The Unofficial Meade 203SC/LXD500 Page
Tear Down and Rebuild the LXD500 GEM

This page has detailed instructions for disassembling, adjusting, and reassembling the LXD500 GEM mount. These GEM mounts are mass-produced in a factory, which means that they didn't get a lot of personal attention and fine craftsmanship applied to them when they were built! The design of the GEM is solid, however, and spending a few hours working on the mechanics of the mount will result in a much more stable, much smoother operating GEM.

* WARNING: Taking the mount apart MAY void your warranty! *
The procedure is very simple, and there are precious few small pieces to lose or things that can go wrong. But, if you're not comfortable with hand tools, or have no experience working with metal parts, either do not attempt these procedures, or get someone mechanically inclined to help you with them! The author of this web page assumes no responsibility for the operation of your mount after following the procedures described herein.

NOTE: The pictures of the process here were taken with a QuickCam Pro while I was working on the mount. Some are not the best quality -- sorry. I hope they get the required information across. If you want to attempt this but have some questions, feel free to contact me and I'll help if I can.

Figure 1
Upon arrival, my LXD500 GEM was in pretty good shape, but had a few problems. The right ascension and declination axes were both too stiff, and were somewhat hard to turn -- this made it very difficult to balance the scope on the mount, since the axes would not turn freely even when out of balance, so you couldn't tell which side was too heavy! The declination clutch was also a bit too loose, so to lock it down I had to crank down really hard on the lock lever, which CAN'T be good. There was very little backlash in declination, but on the RA axis there was quite a bit, and the motors had to spin for about 2 seconds at 32X speed when switching from East to West (or vice-versa) movements. These little problems convinced me that I needed to make some mechanical adjustments to the mount, so I took on the task of tearing it down and rebuilding it. The results have been wonderful, and the mount now has none of the problems mentioned, and tracks smoothly while observing.
The entire procedure will take about 2 hours if you work slowly and carefully. Here's what you'll need to do it:
1) A set of Allen wrenches, both metric and english, including some VERY SMALL sizes that don't come with your mount. You can get a complete set of Allen wrenches at a hardware store for about $5.
2) Some new grease for lubricating the mount. I suggest white lithium grease, which is thinner and smoother than the thick axle grease they put on at the factory, and holds up better under cold temperatures. A can of white lithium grease cost about $3.50 at my local auto shop.
3) Some "crocus cloth" or 600-grit wet/dry sandpaper for smoothing rough metal surfaces
4) Mineral spirits or turpentine for cleaning the old grease off of parts -- synthetic turpentines ("Turpenoid") work well for this.
5) Miscellaneous small tools (socket wrench or adjustable wrench, screwdrivers, etc.) and lots of rags!

Getting Started
Remove the 203SC OTA from the mount, and then remove the LXD500 GEM from the tripod. Remove the counterweight shaft from the mount (after taking the counterweights off of it!). You'll have on your table what is shown above in Figure 1.
If you have the #1702 motor drives attached, remove them now. If you have installed the #812 Polar Alignment Finder, unscrew the finderscope from its place in the RA axis and set it aside, then remove both the front and back covers for the finder scope. Then remove the OTA dovetail mounting plate by removing the two bolts that attach it to the declination head.


Figure 2
The declination axis is attached to the RA axis by a bolt that passes through the declination head, with a bolt on the top (see Figure 2). This bolt has an allen nut threaded into the side to lock it in place -- on my mount, the allen nut was completely loose, allowing the bolt to turn freely. No wonder the declination axis was too stiff! How tightly the declination head is attached is adjusted by tightening this bolt just enough to hold the head steady, without putting too much pressure on it. If you tighten or loosen this bolt and then turn the declination axis, you can find the right tension that allows smooth movement, then lock the nut in place with the allen screw. But first, we need to take the declination head apart and do some work on it.
Remove the top nut using a socket wrench -- the bolt will drop down into the recess in the RA axis where the polar scope goes. You don't need to completely remove this bolt, just leave it where it is and keep its washers with it! Next you need to remove the mounting for the declination worm gear.

Figure 3
The worm gear cover is held on by two Allen screws that pass through the front of it (the third Allen screw, in the middle, is for adjusting worm tension), and by two hex-head bolts on the top of the housing. Remove the two Allen screws first, then the hex-head bolts, and the worm housing with the worm gear inside will come off the declination head smoothly (see Figures 3 and 4).

