Last updated: 17 December 2003

ETX Drive Noise

Date: Tue, 16 Dec 2003 13:01:25 -0500 (EST)
From: Don Sutherland (
Saw a complaint about noisy slewing, a problem which has annoyed me
since I bought my ETX-105 last spring. I have done a lot of testing and
finally, I have noise in the azimuth drive under control.

Unfortunately, it doesn't take much power to make a lot of noise. On
high speed, the azimuth drive on my scope sounded like a coffee grinder.
The problem is caused by a bit of play in the mounting of the azimuth
drive motor, so it can rattle around when it vibrates at high slew
speeds. Also, the azimuth drive gearbox is very solidly mounted to the
telescope base. There is a foam sheet and o-rings between the gearbox
and the telescope base, but the sheet of foam is so large that,
effectively, it is very stiff and cannot absorb the vibrations from the
motor. The noise from the motor is reduced considerably by shimming the
motor so it does not vibrate in its mount and by making the gearbox
mounting more compliant by using small pads of a softer foam, and nylon
mounting screws with rubber o-rings under their heads.

These modifications can be made for very little money, by anyone who is
not intimidated by the words "void your warranty". Following are
detailed instructions. These instructions may not apply to the ETX-90,
but probably do apply to the ETX-125, as the azimuth drive board in my
ETX-105 is labeled ETX-125.

First, loosen the azimuth lock and put the scope on its side on a table.
Remove the batteries so nothing gets fried by accident. Peel off the
three rubber pads and remove the three screws holding on the bottom
cover. Gently lay the bottom cover down, turning the base if necessary
so the wires to the battery box don't get pulled (I got tired of this
and replaced the stock battery wires with heavier gauge wires about 3"
longer). Remove the connector from the azimuth drive board, noting that
the wires approach the connector from the center of the board rather
than the edge. The connector is not keyed so it is possible to install
it the wrong way. At this point it is also worth noting that the color
code on the wires to the drives is red = negative, black = positive, the
opposite of what is usually the case.

Remove the three 4-40 screws holding the plastic gearbox to the base.
Note that the screw closest to the worm gear drive shaft is longer than
the other two. Remove the three 6-32 screws holding the worm gear
bracket to the base. There are three external tooth lock washers between
the bracket and the base. The 6-32 screws engage only a few threads in
the base. I replaced these screws with 1/2" cap screws, but any 6-32 x
1/2" screw would be an improvement over the stock screws. Note that the
holes are tapped to a depth of 16 threads or 1/2".

Gently remove the azimuth drive assembly from the base. Now is a good
time to check that there is no grease in the slots of the encoder wheel.
Remove the three screws that hold the halves of the gearbox together.
Gently work the gearbox halves apart, keeping the shafts in the encoder
side of the gearbox. These ends have flats that line up with
corresponding holes in the gearbox half.

The motor, with the encoder wheel, can be removed from the encoder side
of the gearbox by gently jockeying it around. No force is required.
Slide the motor out of the other half of the gearbox and put one layer
of scotch tape around the motor. Cut away the tape that covers the
ventilation hole in the motor case. Check the fit in the gearbox half
and add or remove tape until the motor slides in and out smoothly, but
without any play. Put the gearbox back together, making sure that the
shafts line up with their holes in the gearbox.

Put a strip of closed cell foam (about 1/8" thick by 1/4" wide by 1"
long) at each end of the foam pad on the base of the telescope. Remove
the three rubber o-rings from the base. Position the drive assembly back
in its place. I used silicone seal to hold the three lock washers in
place on the worm gear bracket. Install the three 6-32 screws. Align the
three holes in the plastic base of the gearbox with the three mounting
holes in the base of the telescope.

Use a 1/2" nylon 4-40 screw (with one of the o-rings under its head) in
the hole at the worm gear end of the gear box. Use two 3/8" 4-40 nylon
screws (with o-rings under their heads) at the other two gearbox
mounting points. Reinstall the electrical connector, carefully noting
the orientation (wires go towards the center of the board). Install the
batteries. Turn on the scope and operate the azimuth drive. Tighten the
nylon screws just enough to keep the gearbox from moving when changing
directions on high speed. Thread locking compound will keep the screws
from loosening. If the screws are tightened too much they will protrude
through the base and may hit an obstruction at some point when slewing.

Besides making the drive run quieter, this modification may also remove
some of the backlash from the azimuth drive train. Be certain to
calibrate the motors and train the drives (or after doing anything that
affects the performance of the drives).

Note that this modification is not applicable to the altitude drive, as
the altitude gearbox and mounting is completely different from azimuth
drive and, in my opinion, is a much better design. The altitude motor is
rubber-mounted and the gearbox mounting screws pass through nylon
bushings with o-rings at either end. The only thing I would recommend is
making sure that the gearbox mounting screws are not too tight. If the
screws are removed, make sure the shorter one is closest to the altitude
bearing, otherwise the altitude axis will jam.

The screws that mount the worm gear bracket to the arm of the telescope
are sheet metal screws, not machine screws. If they are removed, they
should be reinstalled with care to avoid stripping the threads in the
fork arm. Also note that there are no lock washers between the worm gear
bracket and the arm on the altitude drive.

I hope this helps,

Don Sutherland

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