LX-200 GPS Disassembly and Repair of the Right Ascension Gears
By K.C. Kile, Sr.
Two years after the purchase of my LX-200 GPS I began to have slippage on the R.A. slow motion control at various points of rotation. As there was no information available from publication on the Internet and the telescope was out of warranty, I was hesitant in doing or having repairs made. This was a mistake as things only became worse, and eventually the R.A. rotational stops failed. Realizing that this would cause subsequent failure of the wiring due to excessive twist between the base and the fork arms, I had no choice but to either send the scope back to Meade or attempt repairs myself. Due to shipping and potential repair costs, I chose the latter.
As major disassembly of the base was called for, I decided to simultaneously perform modifications to prevent wire wrap from equipment and power leads such as a laser pointer, dew remover, and focuser attached to the OTA. I also had to replace the declination locking gasket, and repaired the locking mechanism, which failed shortly after the R.A. repair. For the sake of brevity and document size, these latter items will be covered in a separate article.
The first step is to lock the R.A. and Dec. so that the OTA is perpendicular to the fork arms and place the cover on the objective; as if putting the telescope in the alt– az park position. Then remove the telescope from the tripod, placing the covered objective face down on a firm but padded surface, and solidly support the lower portion of the fork arms with something that will still allow the base to rotate when unlocked. Next remove the 6 hex head screws retaining the base cover plate. This will expose the two wire bundles from the fork arms coming through holes in a central pivot point, the R.A. rotation sensors, circuit boards, and the R.A. rotation motor as seen in Figure 1. Disregard the additional wiring seen in the diagonally running main bundle, as it pertains to the previously mentioned modification to supply power and prevent wire wrap on OTA mounted devices.
The next step is the removal of the R.A. rotation motor. This can be somewhat difficult as the right hand screw, requires a very stubby Allen wrench, one with a universal joint, or one that is designed with a small ball tip for turning Allen head screws at odd angles. See Figure 2 for screw location.
Figure 3 shows a better overall view of the base with the R.A. motor released.
The third step is removal of the R.A. rotation sensors which are located on a bracket that secure it to the base with two screws. These sensors must be able to be restored to position with some degree of precision as they are optically aligned. First, scribe the outline of the bracket into the base with a sharp point such as a needle, and then outline the retaining screw heads on the bracket with a black permanent marker as shown in figure 4.
This set of reference points will allow repositioning with good alignment on reassembly. The sensors are placed between two rotating semicircular plates that are each attached to their respective halves of the base and must be aligned to allow clearance before sensor removal. This is accomplished by unlocking the R.A. slow motion control and rotating the knob if possible or otherwise the base, while observing for maximum sensor clearance. Once obtained, relock the mechanism, remove the retaining screws and place the sensors with attached wiring in a less sensitive working location. Now on the opposite side of the base, remove the 5 hex screws holding the center cover plate between the fork arms, exposing the single wire bundle that passes through a central retaining nut and branches to each fork arm as seen in Figure 5. The protective sleeves on the wiring to the fork arms were added during reassembly.
The retaining nut contains 2 holes; one on either side of the centered wire bundle, which are used for unscrewing counterclockwise. Place permanent marker reference points on the base and nut for use during reassembly. Lacking the proper tool for loosening the retaining nut, I found that the chuck end of two ?” drill bits will fit these holes nicely as shown in Figure 6.
By placing a length of wood or metal on either side of the bits and rubber banding them together you can create the tool needed for rotating the nut. The variations for making this tool are only limited by imagination and available material. Try to keep the physical pieces and rotational torque as close to the nut as possible so as to not wallow out the holes. Also note that the wires to the fork arms can become twisted around the bits if care is not observed. My solution for nut rotation is shown in Figure 7. This speed square can be obtained at most local hardware or Sears stores.
After approximately 6 to 8 full rotations, the nut should release its grip on the central hub and allow separation of the upper and lower portions of the base. The two pieces must be separated carefully and as straight as possible, as the upper portion of the base fits snugly over a large roller bearing that is fixed in the lower portion of the base. Should the two halves jam, a piece of threaded rod can be screwed into the lower base where the tripod screw would attach and used for greater separation leverage. Maximum separation is only about 8 to 10 inches, due to wire length restrictions as shown in Figure 8. Also note the presence of the upper of the 2 rotating semicircular plates fixed by screws, just below the ring gear that is driven by the R.A. motor.
With the 2 halves separated, I was now able to retrieve the broken parts shown in Figure 9. These are the R.A. base stop plate at the top, its retaining screw at the lower left, and a ?” long piece of steel rod slightly larger in diameter than a pencil lead at the lower right. This rod turned out to be half of the R.A. ring gear stop pin shown in Figure 10.