Figure 4
Once the worm gear housing is off, the top of the declination head and the bronze ring gear can be pulled straight up out of the declination housing. You can see the parts in Figure 3. Note the thick, dark grease that was used at the factory to lubricate the worm gears -- we're going to replace that with better grease!
IMPORTANT NOTE: When you pull the bronze ring gear out of the declination housing, look for the little plastic "button" you see in Figure 5. This button sits between the ring gear and the declination clutch lever, and puts pressure on the ring gear when the clutch is tightened. DO NOT LOSE THIS PART, or you will ruin your declination ring gear by pressing the metal clutch screw directly against it! Sometimes this button will fall out as you remove the ring gear, and sometimes it will remain in the notch for the clutch screw. In any case, get a hold of it and set it aside in a safe place!

Figure 5
Remove the bronze ring gear from the upper declination housing that it's in (it's held there by friction pressure from the grease ONLY). Put some mineral spirits in a pie tin or other container, and brush it liberally over the ring gear with a soft brush to remove all of the thick grease that's on the gear. Set the gear aside to dry while we work on the declination housing itself.
The ring gear rides loosely against the bottom of the declination housing -- how loosely depends on how tight the top nut (described above) is. On my GEM, there were some small metal filings left over from machining in the bottom part of the declination housing, and no lubrication -- that certainly does not contribute to smoothness! Take some of the crocus cloth or very fine sandpaper, and smooth any rough metal edges inside both the top and bottom declination housings, then clean them THOROUGHLY with mineral spirits. Apply a light film of grease to the BOTTOM declination housing where the ring gear rides against the aluminum housing, and to the top housing as well.
The worm gear sits inside a metal housing, held in place by two plastic threaded inserts -- on one side is the drive motor attachment, and the standard slow-motion control lug sticks out the other side (see Figures 3 and 4). If these plastic inserts are screwed in too tightly into the worm gear housing, then can make the worm gear bind, and the worm gets very hard to turn. However, if they're too loose, the worm can move laterally (back-and-forth) within the housing, causing drive errors, vibration, and backlash. Remove the plastic insert on the slow-motion control side (you can fit an adjustable wrench around the outside plastic part, but don't squeeze or turn too tightly, it's plastic!), then slide the worm gear out of the housing. Clean the worm gear in the mineral spirits to remove the old grease. Put some new grease on the end of the worm that fits through the plastic inserts, and on the threaded worm itself, and re-insert the worm gear into the housing. Thread the plastic insert back on, and tighten it ONLY until it minimizes lateral movement, but still allows the worm gear itself to turn freely.
Put a fairly generous coat of grease all along the threaded portion of the ring gear, and insert the ring gear back into the top declination housing. Now go find that plastic button I had you put in a safe place -- here's the easiest way to put it back together and adjust the tension of the clutch at the same time:
Remove the Phillips-head screw that holds the external clutch lever to the clutch screw -- the clutch lever will come right off. Unscrew the clutch screw until there is plenty of room inside the declination housing for the plastic button to fit into. Hold the housing so the hole for the plastic button is DOWN, insert the button, then insert the ring gear/top housing back into the bottom declination housing. Once it's in place, screw the clutch screw BY HAND until it's just slightly tight against the ring gear inside, then back it off 1/8 of a turn. Place the clutch lever back on the top of the clutch screw (make sure and align the slot on the back of the clutch lever with the pin that sticks out of the declination housing) so it's in the UNLOCKED position, and put the Phillips-head screw that holds the lever to the clutch screw back in and tighten it. Now lock the clutch, and insure that the declination axis does not turn. If it's too loose or too tight (you can't close the clutch all the way), take the clutch lever off again and tighten/loosen the clutch screw, and reattach the lever.
Reattach the worm gear housing to the declination housing. Put the worm housing in place, then LOOSELY thread the two hex-head bolts into place on top of the housing. Turn the MIDDLE Allen screw nearly all the way out, then thread the two smaller Allen head screws into place to tighten the worm gear assembly to the declination housing. Now, get your slow-motion knob, and attach it to the slow-motion shaft on the worm gear it the usual place. With the clutch tightened, turn the slow motion control and see how easily the declination axis turns. If it's too tight, tighten the MIDDLE Allen screw -- this will pull the worm gear slightly away from the ring gear. If it's too loose, tighten the two OUTSIDE Allen screws, and loosen the middle screw a bit. Keep doing these adjustments until the worm turns smoothly and easily, and there is little or no backlash when you turn one way then the other. Once you have it adjusted, snug the two top hex-head bolts down into place, but NOT TOO TIGHTLY! If you find you need to adjust the backlash or worm gear motion later, this is how you do it (this procedure is not explained at all in any of the 203SC/LXD500 manuals!).
Congratulations, you've just taken care of the declination axis. If your results are anything like mine, you now have a smooth-turning declination axis with no bumps, little or no backlash, and a well-adjusted clutch. Let's move on to the Right Ascension axis.


