ETX125 AZIMUTH and ALTITUDE REPAIR

I was having a lot of sloppiness in my ETX-125. It is one that I bought second hand and worked well but both the azimuth and altitude lock knobs needed to be tightened very hard. Even so when the scope was set up and locked in home position (horizontal), I could move the end of the tube about two inches! After reading many articles I dug into correcting the mechanics.

When I opened the base to check the azimuth mechanism, I found that all three screws that hold the brass drive gear housing were loose. And just like in all the other articles I found grease in the clutches. I made a few attempt to remove the azimuth clutch but I did not want to shear the wires running through it. I finally succeeded by doing the following. . .

  • Loosen the azimuth lock knob
  • Remove the knob from the hex bolt using a 0.050 Allen wrench.
  • Use the hard case to support the forks and the base up side down.

Note: I also used a pillow to support the battery cage to prevent breaking the wires.

Also I use a screwdriver with a small handle to give me very little leverage and prevent stripping a screw mount and breaking one of the plastic bosses.

  • Unplug the wires that go through the hollow shaft from the terminal block (leave the wires for the azimuth motor attached)
  • With one hand on the delicate wires and the other hand on the azimuth lock knob hex bolt, gently unscrew the hex bolt by hand – DO NOT USE A WRENCH !!! - at the same time check the free-play of the altitude control wires that run through the bolt by gently pulling them up and pushing them back down in the hollow bolt.
  • Unscrew the azimuth lock knob hex bolt about ¾ to 1 revolution checking to see if the threaded clutch disk will come free.
  • Continue to check the free-play of the altitude control wires – STOP if they bind up at all and check that they can be pushed into the hollow bolt.
  • When the threaded clutch disk will come free you can then clean the clutch.
  • I use electrical contact cleaner that you can buy at the auto parts store, it is designed to not harm plastic, alcohol is also a good choice.
  • Cover the electronics and gear train with a cloth to keep them clean.
  • Take a small piece of 220 grit wet dry sand paper and roughen the clutch disk with scratch marks going from center to the outside.
  • Clean everything well before re-assembly.
  • Assemble in reverse order
  • Put a small drop of “thread locker” on the mount screws. Tighten the three screws that hold the brass drive gear housing putting a little pre-load the gears.

Problem solved !!!

Next I turned my attention to the altitude drive

As I mentioned at the start, I could move the end of the scope 2 to 3 inches when the altitude clutch was locked.

When I opened it up I found that two of the three screws were not just loose, I could not tighten them at all. The plastic bosses under the screws were not just stripped, they were broken and destroyed. One of them had just ¼ of the boss remaining and it broke off as soon as I removed the gear box, the other one at least had ½ of the boss remaining.

It took a little (well OK a lot) of head scratching to figure how to repair this one. Something fast, simple, and available to the every day fixer-upper. I don’t have access to the lathes and mills of days gone by when I worked in a machine shop.

I needed a way to form a new boss. And make it stick and stay there !!!

  • Get a ¼” wooden dowel – I cut one to about 3” in length
  • Take a ¾” x 2” strip of aluminum foil and fold it length wise so it is 3/8” wide.
  • Wrap the foil around the dowel as shown

  • Roll the bottom end over to form a base a flange as shown on the right. Be sure you do not reduce the ¼” through hole at the flange base
  • Put this “form” in place where the broken boss needs to be replaced. They can be put were the missing boss is and also over the ½ remaining boss.

  • Mix some epoxy to fill the “form” – I used JB Weld. Take a popsicle stick and cut the end so there is a pointed end. Mix the epoxy thoroughly. I also used a hair dryer to warm the epoxy and make it a little more thin and start the cure.
  • Use the pointed end of the stick it drip the epoxy into the “form” about ¼” high.
  • Go eat dinner or find something else to do for a while.

You might ask how do you locate the screw hole now that the epoxy has covered all evidence . . . I will get to that a little later

  • After about an hour check the epoxy, I found that as it cures it gets quite thin and it ran down the screw hole and the height of the epoxy was much too low.Also the epoxy put a small drop on the foam in my hard case.
  • This was a very good “mistake” because when I wiped the semi cured epoxy from the dial side of the trunion (scope arm) I saw that the existing holes were the perfect location to drill new holes for mounting.
  • Mix another batch of epoxy and fill the form up to about ¼” again.

Note: If I were to do it over again I would put masking tape over the screw holes on the dial side of the vertical arm and mix the epoxy once.

  • Peel of the foil and check that the boss has been formed as desired.

The bosses must be cut to flush or below the surface of the steel gear support as shown in this picture

The gear box will rest on the short metal tabs

  • Take an old hack saw blade and break it to about 4” long. Use it to cut the top off the new bosses. I used the small metal tabs as a guide to set the height of the boss.
  • Vacuum all the little dust and debris form the gear area.
  • Ensure the height of the boss is the same elevation or below the small metal tabs. You can use the back of the hacksaw blade to verify the epoxy is not higher than the metal tabs.
  • Measure the root diameter of the mounting screws, this will give the required pilot hole size. I used a 3/32” drill or 0.0935”
  • From the opposite side of the vertical arm, drill the pilot hole for the screws. It is very important to drill exactly perpendicular to the dial face. If you are accurate the screw hole will be in the perfect location to mount the gear box.

I told you earlier that I would answer how to locate the screw hole after the epoxy covered all evidence . . . I am glad I made that little glue mistake where the epoxy level dropped low.

  • Assemble in reverse order
  • Put a small drop of “thread locker” on the mount screws. Tighten the three screws that hold the brass drive gear housing putting a little pre-load the gears.

Problem solved !!!