TECHNICAL TIPS Last updated: 16 January 2001

ETX-125EC Backlash Tuneup

Came back from a few days in England and decided to have a look inside my "newly repaired by Meade" 125 and see what was going on.

You may recall that after receiving it back from Meade I had tons of az backlash, so much so that when training the drive it sometimes didn't even move off the target object! Removing the baseplate and driving the motors the cause was soon evident. The entire fork assembly that holds the worm gear was rocking back and forth when the motor was driven. In part I could prevent this by tightening down the 3 screws that hold this assembly down (two of them are so inaccessible I can see why the heads have been reported as damaged by some people). However I also noticed something else which AFAIK is unique to the 125, or at least I didn't notice it on various pictures of the 90. The fork assembly also features two allen headed set screws which bear on two metal "ears" which are part of the base (I can provide pictures to overcome my inadequate descriptive text). Their function would appear to be to keep the worm gear closely meshed with the gear it drives. However in my case these ! set screws were not in contact with the ears, thus allowing the rocking motion to occur. The solution was to tighten down the set screws until the rocking motion stopped. A quick test since then showed that for the first time I was actually able to GO (back) TO my first alignment star and find it in the EP. Subsequently I was able to compare my scope with a new unit in the local store, the new unit had those set screws mashed down very tightly indeed - to the point of actually deforming the metal ears!

There does still appear to be some residual backlash which I think is inherent in the clutch itself - I can see the gear wheel start to move (about 2 teeth) before the forks themselves start to turn. However I think this is "normal".

I also called Meade again and asked how they could ship the scope back to me in such a state. No real answer was forthcoming but Scott Byrum did give the official blessing to my fix.

Unfortunately not much has happened since then due to bad weather and me getting a herniated disc - I'm afraid astronomy will have to wait until I can stand and walk without hurting any more.

Steve

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