MAPUG-Astronomy Topical Archive Logo

  Dec Axis, Backlash, & Motor Issues, pg. 2   

MAPUG-Astronomy Topical Archive     AstroDesigns



Subject: LX200 Classic Dec Rebuild/Bearing Replacement  Top

From: Michael Hart, Date: March, 1998

Michael Hart's excellent work on Rebuilding the Dec Drive & Bearings
on Doc G's website.   Note: a new browser window should open over this one.


Subject: LX200 Dec Drive Mods (classic) --part 1 of 6 Top

From: John Ruthroff <> Date: Sept., 2000

This first mod I came up with myself one day when I took the cover off the declination drive assembly and watched as I moved the 'scope back and forth in declination using the hand pad. While looking at your Dec. drive assembly you'll note there is (for some reason that seems hard to imagine) a rubber O-ring that serves as a buffer between the assembly holding the worm gear and the assembly holding the motor and the gearbox.

To determine if you want to do this modification, simply watch the junction where the O-ring is while you move the 'scope back and forth in declination. My jaw dropped the first time I did this as there was a considerable amount of motion with the worm gear and gearbox compressing the O-ring and clearly adding some motion to the assembly that doesn't do anything but make any slop in the Dec. drive worse than it needs to be. I started poking around in here when I noticed that most of my autoguided images had streaking along the declination axis. Using the backlash control setting I was unable to significantly reduce this slop.

Anyway, while I couldn't be sure that the culprit was all the motion happening in this joint, it was worth a try to reduce/eliminate it and see if things improve. I took the Dec. drive out and using pipe cleaners I cleaned the area around the O-ring as best I could (some of the worm gear grease had migrated into that area). I then mixed up some epoxy and let it set for about 20 minutes so it was no longer so fluid. Using a toothpick, I packed the outside of the O-ring with epoxy. After letting it dry for a day, I packed some more in, my objective being to make that joint as inflexible as I could. The idea of the epoxy is not so much that it sticks to the metal but that it's fairly incompressible, and thus will stop those sides of the joint that want to move together. After letting it harden, I reinstalled the Dec. drive and ran the OTA back and forth. I was happy to see that I couldn't visually detect any motion in the joint. I was also very happy to see that my backlash problem had all but gone was now quite controllable with the telescope backlash control. Autoguided images improved dramatically. The telescope has been in more or less regular use for well over one year with this modification and still gives quite satisfactory results. No harmful effects have been noted.

The second mod involves simply removing the *pressure* screw (the one with the spring around it) that presses the worm gear assembly up against the declination gear...taking the spring off, and replacing the screw. I replaced the metal screw with a nylon screw that has a slight amount of give. The good news about this mod is that without the spring the worm gear no longer has the spring induced pressure variation that caused variations in the Dec. drive performance. The bad news is that you must be careful about how much you tighten the screw. Not tightening enough results in slack between the gears, tightening too much stresses the motor. I use the very unscientific method of running the Dec. drive back and forth while noticing the change of the sound of the assembly. It seems to be rather easy to tell by listening when the screw is too tight. That probably sounds rather empirical, but if you give it a try you'll see what I mean.

Where I live we'll have temperature variations from -5 to +90 degrees over the course of a year. Clearly this is enough temperature change to require that the pressure on the screw be adjusted as the seasons change. I just take a small pair of needle nose pliers, take the cover off the Dec. drive, and use the pliers to gently turn the screw while I listen to the pitch of the drive assembly as an assistant rocks the OTA back and forth using the hand pad. I probably do this once every few months. I facilitate this adjustment by leaving the screws off the Dec. drive cover. I use darkroom tape to hold it on, so it goes on and off pretty quick.

These two mods have dramatically improved the performance of my Dec. drive. If you have backlash problems, they may be worth a try.


Subject: Photo Available of Dec Mods

From: Don Tabbutt <>

I have taken the text from John's post and added it to the photo page at:
     Note: should open a new browser window over this one.


