TECHNICAL TIPS Last updated: 3 March 2002

Fix for ETX125 worm gear adjustment

Sent:	Friday, March 1, 2002 16:18:27
From:	boudreau@eng.umd.edu (Paul J. Boudreaux)
I've had my ETX125 for nearly two years with little trouble until the
past few months. I noticed that it had problems in the RA and  the Dec
lock showed some up/down wobble. I have never had the courage to open
this telescope, but after reading the many references on your site,
(Jordan Blessing's ETX/EC Tune up, ETX/DS Technical Tips, Bob's LX200
Web Page :Supercharge your ETX, and many of Dr. Clay Sherrod's web
postings) I decided to find the problem. After removing the OTA, and the
Dec knob and clutch, I tried to remove the inner side panel by taking
out the five screws. To my surprise, it would not come off! After
carefully prying it around the edges I found that it had been glued down
along the bottom edge. When I removed the panel, the inside of the side
arm appeared to be covered in large black particles about the size of
coffee ground. At first I could not locate the source of these
particles, then I saw (by comparing photos posted on your site and
others) that the two Backlash Adjustment screws in the base of the worm
gear housing were just sticking out into thin air. There was no trace of
the plastic tabs that they were supposed to press against. In addition,
one of the three sheet metal screws holding the worm gear assembly was
just hanging in air with nothing to thread into. The second screw in the
worm gear assembly was loose while the third screw was still tight and
held the worm gear housing in place.

Apparantly the plastic base that these sheet metal screws tapped into
had shattered and that allowed the worm gear assembly ro rock against
the two plastic tabs destroying them in the process. To repair the
damage I first reconstructed the broken and partially missing tabs by
using "scotch tape" and children's modeling clay to form a sealed dam to
form each tab. A flat edge piece of wood acted as a form to insure that
both tabs that would be reconstructed would be planar and parallel to
the old tabs positions. I thought the real problem would be to determine
the correct height that the worm gear housing had to be away from the
side wall. Then I observed that although the original sheet metal screws
attached into plastic cylinders that also could act as spacers, they
realy did not matter because the metal back plate has three metal tabs
that actually support the worm gear housing at the proper height. That
enabled me to replace the sheet metal screws completely because there
were three 4-40 body holes in the base plate where the plastic sheet
metal screws had attached. I used three 4-40 stainless steel pan head
bolts to replace these screws. Pan head bolts are required instead of
round head bolts because they ensure that there will be clearance
between the bolt head and the Aluminium Dec wheel cover and lock Knob on
the Dec clutch. The pan heads are shallower than the four notch tabs
that press against the inside of the Dec wheel outer hub. A lock washer
and a 4-40 lock nut completed the repair of the worm gear housing mount.
Following instructions from your site, I adjusted the gear mesh with the
adjusting screws, now that there were tabs for them to press on.

Back in business! No more Dec wobble - things are tight as a drum once
again. Thanks, Mike! Without the help of your site and its many
contributors, I would never have ventured into the world of ETX tune up
and repair!

I attached a JPEG image of the epoxy tab repairs.

Paul Boudreaux

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