ETX125 Dec Worm Drive Mount Repair - Comprehensive

by Ken Toliver (

This guide is for the older models of the ETX that used plastic forks.

My ETX is an ETX125EC, circa 2001, that I added the AutoStar #497 to shortly after purchase. In 2005 I had it SuperCharged by Dr. Clay, a service that I highly recommend to anyone that is hesitant to dive into the inner workings of this little scope. Over the last 15 years it had degraded in GOTO performance and was having binding and slippage problems in the Declination train. As it was approaching 20 years old, I decided I’d take a shot at repair.

This particular ETX125 is notorious for problems with flexure, slop, and backlash in the Declination assembly. If you are experiencing these issues, then I hope this guide may help you. Unfortunately, parts are VERY hard to come by and may force you to work with what you have.

In order to ensure that you address all possible issues, you need to disassemble the entire Declination control assembly:

1)     Lock the OTA roughly horizontal. Working from the rear, remove the OTA by removing the 4 Allen head screws from the rear of the left and right-side OTA Swing Arms. While supporting the OTA, carefully spread the rear of the arms while pulling the tube toward you and slide it out the back.

2)     Remove both the Declination setting circle and the Clutch knobs

3)     Remove both OTA Swing Arms by sliding them inward and out of the forks

4)     Remove the clutch side inner fork cover by removing the 5 screws, exposing the worm drive and motor

5)     Remove the 2 mounting screws that hold the motor box to the fork; there is one at 12 o’clock and another at 3 o’clock. It will slip off the bottom gear of the worm drive easily. Set it on the base while being very careful not to break the soldered wires from the circuit board.

6)     Remove the worm gear by removing the 3 mounting screws that hold the aluminum cast body to the fork

Upon inspection here is what I found:

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As is evident, the mount was toast. One of the 2 plastic tabs for the fine adjustment Allen head screws had sheared off, and 2 of the 3 worm gear mounts were split and broken. It is a miracle that more severe damage to the motor assembly had not occurred.

I removed the 3 screws that hold the steel reinforcing plate to the fork. I then took some needle nose and pulled off the broken pieces as much as possible. A Dremel with a sanding round allowed me to carefully grind down the stubs pretty close to flush. I finished with some 120-grit sandpaper. This is what I ended up with:

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Repairing that cracked plastic tab is important; those little Allen screws in the aluminum body of the worm gear push against them to provide a fine pressure adjustment to finely tune out backlash from the worm drive. I fixed this by using J-B Weld WaterWeld moldable epoxy. Credit here goes to Paul J. Boudreaux, “Fix for ETX125 worm gear adjustment”, on this site.

With those fixes done, time to reassemble using better components. The steel reinforcing plate is secured to the fork arm by three tiny little #4 self-tapping screws. The fork plastic might be 3/32” thick, if that, and provides very little holding power for those screws. I replaced them with (3) 1/4“, #4-40, pan head screws and nuts. Per recommendation from Dr. Clay, I do not suggest reaming out the fork holes with a drill bit so the screws slip through them; just thread the screws into the holes until the plate is tight, then add the nuts to the back with a little dab of LockTite.

Similarly, I replaced the 3 worm drive mounting screws with 1/2“, #6-32, pan head screws and nuts in the same manner. The only difference here is that I needed to replace the two standoffs that had broken. I used Nylon Round ABS Spacers 5MM height for M3 screws. I did ream out the spacers with a drill bit so that the screw could slide through, but not the holes through the fork. Again, better to thread through the fork plastic to provide a tight connection. Here is what the final configuration looked like (note the white J-B Weld fix to the plastic tabs in the background):

Now on to the OTA Swing Arm repairs. These plastic parts are subject to stress cracking due to the torque from the locking knobs for both the Declination setting circle and the clutch (left and right knob, respectively, as seen from the back).

Use soapy water to thoroughly wash and remove grease from all the Swing Arm components; both OTA Swing Arms, the gear and keyed washer, and the Clutch Block (round keyed trunnion bushing that slides against the gear and washer). You may need to use a toothbrush and electronics degreaser to get the gunk out of the gear teeth.