Subject: LX200 Dec Drive Mods --part 2 Top

From: John Ruthroff <>

I can only comment and relate on my personal experiences. I've had the spring out of my LX200 for well over a year now, and it's a 'scope that is used a lot. Before I took the spring out backlash was bad and could not be corrected with the software. After removing the spring my backlash all but disappeared, and when I applied the "epoxy fix" to the *O* ring joint backlash became a non-event.

I am aware of and understand your concern about the possibility that, since some gears may be off center, that the possibility may exist for the pressure to increase as a "warped* portion of the gear is reached. That's why I periodically adjust the tension of the screw that holds the worm to the gear. In my experience, the gear is not out of round enough to make this a great concern for causing damage. Plus the gear is only really used over at the most half it's diameter...when's the last time you pointed the OTA below the horizon?<g>. In fact, how often do we even point the OTA within 20 degrees of the horizon? My point is that we normally only work with half of the gear anyway.

Here's another view to consider. Since my LX200 was basically useless for imaging before I did the modifications (the backlash and retrograde backlash were so severe that the software backlash adjustment wouldn't compensate) and is now quite useful after being modified, to me it didn't make any sense to have an unmodified 'scope that was useless. The modifications were the only way to make it work, and I'll happily take my chances that I won't do any long-term damage. If I do then at worst I'll have to replace a gear or some other components, but again that's preferable to having a 'scope that is unusable.

My experiences in virtually eliminating backlash and retrograde backlash on my LX200 lead me to believe that the *O* ring joint flexibility and the pressure spring are indeed the cause of backlash. In your message you say that you doubt this is the case. Trust me, we are always interested in another theory about any aspect of the operation (or at times non-operation) of the LX200, so please feel free to throw in your opinion about backlash. If you've looked through the archives you've probably notice considerable confusion in trying to understand this behavior. In all sincerity, if you have a theory we'd all like to hear it. Eventually SOMEONE will figure it out.


Subject: LX200 Dec Drive Mods --part 3 Top

From: John Ruthroff <>

I would *speculate* that if your current meter is pegging now that you have a bit too much pressure on the screw. I would think too that there must be a definite pitch change (as in sound) to the motor. You may want to back the screw off some and then watch your current meter to confirm that the Dec. motor loading is the cause of the current increase...I'm sure it is but it never hurts to confirm. The original goal I had when I started fiddling with the Dec. drive was just to get the 'scope to the point where I could correct any backlash with the software. If your trying to eliminate it altogether i.e., with your backlash adjustment set on zero, then that's uncharted territory by me. Someone wrote a post (Doc G?) awhile ago that made the comment that the motors in the Dec. drive are hard to find items, so I wouldn't want to put so much strain on the motor (as may be what's being indicated by your high current readings) as to shorten it's life considerably. I know I made the comment in a post yesterday along that lines that if I have to replace something that's the way it goes, but I'll also avoid it if at all possible. So you might want to try backing the screw off until your current meter reads close to normal when running the Dec. motor, then see if you can beat the backlash with the software adjustment.

To be more specific (after rereading your post) I do not have the spring installed in my 'scope. I'm either fortunate that I have a gear more round than yours, or that the portion of the gear I'm using is very uniform. I snug the screw up against the bottom of the plate and "ear tune" it until it "sounds" too tight. Not very scientific, but it works for me.

There is a procedure for reinstalling the worm gear assembly here: Dec Axis It's one of the entries towards the top. However, if you want a fairly complete understanding of the declination drive, read all these posts. As you can seem some folks have spent a lot of time and energy on this issue. As always it's worth a look at Doc G's site also, those posts in written by him can be found on his pages too.

For "normal" operations, and I assume here you mean visual observing, I'd think that you could put the spring back in...or you could avoid that work by just backing off the screw a bit more. You'll get some of your backlash back, but since visual observing is much more forgiving I would think it would be ok. I've pretty much set mine up as an astrograph of sorts, so I've tried to optimally configure it for imaging only.

