ETX-125 GEARBOX REPAIR - IF ALL ELSE FAILS... Last updated: 31 August 2010

Sent:	Monday, August 30, 2010 10:52:16
From:	M. H. (mark2382 (AT) hotmail.com)
My problem was pretty common -- the support arm that holds one end of
the worm so that it engages with the gearbox broke. And when that
happens, the large gear on the end of the worm disengages from the
gearbox and the scope starts clacking and stops moving. After breaking
two of these precious boxes, even after trying the carbon
fiber/fiberglass suggestion in the article "RA Gearbox Repair", I
decided to take a more radical approach and machine a replacement arm
out of metal.

NOTE: If you do not have a milling machine, lathe, machinist caliper and
a very well stocked set of drills including a "Z" drill as well as
drills > 1/2 inch, I would probably not recommend doing what I did.
Getting all the gears to align is something that is probably beyond a
normal drill press operation.

Here is what I did. 1. I carefully measured the distance to the center
of hole for the long gear axis and the center of the hole of the support
arm. I also measured their diameter so that I could choose the right
drill for both holes later. Make sure you do this, and double check your
work! I was able use a sharpie to blacken the bar and then use a scratch
all to locate these critical points.

2. I then took a deep breadth and cut off the remains of the support
arm.

3. Next, I machined a small block whose thickness was sufficient to
allow me to attach a piece of flat 3/4 " aluminum stock in the same
position as the original support arm. I super glued this block on its
bottom, and on the back and front edges. See photo "mountingblock.jpg"



4. I then machined a new support arm. See photo "Bearing.jpg". 



Note that I added a nylon bearing here to avoid having the large gear
run on a metal surface. This was simply a nylon bearing that I got from
an unusually well stocked hardware store. I drilled the bearing out to
fit the gear and use a cutoff tool on my lathe to cut a grove to hold a
spring clip. Before assembly, I filled the bearing with lithium grease.
The photo "wormside.jpg" show the support arm mounted on the gear box as
seen from the worm side.



Note that you can see the hole where the metal axis for the long
horizontal gear of the gear box is supported.

Again, all these holes take some precision and probably can't be done in
a drill press unless you have an x-y table.

The photo "assembledOutsideScope.jpg" shows another view after I test
assembled it.



5. Photo "Installed.jpg" show the repaired box in the ETX-125. 



Note that the 4-wire connection on the right need to be bent slightly
out to clear the support bar. I also considered adding some type of
brace between the support bar and mounting pole to its right. I figured
that this would eliminate any torque transfer from the support bar back
to the gearbox. But I am going to wait and see if it is necessary.

6. I spent some time getting all the angles to be 90 degrees and to
ensure that the large gear on the end of the worm meshed straight and
deep with the long horizontal gear that is in the center of the photo. I
ended up having to put a .040 inch shim between the support bar and the
mounting block to get the angle right.

So far, so good. I can't see any visible movement in the bear box when I
switch directions on the scope. And it is much quieter. I am a little
worried whether that superglue will hold down that mounting block or
not. If it fails. I will probably figure out how to put a screw into it
and add a brace between the support bar and the mounting pole.

Again, this is pretty much a one way street. If you go down this path,
there is really no return....

Mark

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