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Repairing the most common problems on the LX200 Dec/RA drive.

There have been lots and lots of discussions on the problems faced when your LX200 Dec or RA drive doesn't work correctly.  The most common of these is 'runaway', or a 'buzzing' of the motor, and the second-most common being a drive that is completely dead.

In this discussion, we'll see if we can cover some ways that you can use to be able to make many of the repairs on these units yourself.  There are three things you should know, right from the start.

First, the more familiar you are with electronics and electronic tools, the easier will be the troubleshooting and repair.  This discussion is geared toward those with a minimum of electronics background, so for you more experienced people, bear with me.  

In order to do the trouble shooting and repairs, you're going to have to be comfortable with using a soldering iron, and also know how to use a multi-meter for checking for continuity, resistance values, and DC voltages.

In the event that you just don't know anything at all about electronics, I'd like to make a suggestion for you.  Check with a local electronics supply house in your area and see if they can put you in touch with a local Amateur Radio club.

'Hams', like amateur astronomy hobbiests, are a special breed of people. There are those with minimal skills in electronics, but also those with lots of background and experience,  just like 'Astronuts'.  Also like the amateur astronomer, nearly all of them will go way out of their way to help someone that needs help, be it in their own hobby or some other.  I'm quite sure that you'll easily find several people locally, that would be more than happy to assist you in the electronics needed to repair your scope. It surely beats having to send the parts or the complete scope, back to the manufacturer!

The second you should know is, if you are going to be in need of parts, you're going to find that the online store for Radio Shack <> is one of the most convenient places to go to get most of the needed parts. (There are lots of other places as well, but this is a nice, familiar store to most people in the United States.)

The final thing you should be made aware of, is the fact that nearly all questions related to Dec and R.A. drive problems that aren't covered in this discussion, are thoroughly covered in the Meade-Astronomy Topical Archives: <> as well as in the personal site of 'Doc G' Greiner at: <>.  In fact, there is also a very useful schematic drawing of the drive electronics at Doc G's site as well. This schematic is well worth having. It can be directly accessed at: <>


Beginning at the Beginning

How the Dec and RA units 'do what they do'

In order to properly trouble shoot and repair these units, we have to have a good understanding of how the mechanics and electronics work, and how the two work together.  That's what the following part of this discussion is going to attempt to explain.

Image 1

This is a view of the gear box for an RA drive assembly.  It's identical to a DEC drive assembly except for two small points.  The first difference is that the R.A. assembly uses a 'magnetic sensor' that will be soon discussed, and the second difference is that the cable coming from the main logic board has a different type of connector at the far end, than does the DEC assembly.

At the bottom of the image, you can see the small DC motor that drives everything.  It has two leads attached to it that lead back to the main logic board in the scope, where the proper voltages are applied to it, depending on the direction and speed that it should be rotating at any given time.

The motor is attached to the assembly by two screws, so that a small 8-tooth gear on its shaft, enters right into the gearbox area, and meshes with a 30 tooth gear. You can't quite see the gear on the motor shaft, but the 30 tooth gear with which it meshes is visible. (You can't see it in real life, either. It's blocked from view by the case.)

Image 2

The gear box, by way of the gears shown, reduces the speed of the motor by 60:1 by the time it gets to the worm shown in the previous picture.  From there, the speed is again reduced by a ratio of 180:1 by the worm and worm gear.  Thus, there is an overall speed reduction of 10,800:1, and and a torque increase of 10,800.  No wonder such a small motor can drive such a heavy load!

Not only do these little assemblies drive the RA and Dec, but they also monitor how fast each motor is going and in which direction.  THAT is the area where things get a bit complicated when it comes to repairing the 'runaway' problem.

On the shaft of the motor, not only is the small gear mounted, but there's also a 'disk'. It looks like this:

Image 3

This 'disk' has a lot of slots in it .... 90, to be exact... that are used in the detection of speed and direction as just mentioned.  


Image 4

Here's a side view drawing of how the disk is attached to the motor shaft inside the gear box housing. The drawing also shows several extra items. One is a light source, or Light Emitting Diode (LED), the second is a light sensor that can detect the light coming from the LED, and the third is a plastic guard that keeps any grease on the gears from getting over to the sensor area.  Actually, there are two sensors and two LEDs, side by side.  The light shines through the slots in the disk as it rotates, and on to the light sensors ......... almost.

Image 5

Working in concert with this disk is a 'mask'. The mask has two sets of slots in it. The two sets are offset from each other, as you can see.  At first blush, it would appear that the slots in the mask are identical to those in the disk except for the offsets between the two sets.  Actually, this is true, but the important thing here, is the fact that the offset between the two groups is quite precise.  It causes the two sets of slots to be offset by an additional 1/2 the width of a slot.  

The method in which these parts, as well as the light sources and light detectors, work like this:

The mask is mounted so that each set of slots is directly over one of the holes that the light sensors are in.

Image 6

Here is a view of the side of an RA gear box.  The two holes shown, are the two holes in to which the light sensors are mounted.  The slotted mask is installed just inside of the side piece as indicated by the white outline drawn over the holes.  If you look closely, you can see two of the slots of the mask.  The inset shows the slots in red, to highlight them.

From the drawing earlier, it should now be apparent that the light that is supplied by the two LEDs will shine through the rotating disk, and as the slots in the disk align with the slots in each sensors' mask, light will fall onto the light sensor.

Image 7

Above, we see the opposite side of the gear box.  We can see the location of the two LEDs that supply light through the disk and masks, to the light sensors. They're just below the motor.

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