I've never thought about flipping the gear over. Take a look at the posts that discussing removing the gear (it's part of the replacing the bearings mod) doesn't sound like a procedure that's for the faint of heart<g>.

While were on the topic, as soon as I power the 'scope up I always reduce the motor speed to "3". I can't help but think that will extend the life of the motor, and it also knocks down the 'scope noise considerably.

Looking through the MAPUG archives yesterday, I found a post by a guy who found a place to buy the gears in the LX200. If one does have a seriously out of round gear, it may be worthwhile to run this down and see if the retailer has a "quality grading" of their gears and thus look into purchasing a gear that's not as out of round as the one on the 'scope may currently be. The post is towards the end of the list in "Miscellaneous Items."

Glad the epoxy fix seemed to help. My Dec. drive is going back to Meade tomorrow as part of the RS232 repair. It will be interesting to see if they chisel all the epoxy out and I have to do it all over again when I get it back<g>.


Subject: LX200 Dec Drive Mods --part 4 Top

From: Doc G

> I'm sure every bull (worm) gear is different in all scopes. My scope Dec. has a
> .010" run out which is pretty typical. Others might have considerable
> less which the spring might not be needed. It is real easy to tell by
> taking all the movement from the tilt plate and run the Dec. motor. If it
> binds you better put the spring and loosen the adjustment. If you don't
> have this problem, you don't need the spring. The spring takes the
> runout of the bull gear and keeps the worm gear meshed. Sticktion on the
> Dec. bearings cause, retrograde and the tight tilt plate and spring
> prevents, backlash (If adjusted correctly).

I have another suggestion for improving the worm to worm gear (bull gear) contact. This applies if you have a permanently mounted telescope. If the worm wheel is out of round or off center, there will be one side that is tighter and one side that is looser against the worm. Find the center region of the tight side and set the scope to be tightest at the declination that you use the most. Then set the tightness of the platform snugly against the worm wheel in this position.

Then you will have best adjustment for the region of the sky you use most and good adjustment elsewhere. You will never use the loose side of the worm wheel as long as you never loosen the Dec. clutch. It is never necessary to do so with a permanently mounted instrument anyway. You only need half a worm wheel anyway. I have designed and built just such a Dec. platform which uses only half of a worm wheel.

As long as you stay on the same half of the worm wheel, you will have minimized the need to have a springy platform for the worm. The amount of slop in the standard worm platform is way to large on all the LXs I have seen.

This is just one of those tiny adjustment tricks that make the drive work a bit better.


Subject: LX200 Dec Drive Mods --part 5

From: Nikolidakis George <>

I had a considerable amount of declination backlash just from the first day I got my new LX200 12" telescope and it was the first problem I had to control. I am now happy to say that I have complete eliminate the problem. Please visit my page where I explain with photos where the problem is and how you can eliminate it. Just hit the URL below because an image is better than thousand words.

   Note: should open a new browser window over this one.


Subject: LX200 Dec Drive Mods --part 6 of 6 Top

From: Doc G

Steve Glazener wrote:
> > In altaz mode you only need 1/4 of the worm wheel. But I'm not sure this matters
> >I think it matters a lot. Tuning up a 90 degree section of a
> >worm/wormwheel combination is much easier than tuning up the whole
> >wheel. --Doc G

> Doc-- I agree, but how does one 'tune up' the OEM gear assembly ... 'lapping' the
> gears (as posted by someone earlier on this list) is 1 process that comes to mind??

I did not have in mind something as dramatic a lapping the worm wheel. What I have in mind is something very simple. The worm wheel will generally be off center rather than out of round. That is elliptical. If it were elliptical the high and low portions of the wheel would have a second harmonic variation. That is, it would have two high and two low spots.

