12" LX200 Classic RA Problem and Resolution.

The Problem:
I turn on the scope and it first tries to go east real quick (for a second). Then it keeps slewing west until the keypad comes "online" It did this once and then stopped and seemed to be working OK for a little while (though slewing east was a little slow). Then I restarted and it did the same thing but this time slewing in both direction is whacky. The scope had been outside for a few weeks (in a waterproof cover) and there was some condensation so I wondering if that has anything to do with the problems.

I took the scope inside and opened it up to look for any obvious loose connections. After that the behavoir stayed the same in that it would do the whacky loud startup and then it would slew West fine but struggle to slew East.

I also found that if I left the scope on for 30 minutes or so the problems would go away. However, after the scope had been powered off for a while the problems would return. The behavior was very consistent.

The Fix:
First of all, there is an excellent article by the late great Bruce Johnston that explains the working of the LX200. I recommend you read through this and absorb what you can (don't need to understand it all):

I reported the problem on a Yahoo! Group and Alan Sickling was kind enough to spend the time with me offline to help me get this fixed. I can't thank Alan enough. He really knows his stuff. Hopefully this page can help someone else in the future.

I sent Alan some wav files so he could hear what my drive sounded like. I only had a crappy microphone so crank your sound and igore the white noise at the beginning of most of these.

Here is what it sounds like when I boot up after the scope has been turn off for a while:

This is what is sounds like "cold" slewing full speed West:

.. and full speed East (noticably slower)

Here is what is sounds like booting up again after the telescope has been on for a while (sounds "normal"):

Here is what it sounds like warm slewing East, West, East, West:

The following is text sent to me from Alan Sickling.

I think we have enough evidence here to indicate that you have PROBABLY suffered some sort of drift in the pot resistor values. This is not an uncommon condition. It can occur as the components age, or due to the effects of humidity or temperature shifts in the environment, or movement and vibration.

Firstly, I am going to tell you the quick and dirty way to get your RA drive back into shape. I can tell that it is not far out of trim from your reported symptoms) and this MIGHT work for you. All you do is switch on the scope and twiddle a screwdiver gently in the two RA pots until the drive settles down and starts behaving itself properly. However, you do that at your peril, and if you get either of the two pots into the wrong region of their tracks then the drive could go completely haywire and you may not be able to find the sweet spot in the settings to get it under control again. This is the way that the Aussies got it sorted. I warned them of the danger, but they said all they were worried about was getting it to work that night and if it ďblows up tomorrow, then thatís someone elseís problemĒ. Thankfully, it didnít. But I donít recommend you tackle it that way. It might work initially for a while, but if the adjustment is not correctly at the centre of its range, it may drift out again in a fairly short time.

So letís do it the correct way.

Herewith the promised photos, drawings and instructions.

Attachment 1 is just a drawing of the relevant bits of the RA motor drive pcb. It notes the key pins and components which we are going to be dealing with. Unless you have a problem getting the adjustments to work, you wonít need to bother with the two signal test pins on the 3-pin header block. These show the waveforms coming off the photodetectors and unless you have a very strange condition you wonít need to look at these. You will need only the earth pin.

Attachment 2 is a photo of the simple leads I have made up to make the adjustment task easier and safer Ė it will help you to avoid making any damaging shorts with the instrument probes. I would advise you to do this as well. It will ensure that you will not damage anything else in the drive. (Having said that, there is nothing on the board which costs more than about 50c, but you donít want the inconvenience of another fault.)

The triple-wire lead is to get signals from the 3 test pins on the long side of the pcb, out safely to your instrument(s). You can terminate the outboard end any way that is convenient to you. Just make sure that the arrangement doesnít allow shorts. The end which plugs onto the board is just a 3-pin/0.1Ē header Ė mine came from an old PC fan lead.

The other, single-wire, lead is to collect signals from the edge connector on the pcb. You will see that you need to connect to the 2 end pins on the connector strip. Look carefully at my lead. One end is made from a stiff piece of single-strand wire, which was formed into a loop by wrapping it tightly around one of the pins. To measure what is going on in the wires of the connector, you must remove the connector, slide on the wire loop, and then replug the connector. Make sure the loop of wire does not touch any adjacent pins. As before, the outboard end can be terminated any way you like - to suit your instrument. Only attach and remove these connection leads if the power is OFF.

I think that should all be clear, but ask if it is not.

Other things you will need are Ė 1) a screwdiver, with an end which is a good fit in the potsí slot. The pots are the large square plastic components, usually blue in colour, and 2) a digital multimeter, set to 5V or 10v D.C. range. Perhaps also some connection wires with croc-clips to connect your meter to the probe wires.

