Hoo boy, did I have a sick DEC axis. l think I only started noticing it when I started guiding long-exposure shots as it was on the most part livable for the visual stuff.

I had a few problems:

1. Reversing direction quickly back and forth several times would actually “crash” the LX50! The power led would start blinking and all I could do was power off, wait a few seconds and power on to get it to respond. Kinda’ like an operating system or two that I won’t mention here! ;o)
2. Speed of motion on the DEC axis had nothing to do with the RA axis. Either it was Speedy Gonzalez or the motor would actually come to a grinding (seriously – grinding) halt.
3. When I reversed direction it would take a long time before whatever was in the FOV actually moved. It was like slack was being picked up somewhere in the tangent arm.

None of this is very good when you’re trying to guide a photo on a $%&# guide star you can barely see and I won’t even go into what an autoguider will try to do…

Problem 1 was the easiest to solve. It was pretty obviously power related but after trying fresh batteries, NiCads, even a blasted 9v the “blinking light” weirdness persisted. After some good advice (always!) from the folks at SCT-User I decided to give it a go with a little 12v, 1A power brick. That did the trick and I’ve never had any problems since. Seems the problem is
related to current and how much current batteries can sustain. Phil Chambers explained what was going on much better than I ever could in a post to the SCT-Users group:

“The issue, as Rod said, is the gearing which loads the motor down. What is probably happening is the motor is loaded, its cold so it moves a little harder so the motor needs more current but as it gets more current, the voltage drops and the spiral is to stall the motor. You must realize that a DC motor uses MAX current at stall. As it goes faster it uses less current. The alkalines simply have to much internal resistance to play this game. As the current starts to rise, the alkalines voltage starts to drop…”

Thanks for the explanation Phil. Since I do all my observing from my back yard this so far hasn’t been an inconvenience. Eventually I’m going to have to deal with powering the scope properly in the field but I’ll do a write up on that bag of beans when I have to face it! :o)

Problem 2 is the #1 gripe about the LX50’s DEC and whenever you mention you have a DEC problem with this scope on most newsgroups about 50 people will step forward and say in one voice: “Scopetronix LX50 DEC Fix Kit” [Link to the Internet Archive – Unfortunatley ScopeTronix went out of business some time in 2008.]

I am now one of those people! 😀 Wow what a difference! I had never noticed how tacky and plasticy those Meade gears were until I had the kit installed on the scope. (I bought the 108 tooth aluminum gear version)

In my opinion if you’re having DEC problems ordering the kit should be the first step. Mechanically the motors no longer “freeze” no matter what you do to them. You can stop the motor turning by grabbing the 108 tooth gear but let go and the motor just keeps on chuggin’ along. I was impressed already.

As far as guiding is concerned the DEC speed is much slower and smoother. At 2x I now have to press and hold down the button on the key pad and just watch the star glide where before it was more like tap… Oh $%&# where’d it go?!

Actually the DEC speed is now slower than the RA speed and as a matter of opinion that suits me just fine! At 32x for me the speed is still quite acceptable as I’ve always gone either straight for the DEC knob or just unlocked the thing to slew more than a few degrees anyway.

A Final note on the motor revision and ROM version. Its stated on a lot of web pages including the Scopetronix page that the best combo is the 16 2/3 motor with the newer ROM (v6.0). My LX50 was bought in late 1999 and I have the newer 33 1/3 motor and the new ROM version and in my case the Fix Kit works great. So far I’ve guided a few shots with the kit installed but have yet to test it with the autoguider.

Problem 3 didn’t take long to locate but I had a heck of a time figuring out how to cure!

Even after installing the DEC Fix Kit I had a lot of back and forth motion along the long DEC screw the tangent arm attaches to.With the DEC locked I could grab the tube, shift it back and forth and watch the DEC knob move slightly in and out.

I won’t begin to go into the amount of little corrections I tried to make or how many times I removed the DEC screw to try to figure out what was going wrong.

Removing the DEC screw is a pretty basic procedure: With an allan wrench remove the DEC knob by loosening the screw that holds it in place. Do the same to the locking nut and push the tangent arm forwards (i.e. the OTA moves north) to disengage the DEC gears. When the gear is loose you can spin it easily until the threads run out on the nut attached to the tangent arm then just pull it out slowly. Do yourself a favor and note where all the little washers go, in which direction, etc. :o)

I tried washers to take up the slack but all they would cause is binding making the motors work harder and make the DEC motion very irregular.

Eventually I noticed the brass ring on the gear side (seen in the top picture with the 108 tooth gear removed) would be pushed out in one direction until it could go no further before the DEC arm would start to move and then be pulled back in when moving in the opposite direction.

AHA! This little ring has a flange on one side that keeps it from sliding through the hole in the fork arm. I had the flange pointing _outwards_ and doing a pretty useless job of staying in one place when the screw would naturally try to push or pull against it as the locking nut is on the inside!

So once again I took the whole thing apart so I could reverse the ring and see what would happen. In the second picture you can see the brass ring in a “corrected” position. There is a washer between the ring and the locking nut to keep the locking nut from binding. After about 2 seconds of slewing in DEC I was positive that this was the gremlin I had been hunting down. DEC motion was smooth in both directions and while the slack was a _lot_ less some was still there and probably could be reduced. With the DEC locked pushing the OTA in the N/S direction would produce minimal movement.

The easiest way I found to reduce the movement that was left was simply to take a cheap pen apart and use the spring to push against the DEC knob. The spring takes up whatever slack is left and so far has done a really good job. There is no binding and the slack between directions is as small as I can make it without replacing the screw and nut with a finer more accurate thread. The spring should be fairly strong. I tried a couple of pens before deciding a spring that I pulled apart slightly so it would have a bit more tension. In the whole process experimentation is important and, as usual, YMMV (your mileage may vary)…