But if only off centered, it has one high spot and one low spot at the opposite side of the wheel. Thus if you set the worm at the high spot, at its extremes of motion which will be at worst 90 degrees away and at best considerably less than that. This means that the lifting of the wheel from the worm will be at worst 1/2 of the peak to peak run out. Generally you do not move the Dec. from horizon to horizon (South to North, but only from say 20 degrees above the South to 40 or so degrees above the North. Thus if you set the high spot on the wheel at 70 to 80 degrees above the South horizon you will have a run out of only about 1/3 of the peak to peak run out. This is a big improvement over using just any part of the wheel at random.

Also you will never get binding tighter than the high point of the wheel since that is the tightest it gets against the worm.

This is the way I have set the wheel on the 12" LX which is permanently mounted. I think it helps control Dec slop. Ideally one would center the wheel. This could be done by centering the wheel and locking it in place with the Dec. clutch or by permanently bolting the wheel to the moving part of the Dec. assembly. I have not gone to the latter lengths because I am satisfied with the technique described above.


Subject: LX200 Classic Runaway Dec Motor Problem --Part 1 of 4   Top

From: Doc G, Date: June, 1998

Well, this should give all you MAPUGers who have had runaway dec drives a chuckle. It has just happened to me. (alleged dec expert??) This morning I went to the Madison Astronomical Society site for our annual picnic. I got there two hours early to clean up the observatory and get the LX200 polished and running for Solar observation. When I turned on the scope. blewwy!! running dec with no control whatever. Dec plugs check all three of them! Nothing Turn it on and turn it off a few times. Nothing!! Talk to it. Nothing!! Talk in uncertain terms. Nothing!! Kick the pier. Nothing!! (bent toe!) Took off the dec drive cover and wiggled everything carefully. Nothing!! Wiggled everything harder. Ah Ha a momentary stutter. Turn off the scope. Check every wire visually very carefully. Ahh Haa!! Broken wire on one of the encoder lamps. The wire was broken right at the lamp. (in fact it was the lamp lead broken right at the seal) Very difficult to resolder. But did resolder using tweezers and hot soldering gun. Turned on scope. All fixed!! Thus another way to have and find a dec drive runaway. The picnickers enjoyed viewing the Sun just fine. Scope continued to work perfectly all day. They never knew the trouble I've seen. :-) More LX200s sold on the spot. :-) The scope tracked the Sun with minor centering corrections for hours, even without changing the frequency of the drive. Great spots with nice detail using the Thousand Oaks filter.


Subject: LX200 Classic Runaway Dec Motor Problem--Part 2  Top

From: Robert Preston

>The Declination motor and telescope motherboard have both been changed.
> Bad dec cable connection. Work the outside dec cable connections

The cable connections mentioned above are not the only possible cause of this problem. The basic problem is likely to be loss of encoder output pulses. When that happens, the scope thinks it is mis-aimed (and it IS, because it can't possibly aim without encoder signals) and it commands the motor to move at top speed to re-establish the aim. Without encoder pulses, it can't ever re-establish aim, so it will show the classic "runaway dec" symptom. There are numerous points where the encoder signal can get lost. The 5v supply for the encoder LEDs on the mini-PC board attached to the dec. motor can fry. The wires to the LEDs or from the detectors can break off. The encoder wheel itself can get trashed. The variable bias resistors on the mini-PC board can be set improperly.

There can be a piece of crud lodged between one of the LEDs and its detector (there are two of each). Etc., etc. If it is true that a motherboard and dec motor assembly change did NOT fix the problem, then many of the points of possible failure seem unlikely.

What's left are:

  1. The feed through plug in the fork has a crushed prong on one side or the other.
  2. The coiled dec. cord has an intermittent break, most likely at one plug end or the other.
  3. There's pigeon crap in the dec. jack in the front power panel.
  4. The front power panel DEC jack has a bad solder joint to the PC board.
  5. The ribbon cable to the front power panel isn't fully seated or may have an intermittent defect, again most likely at the connector. (This assumes that, during the motherboard and motor swap, the new and old parts did not get mixed up and the wrong one reinstalled, and that during the swap, the wires to the encoder LEDs and detectors on the new motor were not accidentally damaged, and silly stuff like that.)

