ETX TECHNICAL TIPS Last updated: 6 July 1999

ETX-90EC DEC fix

Subject:	 Reducing dec axis backlash
Sent:	Sunday, July 4, 1999 11:44:49
From:	tietz@psych.ucsb.edu (Jerry Tietz)
In the process of replacing a broken dec axis bracket I discovered that
the dec motion backlash is greatly reduced if the thin metal disk in
back of the screw-on dec lock knob is removed. You might try this to see
if it works since it is so easy to do.

Jerry Tietz, Ph.D.
Psych. Dept.
Univ. of CA
Santa Barbara, CA 93106

From: Tony von Ruden (vonruden@netcom.com)

The right tube adapter failed on my ETX-90EC for the SECOND time yesterday, and exasperation being the true mother of invention, I think I up came up with a solution to fix the problem once and for all.

Last time it failed, the shaft sheared off at the edge of the press-nut. This time, mercifully, the press-nut simply came loose, leaving me with something to repair. The 1/4" hole that the press-nut fits into is 3/4" deep, although the nut itself is only 1/4" high, and with the knurled length only about 3/16" long.

After a trip to the local hardware store, I made myself a brass replacement that is 1/4" outside diameter X 3/4" long, with a 10-24 thread tapped all the way through. I cleaned the new threaded insert and the hole it would go into with isopropyl alcohol, then used 5-minute epoxy to glue the insert into place. I added extra epoxy to the bottom of the hole in hope that it would ooze up into the lower threads, adding some extra holding power.

The new assembly seems to work like a charm. The bond between plastic and metal is now at 4 times the area, and is adhesive as well as mechanical. The stress concentration caused by the edge of the press-nut has been eliminated, or at least moved down to the base of the hole, where the stress can be distributed throughout the face that supports the gear. I estimate the repaired part is at LEAST 10 times the strength of the original. Time will tell -- I'll keep you informed.

I'm including a photo showing the new new part next to the old, before insertion.

The photo also shows the clutch assembly, for the benefit of those who haven't yet had theirs fall apart. The cast iron drive gear is sandwiched between the black plastic right tube adapter (RTA) and a black plastic hat-shaped piece (HSP). The shaft with keyway and threaded insert (missing in photo) is part of the RTA. A white plastic bushing is in place over the HSP in the photo, but you can see the top, keyed edge, and the bottom face next to the gear. When you tighten the DEC clutch knob, you are pulling on the insert and RTA, and pushing on the HSP, squeezing the gear -- it's a simple friction clutch. The HSP has a tab sticking out of it that hits the limit stops molded into the housing. It is keyed to keep it from rotating with respect to the RTA.

Well, enough messing around with the hardware -- It's time to try upgrading my Autostar firmware. Thanks again for maintaining your terrific site!

Tony von Ruden

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