Now, before you start, some basic rules. Make sure the scope power is OFF whenever you attach the connection leads onto a pin of the edge connector. Do not power back on until you are sure the wire loop is touching only the target pin. Make sure the scope is safely and securely held on the workbench. And donít forget to loosen the RA clutch. For good measure, disconnect the Dec lead from the fork socket. This way the scope is not going to go walkabout, whatever your drives do! Look carefully at the centre boss of the two RA pots - the circular part which has the adjustment slot. Is this gunged up with wax or paint? If it is, get rid of it by scraping it away with a scalpel or sharp knife. The idea here is to free the movement ready to be able to adjust the resistance. Only bodgers (like Meade) use this sort of locking arrangement. It isnít necessary and it isnít professional. Note carefully where the slots are orientated and mark the starting positions of the two pots as accurately as you can. Now use the screwdiver on each pot in turn and swing the pot quickly to and fro to drive the wiper over and over the resistance track, from end to end. Give each pot about a dozen end-to-end wipes. What this does is to clean both the track and the wiper contact, removing any dirt, oxidation or corrosion. Finally, restore both pots as accurately as you can to their starting positions.

Adjusting : Start with the scope power OFF. Start using just your meter. STEP 1. Connect the meter black (neg) lead to the ground (0V) pin of the 3-pin header. Connect the meter red (pos) lead to pin 1 of the connector block. This will check comparator 1 output. Switch the scope ON and run the RA drive at a moderate slew setting Ė either (and both) directions. Measure the voltage on pin 1. If all is well and the motor is running reasonably as it should, the reading should be in the range 2.3V to 2.5V, whichever way you slew. The ideal voltage is 2.42V. If the voltage is not as stated, use the screwdiver to turn the Pot 1 a little (either and both ways) and see if you can get the reading to settle at 2.42V. If at anytime the drive goes into runaway, donít panic, just try to get the setting back where it was and let it settle again. You should be able to get it to a position where the drive motion suddenly settles to a smooth action and the current registered on the front panel bar-leds becomes a minimum. If you still canít settle the drive down, then we do stop 2, which is : STEP 2 : Switch off, transfer the signal probe to the other comparator (pin 2 on the edge connector) and repeat this whole adjustment operation for Pot 2.

When you have finished both of these stages you should have the drive pots set roughly to the correct values, but perhaps not optimal. However, in playing around with the settings you will have learned what the drive does and how it sounds with varying settings of the pots. You will also almost certainly have experienced what happens when you turn the pot to a position where the setting causes the feedback loop to go unstable and the drive goes beserk.

Now to optimise the settings. Repeat both the steps above, but swing each pot in turn, at each step, away from its new position until you find the point where the drive suddenly goes beserk. Note the position of the pot at that point. Turn the pot back in the opposite direction and back into the stable region. Continue going in that other direction until you reach the point of instability in the opposite direction. Note that position also. You have now found the extremes of the settings for stability on that pot. Now carefully turn this pot to be exactly central between these two extreme positions. Repeat for the other pot. Your drive should now be both stable and well centralised within the feedback loop stability margins. This means, in practice, that the pots will have to drift quite some way, in future, before the drive fails again. Given fair conditions, it may never need re-adjusting.

When you get to this point, power the scope on, let it run to find the index mark and then just listen to it tracking (ear pessed hard onto the base casting). If youíve completely eradicated the instability in the feedback loop, the sound should be silky-smooth, quiet and very evenly paced. It is unmistakeable, really. My 12Ē runs so quietly that I usually cannot hear it at all at night and I sometimes have to get my ear cold just to reassure myself that it is still running!

If, unfortunately, you find that you cannot get any combination of pot settings which completely return the drive to smooth operation, then we may have to resort to the oscilloscope. But I think you will not need to do that. I havenít had a failure yet. Donít worry if you do have to have a second go Ė itís not difficult. Sometimes the scope can let you see oddities which the meter cannot pick up. In that case we will still get a fix, but it might take an hour or two longer, thatís all. Attachment 3 just gives you an idea of what the circuit waveforms look like if you do have an oscilloscope. I have some photos somewhere on file but I canít find them at the moment.

Good luck!

Postscript by Alan:

1. Adjustments of the two pots are somewhat interactive, so cycle around the two to get an optimal result.

2. I find an oscilloscope does the job faster than a meter as you can interpret the drive's actions better watching the waveforms. Optimisation is quicker too - just get the comparators to produce an even 1:1 m:s ratio. See Attachment 4

3. For the RA drive, you don't need to open up the baseplate. Access to the drive can be obtained by removing the base front control panel, flipping it carefully out of its aperture, and the drive pcb will be seen below. So it can be done with the scope still mounted on a wedge.