Get out those high-powered reading glasses and a good strong 200 watt inspection lamp and look very closely for suspicious connections. Intermittent problems like that are a real challenge. Maybe by wiggling the responsible wire you can force the system to misbehave, and that will show you where to look for the problem.


Subject: LX200 Runaway Dec Fix--Part 3  Top

From: Aaron Barson

Just found a new reason for runaway Dec drives on the LX200 classic. Mine started this behavior during a private star party and caused much embarrassment. It had never done this before and I was genuinely puzzled. I searched the web for info and found a multitude of reasons and started searching my unit for a cause. All the cables and connections checked out. Took the cover off the Dec drive and checked the usual connections to the encoder pickoff and it seemed intact. Continuity was good and I then concentrated on a small translucent connector that attaches the wires to the PC board. By wiggling it the drive would calm down. I figured the contact was bad or the wire stripper integral to the connector was bad. Reset the wire several times and this seemed to fix the problem. Suddenly it went runaway again.

Discouraged, I took off the PC board and turned it upside down to see if the soldering was bad to the prongs connecting the above mentioned wire connector to the PC board. It seemed fine and I checked the continuity backwards from each solder connection on the PC board through the wires and plugs back to the plug that goes into the RA drive box/control panel. All was good. At this point I thought the PC board might be bad. I then started to reassemble the PC board and, lo and behold, the problem was found. There is a small thick plastic washer that holds the PC board up off the housing holding the dec drive. This washer was offset off from the mounting screw hole and was just close enough to the screw that it held the PC board off the metal dec drive housing most of the time. With moving the scope, the bottom of the PC board (where all the solder connectors are) would make intermittent contact with the metal dec drive housing. This would short out one or several contacts and the computer would lose encoder signal and instantly start searching, i.e., runaway. After remounting the PC board with the plastic washer in its normal position aligned with the mounting screw, NO MORE RUNAWAY DEC DRIVE! This was evidently an assembly problem at Meade as the washer is small and the space tight.

Since I had not found this particular problem listed in the usual web sites, I thought it might save someone sending Meade a perfectly good PC board for replacement when it was the proverbial "3 cent washer" malposition that caused the problem. I could see someone sending the PC board back thinking it was bad, getting a replacement, installing it with the washer in the correct position and thinking a bad PC board was the problem when, in reality, their correct reinstallation of the new PC board and washer was really what fixed their problem.


Subject: LX200 Runaway DEC Fix--Part 4  Top

From: Alistair Symon <>

As some of you may know from visiting my website I have been taking CCD images with my LX10 for over a year. Recently I decided that I had probably pushed the LX10 and its capabilities as far as they could go and decided to order an LX200 10 inch so that I could get the benefit of PEC and the more accurate drive for long exposure images. My LX200 arrived just over a week ago. I set it up, powered it on and everything seemed OK. Then I powered it on a second time and the DEC drive started to judder backwards and forwards very quickly. Suspecting a loose connection, I replugged all the DEC cables and tried again. Once more everything was OK. I powered it on a third time and this time I got the full blown Runaway DEC problem. I replugged the DEC cables several more times but the scope never powered up successfully again.

