind the level and you see the azimuth clutch locking lever -- the silver thing with a round end and a lever.  To lock the azimuth clutch and allow the motor to drive the scope, you push this lever until it gets snug then give it just another slight push.  The problem is that this lever is not metal as it should be -- it's plastic.  Some Meade owners report that this lever has broken.  Consult Mike Weasner's Mighty ETX Site for tips on how to remove and replace a broken azimuth locking lever.

Declination Locking Knob

The declination clutch is locked by means of a big knob on the one of the fork arms that supports the scope -- as you stand behind the scope and face in the direction of the scope, the locking knob is on the right-hand fork knob.

The big knob in the lower left of this photo is the declination clutch locking knob.  To lock the clutch, tighten this knob.  The knob is plastic.  Inside the knob is a hole into which is pressed a brass stud with size 10-24 threads; the threads screw into the clutch locking mechanism -- tighten/loosen the knob, the clutch tightens/loosens.

The Problem

I do not know how or when this happened but one day I tried to loosen the clutch so I could disassemble the clutch assembly for lubrication and I could not loosen the clutch.  I tried tightening it and found that the knob simply turned and turned -- did not loosen or tighten.

The Diagnosis

I figured that one of two things was wrong:

1. The brass stud had stripped out of the plastic knob, or,
2. The threads in the clutch mechanism were stripped.

Either way, I needed to remove the knob to fix whatever was wrong.  I did this by slipping a stiff-bladed putty knife under the knob and prying outward while turning the knob.  This worked -- the knob broke off the stud -- the knob popped off onto the floor while the stud remained threaded into the clutch mechanism.  This is what it looked like after the knob popped off (broke off, actually).

This photo shows the brass stud that normally is pressed into the knob -- with the knob broken off, only the stud remains.  I turned the stud with a pair of pliers and the clutch released and tightened depending on the direction I turned the pliers.  This told me that the clutch mechanism was fine and that the problem must have been that the stud was stripped out of the knob.

I removed the stud and laid it on the knob for this photo.

Normally the stud would be pressed into the hole in the center of the knob.  I called Meade and ordered a new knob.   This is what a new knob looks like from the inside.

I put the new knob on my scope -- but -- I thought it would be wise to repair the old knob in some way that may be stronger than the Meade knob.  After all, pressing a brass stud into a plastic knob is really not the best way to do this.  The plastic is much softer than the brass so it's not a surprise if the stud strips out.  Also, the knob diameter is MUCH more than the stud diameter so, when you tighten the knob, you are putting a lot of force on the stud.

My Repair for the Declination Knob

This is how I fixed the broken declination knob.

1. Get some kind of 10-24 machine screw -- 1-1/2 inch long -- and a nut.  I used a hex head 10-24 machine screw.  You could get a flat-headed machine screw with screwdriver slot, don't have to use the hex head as I did.
2. Set the broken knob on a wooden surface or on a drill press plate, with the outside side down (with the Meade decorative logo).
3. Drill a 3/16-inch hole dead center in the knob, all the way through the knob.  The best way to do this is to drill from the inside out -- drilling down into the hole where the stud came from.
4. Put the machine screw through the hole with the head on the outside, threads on the inside. 
5. Tighten the nut onto the machine screw to lock it onto the knob -- apply Loc-Tite to the threads before installing the nut to lock the nut onto the machine screw.
6. Use your Dremel tool to cut off the machine screw so that only 5/16-inch sticks out from the inside of the knob.

Look at these photos.  The photo below is the inside of the broken knob that I modified.

This is the broken knob with my modification applied.  Lying in front of the knob is a spare hex-head machine screw and nut.  Note in the center of the knob that 5/16-inch of threads sticks out and the machine screw is secured by the nut, which sinks down into the plastic when it is tightened -- put a pair of pliers on the head of the machine screw and a pair of pliers on the nut and tighten, after applying Loc-Tite to the threads.

Below is the view from the outside.

Now -- purists may not like the idea of the hex head sticking out of the knob -- but consider this.  The machine screw will not break -- there is no way you are going to tighten the knob enough to break the machine screw.  If the knob ever strips loose, all you need to do to open the clutch mechanism is to stick an Allen wrench in the hex head and loosen/tighten it.

This fix should work on all ETX scopes -- ETX-60, -70, -90, -105, and -125.  I don't know if other Meade scopes have a similar arrangement but, if so, this may work on other models.

Back to my ETX-90 page.