The ring gear stop pin you see in Figure 10 is the repaired view. This visible part had been sheared at the base of the ring and is the broken half shown in Figure 9. Mark the ring and gear for alignment on reassembly. To remover the stub, the three screws holding the ring to the base were removed, the ring slid off, and the remainder of the pin driven out from the inside with a drift. Be careful not to enlarge the hole with the drift. Although most, if not all of the screws used in the telescope are metric, this pin is not. It turned out that the chuck end of ONE of my ?” drill bits (several others I tried were too small) was of the proper diameter and required hardness to use as a replacement pin when ground down to the needed ?” length. Drive in the new pin using medium strength Loctite for additional security and reattach the ring. I now turned my attention to the problems in the lower half of the base. Figure 11 displays the inside of the base with the missing rotation stop plate and retaining screw.
Take note of the lower of the 2 rotating semicircular plates with the attached stop block. Although the screws on the back of the plate that hold the stop block to the plate were tight, the block was free to move to a considerable degree, indicating that the countersink holes in the plate were too large. Play was so great that temporary remounting of the sensors followed by plate rotation resulted in observation of the block striking the lower of the two sensors as shown in Figure 12.
The only solution was to place thin nylon washers under the screw heads on the back side of the plate to shim out the slack. These washers must be extremely thin to prevent the screw heads from protruding excessively beyond the countersink holes and binding the plate to the base during rotation. As it was impossible to find manufactured washers to fill the need in a reasonable length of time, I made my own from small nylon washers that I sanded down on very fine Emery paper to suitable thickness. By suitable thickness I mean something in the order of tissue paper, as I have no instrument capable of making that small of a measurement. Again I used medium strength Loctite on the screws and trimmed the protruding excess nylon flush with a razor blade as seen in figure 13. I then checked for sufficient rotational clearance with both the sensor and the base.
The repair of the base stop plate was next, and relatively simple. However, I did find it necessary to remove some burrs on the ends of the base stop plate that were engaging the rotation stop block prior to reaching it's centered stop pin. I replaced the screw shown in Figure 9, with an M5-.8 x 20 mm obtained from Home Depot. I also used an appropriate sized lock washer and did not use Loctite as the fit with the longer screw and lock washer was quite firm. Figure 14 displays the completed lower base repairs.
The next item requiring repair was the R.A. slow motion control. In order to gain access to these gears located in the upper half of the base, it is necessary to remove 4 screws from the inside top of the ring gear as depicted in Figure 15. Note that one of the screws is not visible.
The ring gear assembly can now be pulled away from the upper half of the base exposing the R.A. slow motion control gears as seen in Figure 16.
The mess created by the failure of the R.A. slow motion control consisted of hundreds of small metal shavings distributed around the housing and gears as shown in Figure 17. This had to be cleaned thoroughly to prevent possible contamination of the main load bearing. In addition, the gear teeth had to be scrubbed to remove particles that could potentially recreate the problem. Figure 18 displays the cleaned assembly.
Figure 19 shows the knob on the R.A. slow motion control removed, so that you can see the shaft hole is slightly offset from the center of the sleeve. This allows rotation of the sleeve to provide adjustable gear tension and height to correct for manufacturing variances. Apparently the set screw holding the rotational sleeve became loose, allowing the gear to rotate and / or drop to a partial and intermittent mesh condition. This may have been caused by the lack of using Loctite , a piece of contamination jamming the gears, or a combination of both. Once the friction of the R.A. slow motion control was lost, the mass and rotational velocity of the OTA, when excessively rotated by hand was great enough to shear the ring stop pin and dislodge the base stop plate. A contributing factor to the base stop plate failure is that the retaining screw was too short and only holding by 2 or 3 threads.
With all parts cleaned of debris, reassembly can commence by proceeding through this document in reverse order. Be sure to LIGHTLY grease all parts that had grease removed during cleaning. Also adjust the R.A. slow motion control sleeve with partial assembly of the ring gear to the upper half of the base using only two screws instead of all four. Check for possible jamming on FULL rotation in both directions and adjust tension before final assembly with all four screws. I used lock washers on these screws holding the ring gear to the upper half of the base shown in Figure 15 during reassembly. Align the rotating semicircular plates for maximum clearance prior to reattachment of the R.A. rotation sensors. I found it necessary to replace the central wiring sleeve shown in Figure 17, as well as the two sleeves running through the lower rotation point seen in Figure 3. I also added sleeves to the wires going out to the arms shown in Figure 5 to prevent chaffing on the cover plate. This was due to the increased size of the wire bundles caused by wiring modifications mentioned at the beginning of this article, rather than damage during the repair. This sleeve material can be purchased at either Radio Shack or most automotive supply stores should you need to do so. Use the reference marks recommended earlier, to adjust parts placement and tensions during the reassembly process.
For those wishing to prevent wire wrap and / or supply power to externally mounted OTA devices, refer to the “Modifications to Prevent Wire Wrap and Supply Power to LX-200 GPS OTA Mounted Devices” article BEFORE reassembly.