Now inspect all of the plastic components. In my case there were stress cracks in both OTA Swing Arms near the #10-24 threaded bushings, especially the arm for the Declination setting circle. It was clearly only a matter of time before this would fail. When tightening the knob, it spreads the trunnion in the fork and adds friction and drag, contributing to OTA “jump” or binding when trying to change direction while slewing in declination up or down. In the photos below, you can see the radial stress cracks at the bottom from the rear. Also pictured is the “after” repaired swing arm:

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To fix this I added a 3/8“, #10-24, flathead screw with several washers in the back, then filled the entire well with Loctite Epoxy Five Minute Instant Mix. This will ensure the brass sleeve can never be pulled out the other side and bonds the entire trunnion into a single piece. I also brushed a little SuperGlue on the visible surface cracks. After it dried, I used extra fine steel wool on the trunnion to smooth any edges from the stress cracks and allow it to move freely in the fork. A WORD OF WARNING: Be sure that you use washers to ensure the screw just barely engages the threads in the back of the brass sleeve. Dry fit it to the fork arm several times to be sure that your Declination setting circle knob can screw in sufficiently from the other side to clamp and hold the circle. If it bottoms against the new screw, back the screw out some and add another washer. Be careful; once you epoxy this there is no going back. Since I had the Five-Minute Epoxy open, I filled the back well of the clutch side OTA Swing Arm at the same time.

The clutch side OTA Swing Arm had far fewer stress cracks but had other problems to be addressed. The Clutch Block is a keyed bushing that slides over the keyway in the OTA Swing Arm and holds the gear and washer in place. It is designed to be a press fit; that means it should hold firmly when pressed against the gear washer; however, mine had developed a little play in it. You can test this by sliding it in place and then trying to turn the block by grasping the stop as is shown below:

To fix this I had to get creative. I took a 3” length of Teflon tape and cut it lengthwise with a sharp blade to about 1/4“ width. Wrap it twice around the far edge of the post and then make a single cut above the keyway pushing the tape down into it to secure it. It took a few tries, but this is what it looks like:

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Now carefully place the block onto the post and push it all the way on. YES, it will bunch a bit and resist as a result of the slit in the tape, and if it goes on funny just take it off and try again. On my second attempt the result was a rock-solid clutch block with absolutely zero play.

Time to put it all back together. I used SuperLube for grease; it is a silicone-based grease with PTFE and excellent heat characteristics. Put a little dab on your finger and spread a light film inside the fork holes. Use whatever you have left on your finger to lube the trunnions even lighter; chances are what you put in the fork holes was more than enough. You don’t want any excess. I had already lightly lubed the worm drive with a little flux brush before re-installing; it was just easier. The gear is the important area to be really careful; once you lube the worm drive there is likely enough for the gear. I used a toothpick and laid a very light line of lube around the centerline of the gear teeth avoiding the edges entirely; the centerline is the only place the worm drive contacts it. If you put any more on, it will work its way to the edges, behind the washer, and between the gear and the clutch surface. Over lubrication of the clutch side gear causes the clutch to slip, further leading to users over tightening the clutch knob, leading to breakage. This will cause stress cracks, pull out the threaded insert, or even break the trunnion loose from the OTS support arm. I am not going to go into more detail on how to lube but refer you back to this site. “Part 4 - Eliminating Common "Rocking" in ETX Altitude Axis”, by Dr. Clay Sherrod, provides excellent tips on this most common problem that leads to failure. Perform the fixes outlined therein.

The final adjustment goes back to that worm gear and those 2 Allen screws that are used to fine adjust the backlash on the OTA Swing Arm gear. Re-assemble everything without putting the OTA back in place. Hand turn those swing arms back and forth multiple times to work in the lube. Then lock the clutch (it should require less effort than before, especially if you followed Dr. Clay’s tips). Gently rock the locked OTA Swing Arm to see if you find any movement (backlash) from the teeth of the gear not having enough contact with the worm drive. If there is, unlock the clutch, remove the OTA Swing Arm, loosen the 3 support screws of the worm drive block, then adjust the tension of the worm drive against the gear using those 2 Allen screws. This is a fine adjustment; don’t rush it. The backlash can be eliminated, but you don’t want it so tight that the motor will stress trying to turn it and potentially live a short life. Once you re-tighten the 3 support screws and place the gear back against the worm drive, you may find that the tension has changed requiring another adjustment. Work in 1/16 to 1/8 turn increments re-checking the tension each time by reassembling and repeating the backlash test by hand. I must say, a standard Allen wrench did NOT clear the worm drive to get into the head of those tiny screws. I had to take my trusty Dremel tool and grind off the short end of my Allen wrench to about 1/4“ to get it in.

Once you are satisfied with the backlash, run the Dec motor back and forth several times from stop to stop to spread the grease on the gear, then pull it back out and take a look. If you have any excess grease reaching to the edges of that gear, take if back off, de-lube the parts again, and start over until you have minimized that grease. Otherwise trouble will ensue.

Once finished with all these repairs, my ETX is now rock solid in Declination. Yes, there is a little rocking of the OTA, but that is simply inherent flexing from the substandard plastic forks. The slop, binding, and backlash is gone. One more breath of life has been breathed into The Mighty ETX!

I want to thank Dr. Clay Sherrod and Mike Weasner for their ongoing support. Fifteen years later they are still responsive and supportive of the ETX community.


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