Last updated: 17 December 2003
Date: Tue, 16 Dec 2003 13:01:25 -0500 (EST) From: Don Sutherland (firstname.lastname@example.org) 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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