RIGHT TUBE ADAPTER FIX
Last updated: 27 August 2013
Subject: Reducing dec axis backlash Sent: Sunday, July 4, 1999 11:44:49 From: email@example.com (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 (firstname.lastname@example.org) 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
Subject: RE: Meade ETX-90/EC Right Tube Adapter Sent: Friday, March 19, 2010 07:46:12 From: Tim Magrino (email@example.com) I wanted to follow up and let you know that I was able to repair my right tube adapter following the procedures from your site with some slight variations. The whole thing cost me 48 cents in parts, which was a lot cheaper than the $165 that Meade wanted to repair the telescope. Since the shaft of the tube adapter was sheared off at the base, I had to drill a hole completely through the shaft and screw a 1/4 inch furniture fastener with a 8-32 screw hole into the broken end of the shaft. The furniture fastener screws into the plastic shaft with an allen wrench, so you have to secure the shaft somehow while screwing in the fastener. I held the shaft with a pair of pliers with a towel between the shaft and the pliers to prevent the shaft from getting damaged. I also had to trim off a triangular shaped piece of plastic from the inside of the tube adapter base to make room for the screw. I used an exacto knife to trim the plastic. That allowed me to secure the broken shaft back on to the base by screwing them together. I didn't use any glue because I was afraid of getting glue in the threads of the brass fitting on the other end. The repair seems pretty solid without using any glue. I bought the 1/4 inch furniture fastener and 8-32 screw at Ace Hardware for a total of 48 cents. I hope this information will be helpful to anyone else that needs to repair a broken right tube adapter. I am including a picture of the repair. Thanks for your help. Tim
Subject: ETX-90 Alt/Dec Clutch Repair/Enhancement Sent: Friday, April 30, 2010 08:18:23 From: Nohr Tillman (firstname.lastname@example.org) I bought my ETX-90 used, and it has exhibited not-so-good clutch clamping with the Altitude/Declination fork arm. Knowing this is a common thing, I disassembled the scope and arm to improve this clutch mechanism. The "clean and sand the surfaces" approach had been performed before I got the scope, and I found the plastic arm surface was not sanded uniformly. So the gear did not seat on the surface perpendicular. It did not have good grab at all even though the surfaces were clean and free of grease. Pictures are worth a thousand words so I'll let these two do most of the talking. In short, I trued up the plastic surface some with automotive valve grinding compound between the gear and the arm surface. After cleaning that off with alcohol and soapy water, I expoxied the overhead-valve spring shim (1.25 dia X 0.15 inch thick) to the arm. This further fixed the flatness and turned the clutch into a true friction clutch with the effective clamping surface at the outer diameter. It works very well with much less clamp load required to positively lock the arm to the gear. Hope this helps someone else too, Nohr Tillman Troy, MI, USA, Earth
Subject: ETX -90EC repair? Sent: Monday, August 19, 2013 13:55:06 From: Ned Thanhouser (email@example.com) Hi, I have a ETX-90EC and the vertical declination gear or pulley belt seems to be broken (I.E. no resistance or motor drive in this axis). Mead reports no support. Recommendation? Thanks. Ned Connected by DROID on Verizon WirelessMike here: It sounds like the Right Tube Adapter is broken. You may be able to repair it yourself. See the article "ETX-90EC DEC fix (Right Tube Adapter repair)" on the Helpful Information: Telescope Tech Tips page. Alternatively, you can contact Dr. Clay Sherrod for his Supercharge Tune-Up Service (link on the ETX Site home page).
Thank you so much...I shall endeavor to fix it.
Disassembly reveals the problem as described in the article you referred to below "ETX-90EC DEC fix (Right Tube Adapter repair)." The DEC shaft is sheared off at the edge of the press-nut. See attached images. The fix appears to be to just epoxy these two parts back together...yes? But I am very concerned about getting epoxy where it shouldn't and bugger the whole assembly. I don't need a new bushing as it is still secure, although Tony von Ruden's solution appears more robust with the longer bushing. Can you please confirm? Any where to get a new part? Thanks so much for your help!Mike here: Epoxy may work although there is a lot of torque there. You might try Telescope Warehouse (link on the Astronomy Links page) for a replacement RTA but I doubt he has any without purchasing a new mount.
Thanks for the suggestion, I'll keep you posted. Ned
And an update:
Hi Mike, thanks for all your helpsince the RTA sheared off flush with the original press nut (right in pix below), I was able to find a replacement 1/4" x 10-24 press nut (http://www.winkshardware.com/) that is 3/8" vs. 1/4" deep with flanges that fit into the RTA (with some persuasion) into the tube below where it had sheared off. That plus some 5 minute epoxy seems to have done the trickeverything works now and I've added a note on the instrument reminding me and others NOT to over tighten the clutch! Thanks below for your help and excellent site. Feel free to post.
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