LXD75 Mount Tune-up

Out observing the other day, I noticed a little more play in the RA axis than I was comfortable with. The problem had been building for awhile, and the play in the axis had now reached that magical point of frustration where I had to do something about it.

Now, up until this point, I had avoided taking the mount apart at all, for fear of messing something up so badly that I had to send it back to Meade-- and be scopeless for a month or longer. A search of the web, however, revealed a wonderful site all about eliminating backlash from an LXD75. Reviewing the directions, complete with pictures, I decided the job was manageable with minimal risk. And of course, you never really understand how something works until you tinker with it, so it was time.

I won't reproduce the instructions here in their entirety. I will, however, expound on a few issues I ran into as a first-timer opening the mount, so hopefully you can avoid making the same mistakes I did.

Getting Started

First, as suggested, I removed the OTA from the mount and placed the mount in the polar home position. Next I removed the declination axis motor assembly and the RA axis motor assembly. They are each attached via a single socket head bolt. The declination motor assembly was easy-- the bolt is easily accessible with the OTA off, and its short. The RA motor assembly is a little trickier-- the bolt is installed way up in the inside of the equatorial head. My allen wrench was just barely long enough to reach up and in and loosen it. When removing it, I was worried about whether or not I would be able to get it back in with the same tool-- as it turned out, reinstallation was easier than I imagined it would be. I placed the two motor assemblies aside for safe keeping.

With the motor assemblies removed, the worm gears are visible, as well as the the adjustment bolts. Here's a closeup of the dec worm gear:

On the left is the drive gear. In the middle are the three allen head bolts used to adjust the worm gear, and on the right there is the nut that holds the retainer collet. In any event, the next part is where I made my first mistake. The instructions say to lock down the axes using the manual clutches (the little black tab things), and attempt to move the counterweight bar against the clutches, to check for end play.

The instructions read:
Take hold of the counter weight bar and gently work the axis in both directions, if everything is perfectly tight & there are no backlash or end play issues the axis will not move. You shouldn't be pushing or pulling so hard on the axis via the counter weight bar you can move it against the clutches, just enough pressure to determine if there is any looseness or play in the axis. If you cannot get your clutches to tighten down enough to resist some light pressure against the axis without slipping we are going to need to fix that now. This is a simple fix, there is a Phillips head screw in the top of each black clutch lever, remove it pull the clutch lever off & reposition it so you can get the clutch tight enough on the axis to resist the light pressure needed to check the worm gears. Put the Phillips head screw back in the clutch handle head and secure it, your done with that!

Now because my RA clutch didn't feel like it was tightening as much as my dec clutch, I thought it needed adjusting. I mis-interpreted these instructions thinking that the endplay I was experiencing was caused by this looseness, when in fact it wasn't. I realized later that they're saying that you shouldn't be able to force the axis with the clutch engaged, with a moderate amount of pressure. So I set about attempting to remove that Phillips screw. Let me tell you, I'm glad this wasn't the issue, because I never did get that screw out. I couldn't separate it from the black tab, and the screw began to strip. In between searching the web for screw extractors, I re-read the instructions above again, and realized that my clutch was functioning just fine. Oops.

Adjusting the Retainer Collet

Ok, time to move on to the next step. The next step is to adjust the the worm shaft retainer collet to eliminate any endplay. Wha? Yeah, that's exactly what I was thinking. I read this paragraph quite a few times. They say to put your finger on the end of the shaft, and jiggle the counterweight bar. Yep, I could feel it, so that's what endplay is-- the shaft moved freely in and out of the collet what felt like, oh 1/16" or so. It was easy to notice. Ok. So the instructions say to use a 17mm wrench, and remove the lock nut from the retainer collet, hand tighten the retainer collet, and re-fix the lock nut. Now having never opened a mount before, I didn't know what any of these pieces were, so let me explain it quick. The worm shaft passes through the mount. The retainer collet is a small round threaded piece that screws into the mount and (I believe) tightens down on the shaft as it goes in. It has two cuts in the end, so you can use a flat-head screwdriver on it (but you shouldn't-- read the instructions!). Finally, the lock nut threads onto the retainer collet, and locks it in its desired position. Here's a picture of the retainer collet on the left, and the lock nut on the right:

Unfortunately, I didn't understand the parts, and my lock nut was stuck to my retainer collet-- so when I removed the lock nut, the retainer collet came with it, in one piece. I thought they were one piece. This turned out to be a big oops, because the next step says to hand tighten the retainer collet back down, and check to see if the endplay is removed. I hand tightened the retainer collet/lock nut assembly, but it had no effect (presumably because the lock nut was preventing the retainer collet from screwing in far enough). So I used the wrench, and tightened just a little more. Nothing. So I tightened harder. Ok, that did it! I was ecstatic. Problem solved. Unfortunately, when I tried to turn the shaft via the drive gear, it wouldn't budge. I had over tightened. Ok, loosen it up some. Except when I loosened, just the lock nut came. The retainer collet was locked in. Oh look, they are two separate pieces. Oops.
Now I had a problem. The retainer collet was stuck, and it was in so far that I couldn't use a screwdriver to remove it-- the worm shaft protruded too far. What I needed was some kind of screwdriver with a notch cut out in the middle. I slept on it, and it came to me overnight-- a washer has a hole in the middle. If I found a washer with the right size hole, and cut it in half, I could use that to back the retainer collet out. Excellent! The next morning I measured the shaft and collet. The shaft has a diameter just shy of 1/4". The collet has an inside diameter of 5/16", and an outside diameter of 7/16". Perfect. Off to the hardware store I went.

Let me digress a moment to talk about hardware stores. Here in Central Florida, we basically have the big box stores (Lowe's, Home Depot) and the somewhat smaller Ace Hardware. I think there might be a True Value or something around, and maybe a family owned shop or two, but not near me. I usually go to Lowe's, since they generally have what I need. But for hardware, I realized that Ace is far and away the better choice. At Lowe's, all of the smaller hardware is packaged somehow-- so that you can't test fits without breaking a package open (which I'm sure they frown on), and if you need one washer, well, tough-- you have to buy four. At Ace, they have the hardware all loose, and you can test as much as you want and buy only what you need. Their selection is much larger to boot. From now on, for hardware, I'll go to Ace.

Back to the mount. They make what are called fender washers, which looked plenty strong for what I needed. I bought (four... this was before I had the hardware revelation above, and I was at Lowe's) 1/4" x 1-1/2" fender washers. This means a washer with a 1/4" diameter hole, and a total diameter of 1-1/2". Perfect size. I clamped a washer to the table and grabbed my trusty hacksaw. Here's the cut started:

I was having a little trouble getting the washer to stay put on my table, which has rounded sides, so I clamped it to a piece of wood instead, and finished the cut, giving me two halves. As it turned out, I needed them both. Alright! Perfect. Time to remove that pesky collet. Wait a minute. What's going on here. Oh bother. The washer didn't fit in the slots on the retainer collet-- it was just a little too wide. I grabbed the file, and started filing. Boy was that slow going. About twenty minutes later I looked up and saw this:

Oh yeah. That's what this job needs, power. I knew I'd find a use for that bench grinder eventually. I fired it up, donned my safety goggles, and started grinding the end. Unfortunately, I over ground the first half on one side, so it was no longer straight-- and I couldn't engage both sides of the retainer collet. Luckily I still had the other half, which I ground down more carefully. The finished product:

Note the flat side-- I had to grind off one side of the tool to allow it to turn past one portion of the mount. Now, finally, time to remove that collet:

Worked like a dream. Collet came right out. Perfect!

I continued now with the original instructions, hand tightening just the retainer collet this time. I found I had to tighten just ever so slightly past hand tight (I used my tool) to get it to mostly eliminate the end play. There was still a little bit of movement, but that will be taken care of in the next step. For now, it was just tight enough to remove most of the movement, and I could still turn the shaft with the drive gear easily by hand. Time to put on the locknut. I chose to take the plumber's ring route, and put a #9 plumber's ring on the retainer collet. And I threaded the lock nut on and tightened. The first time I over-tightened it. It pushed the o-ring out of the way on one side, looking quite odd, and tightened down the retainer collet itself. So I had to loosen everything up again properly, and re-tighten. This time I only tightened the lock nut until it was fairly tight, but before it distorted the o-ring. I believe this is correct-- everything felt solid, the shaft turned easily. I'll keep an eye on it and make sure it doesn't loosen up-- if it does, it should be fairly obvious; I'll start experiencing end play again.

Finishing Up

Ahh, through the rough stuff. The last part was easy for me-- adjusting the three allen head screws to eliminate that last bit of backlash. I followed the instructions, loosening the middle one 1/8 turn, and tightening the outside two 1/8 turn. This proved to be to much tightening-- I could no longer turn the drive shaft-- so I loosened the outside ones just ever so slightly. This eliminated the remaining motion when I tried to jiggle the counterweight bar. Perfect!

All that was left to do was to reattach the two motor assemblies. I made sure the gear on the shaft and the gear on the motor made good contact, and reattached the socket head bolt holding the motor assembly. I only needed to adjust the RA axis-- my dec axis had no discernable motion, so I left it alone. With the drives reattached, I attached the handbox and powered up the mount and-- the moment of truth-- slew! It slew just fine. Fast, slow, RA, dec-- everything sounded smooth and good. Excellent!

Hopefully this sheds a little more light on this simple tune-up procedure for those of you out there who are getting to that point where you think it might be time to get in there and tinker, get your mount working like new again. I say go for it! I learned more about my mount in a day than I had in the past few years of just observing.

Links:
LXD75 Maintenance 101