Figure 6
The RA housing is held in place by a threaded ring that screws onto the back of the RA shaft. This ring is the black knurled ring that holds the polar scope cover in place. The ring itself is held in place by three Allen screws -- you can find the holes for tightening/loosening these along the edge of the ring. These Allen screws have TINY heads, so this is where you'll need those additional Allen wrenches! Loosen all three Allen screws in the rim of the ring, but don't loosen them so much that they fall out (they really, really small -- and easy to lose). Once they're loose, you can turn the knurled ring counterclockwise to remove it. This ring serves the same purpose as the hex nut with the Allen screw on the top of the declination axis. You screw it down until the tightness and friction of the axis is just right, then hold it in place with the Allen screws. Mine was too tight!
Under the knurled ring are three washers -- a thin steel washer, then a brass washer, then another thin steel washer. Remove the washers, and keep them in the correct order. Once the washers are removed, the rear RA housing will slide off of the RA shaft, as in figure 6. As with the declination housing, smooth out any rough edges inside this housing, and clean it thoroughly.
Notice the nylon washer at the back of the RA ring gear...the declination axis has no such washer, and this is the ONLY plastic bearing surface in the entire mount! After you remove the nylon washer, proceed just like you did on the declination axis. Remove the clutch lever, then the worm gear housing, and slide the ring gear out of the front RA housing (DON'T FORGET ABOUT THE LITTLE PLASTIC BUTTON!). Smooth any rough metal surfaces inside the front housing, and clean it well. Clean the old grease off the RA ring and worm gears, re-lube everything with the new grease, and reassemble the whole lot -- adjusting the clutch tension and worm gear backlash/tension just like you did on the declination axis. Don't forget to put that nylon washer back on! When you go to put the knurled ring back on, tighten it only a little bit at a time, testing the motion of the RA axis after each adjustment. When it's tight enough to hold the RA shaft in place and keep it from shifting laterally, but not so tight as to impede smooth movement of the RA axis, tighten down those teeny little Allen screws, and lock it in place.
Notice how much detail I went into on the declination axis, and how short the RA axis section is -- this shoudl be reassuring to you, because other than the shaft attachments they're virtually identical. Once you've worked your way through the declination axis, the RA axis will seem like a piece of cake.
Finishing Up
Congratulations! That wasn't so hard, was it? Frankly, I was scared I would mess something up when I first started to do this -- but the mount is well-designed, and is pretty easy to take apart and put back together. Adjusting the clutch tightness, the worm gear backlash, and the axis lock nuts are the most important parts of this procedure...if you're happy with the thick grease they used originally, you can just do those adjustments and save yourself some time. Once the entire GEM is put back together, put it on the tripod, add the scope, and try it out. If your results are like mine, it will be much easier to balance the mount with the scope on, since an out-of-balance condition now swings the axis by itself when the clutches are loosened. It's much easier to point the scope where I want it to be now, since the axes move more smoothly and easily, my clutches hold better without having to crank them down so hard, and I have had no trouble with my motors driving the mount at all speeds when it's properly balanced. Tracking accuracy has improved somewhat as well, but I can't quantify that since I didn't measure it before I did the rebuild.
With the well-designed and simple mechanical construction -- and BRONZE worm gears -- of the LXD500 mount, I'm actually more impressed with it after the rebuild than I was before. Those minor, annoying problems before the rebuild made me think that the mount might not be worth keeping. Once I opened it up and saw that it was just a lack of attention to detail and poor lubrication that caused my problems, I was more hopeful that it would work well. And once I was finished, I've been very happy with the results. Combining the mount rebuild with a new wooden tripod has given me a very stable, smooth, solid platform for the 8-inch SCT for a reasonable price and a small amount of work. That's good value in my book ;-)

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Questions? Comments? E-mail me! lefevre@midway.com