LX-200 - Dec Clutch Problem and Solution

I recently acquired a second 10" LX-200 "Classic" telescope to mount on a pier in my backyard so that I can use it without having to do lots of realignment whenever I want to use it.  I discovered that this new scope had two problems.

Problem 1: 

When the sun would heat up the telescope, the Dec motor would run away, i.e., run even though no button was pushed.  I tested with a spare Dec motor acquired years ago.  Same result.  So I took everything apart.  Set up the scope on a table with the motherboard cover removed so I could see the board.  Used a hair drier to heat up everything (carefully - don't want to melt anything - Sunlight here only gets up to 122 in the shade and a telescope motherboard enclosed in a black housing sitting in the sun outside probably cannot exceed 190 degrees F?  Not quite the boiling point of water...)  Hair drier test did not reveal anything.  Motor did not run away. 

After fixing problem 2 below, I mounted the telescope outside again in 105 degree weather.  After scope heated up for a few hours, I turned it on and the Dec motor did not run at all.  Hand controller had no effect on it.  Powered down, swapped Dec cable from the other LX-200, powered up and it worked fine!

So I replaced the RJ-45 connectors at both ends of the Dec cable and everything is now running OK.  No sensitivity to temperature at all now. 

Problem 2:

This problem was VERY strange.  This problem was tricky.  Almost impossible to analyze!  The real problem was totally unexpected!  I wonder if this problem led to all the Dec clutch replacement methods and kits that people are selling?

I noticed that if I pushed or pulled on the front end of the telescope in the Declination direction that the scope would move a degree or two.  It acted like the clutch was slipping a little bit.  This problem of course would lead to all sorts of Declination pointing problems if not resolved.  Like every time a heavy camera was mounted or if you inadvertently bumped the scope with your head.  (That's how I discovered the problem.  :-)  ) 

So of course I started taking things apart.  You have to remove the Dec motor cover first.  Then there is a plate over the main gear that just pulls out.  The grease between this plate and the main gear can create suction so I used an old towel to get a grip on it and pull it loose.  Then you can see the two bolts that hold the main gear onto the shaft.  I removed the two bolts and pulled the gear off.  You have to push the Dec motor a bit so that the worm gear is not pushing on the main gear in order to get the main gear out. 

If you have read down to here and have this slippage problem, you DO NOT have to do the following to fix it.  I had to do it to figure out what was wrong though.  I will show you soon enough what I had to do to fix it.  The fix is simple...  You can do it easily...

I could not yet get a good view of the complete shaft.  This was because the fork arm covers the shaft.  So I removed the fork arm by removing the four bolts at the base of the fork arm.  After the fork arm was removed, (there is a plastic bearing that I was very careful with that is a cylinder that is between the fork arm and the shaft)  I could then see the shaft that the main gear was connected to.  You can also then see three items that are on the shaft.  Basically they are a roller bearing or thrust bearing that is between two large washers.  I removed the three items carefully and inspected the shaft.  I was puzzled by it and wondered if it was bolted to the inside of the tube.  Flashlight showed nothing on the inside of the telescope tube.  I was momentarily puzzled by this so decided to remove the three bolts holding the item in the following image:

So, surprisingly, those three bolts are all that holds the above item to the Optical Tube Assembly.  The above is a view of the shaft.  The inner screw hole is what the large Dec knob that tightens the clutch goes into.  The two outer screw holes are the ones that hold the Main gear tight against the shaft.  The above item appears to be a single piece of cast aluminum.  I could not see any way for the shaft to rotate against the rest of the above item.  Yet it seemed like it MUST be rotating to allow the slippage I was seeing.  I was not positive that the above item was really one piece though.  Maybe somehow part of it could rotate against the rest of it?  The problem was how to test it. 

I then realized that to be sure, I had to test it with the main gear attached and see what was rotating against what.

So I then assembled the pieces that matter:

So in the above image, you see a side view of the shaft and the black plate it is part of.  The main gear is bolted to the shaft using the two screws that I took out previously.  Then on top of that is the other plate mentioned before which I think constitutes the clutch.  Then the Dec clutch knob is above that screwed into the center hole of the shaft.  I tightened it down then carefully positioned the left end of the plate in the above image into a vice but DID NOT TIGHTEN THE VICE.  That is cast aluminum!  I didn't want to damage it!   But I used the vice to stabilize it so I could twist the main gear and see if anything rotated.  Surprise! Surprise!  Something DID rotate!  It shouldn't but it did!  I looked carefully at the clutch/main gear and they were not slipping against each other!  Impossible! 

This next image shows the screws holding the main gear to the shaft.  Look at the two screws (there is an allen wrench in one of them.)  The heads are cone shaped to exactly fit into the cone shaped counter-sinks.  If they are tight, there is NO WAY the main gear can rotate against the shaft. 

Well they were tight but the main gear WAS rotating about the shaft!  How could that be?  Then it dawned on me.  The *$#!&#$ screws were too long!  They were tight.  Tight against the bottom of the screw holes but not tight against the Main gear!   If  they are not tight against the main gear, then the gear can rotate a tiny bit about the shaft!

This following image shows the screws tight against the bottom of the screw holes without the gear attached.  They are in all the way.

They are just flat-out too long!   When the Main gear is in place, they bottom out and do not allow the Main gear to be completely tight against the shaft. 

Solution.  File the two screws down a bit, i.e., remove about two threads.

I did that.  Reassembled everything.  NO MORE SLIPPAGE.  The Meade clutch works just fine.  Meade either did not drill the screw holes deep enough or somebody provided screws that were longer than specified. 

Tough problem.  Hard to analyze.  Counter-intuitive. 

So, here's all you have to do if you have this problem.  You don't have to disassemble the fork arms or remove the above plate like I did.  Just remove the Dec knob, the Dec cover, remove the clutch plate, remove the two screws, file them down (reduce their length), screw them back in, replace the clutch plate, replace the Dec cover and the Knob and you are back in business.

In my case, since I removed and reassembled all that stuff, my scope is now not properly aligned with respect to the Dec Axis.  I aligned the Dec axis on the other scope years ago using Polaris so I will have to do that tonight for this scope.  Having the Dec axis properly aligned GREATLY improves the pointing accuracy!  21 Sep 2008.

Copyright 1997 - 2008 Howard C. Anderson