After reading the section on Runaway DEC in the MAPUG Topical Archives, I inspected the connections to the DEC motor itself. They looked OK. At this point I gave up and called the dealer (BC&F). They were very helpful and offered to replace the scope immediately with one that they would test first. So yesterday I drove up to London to pick up the new scope. When I got it back home I powered it on and guess what, the same runaway DEC problem again! However, because BC&F had tested it in the shop before I picked it up I knew the problem couldn't be with the electronics and motors in the scope. The only thing I hadn't exchanged was the DEC motor cable and the handset.
My prime suspect was now the DEC motor cable. I used a circuit tester to check the connection of each wire in the DEC motor cable. Every wire except the red one registered a complete circuit. The most likely explanation for the broken circuit was that the gold tabs in the jack plugs had not been pushed far enough in to the red wire to make a good connection. I used a screwdriver to force the gold tabs on both plugs further into the red wire. The circuit tester now registered a complete circuit with the red wire. I tried the LX200 with the fixed cable and it now worked perfectly. Problem solved! BC&F are now sending me a new DEC motor cable. I am sending this tale to the group as another example of a problem that can cause Runaway DEC and yet is relatively simple to fix. If only I had thought of testing the cable before going to the trouble of exchanging the scope!

I am looking forward to trying out the LX200 with my MX5 CCD camera and seeing how it compares with the LX10. I have really enjoyed using the LX10 and have no plans to sell or retire it. My wife Michelle plans to use it for her variable star work and as it is much more portable than the LX200 it will get used on those nights when it may be necessary to make a hasty retreat indoors due to the onset of showers.
Visit my website at <>


Subject: Other Dec Runaway Causes --part 1 of 2  Top

From: Bruce Thompson <> Date: Mar 2002

OK, a little while ago I brought about a situation where our LX200 wrapped it's Dec cord very tightly. There was a bad electrical smoky smell by the time I noticed what was happening and off course I shut down the power to scope. At the time I blamed the smell to motor strain or tortured electronic components. Later, when I repowered the scope the Dec motor took off! A lot of panic!

It turned out that the sideways stress on the Dec cord where it plugs into the control panel had caused a few of the terminal connectors in the control panel socket to bend inward. This resulted in poor or no contact being made on one or more of the connector terminals.

Also, the bending of the connectors terminal wires was so severe that it seems possible that adjacent connectors may have actually shorted and produced the tortured electrical smell that I had detected briefly.

Swapping cables will not solve this problem if the host socket is damaged. Have a very close look at the cable sockets! I was able to repair mine by carefully teasing the socket wires back into place with a hook tool fashioned from a very small 'safety pin'.


Subject: Other Dec Runaway Causes --part 2 of 2  Top

From: Manning Butterworth, Date: Jan 2003

After checking all the causes I found posted in the MAPUG Topical Archive and still not resolving the problem, I examined the internals of the LX200 hand controller. I noticed the ribbon cable connector seemed not to be fully seated on the connector pins. Using a pair of angled needle nose pliers and pressing the cable connector down onto the pins as well as I could, the cable connector seated itself better and when I powered up the LX200, the DEC motor behaved normally and has since. I don't think anything in particular loosened the connector. The contacts may simply have oxidized over time and no longer provided an adequate connection. In any case, there appear to be a multitude of possible causes for DEC runaway, and the ribbon cable connector in the hand controller can be one of them.


Subject: Dec Axis Encoder Wiring Failure Fix  Top

From: John Mahony <> Date: Sep 2003

The next thing to check is the leads on the encoder LEDs and sensors. These are prone to breaking. The LEDs and sensors (which resemble LEDs) have solid wire leads like most electrical components, but Meade mounts them head first in holes near the encoder mask, leaving the leads sticking out. Then flexible stranded wires are attached to the ends of the solid wires, with the other end of the stranded wire connecting to the circuit board. So the solid leads are only physically supported at the LED/sensors themselves.

The motor/gearbox is attached to the rest of the worm block (which holds the circuit board) with a rubber O-ring at the connection, so it can move relative to the rest. Any vibration/motion in the flexible wires moves the solid leads, which naturally (basic leverage) bend mainly at the LED/Sensor. All that flexing at one point breaks them. Difficult to fix too, because all that's left is a little stub of solid wire sticking out of the LED/sensor. Hard to solder a new wire to.


Subject: Classic Declination Drive Analysis Available  Top

From: Doc G, Date: June, 1998

I have added a small article with photographs to my website on the electronics of the declination drive. It is located under Electrical Concerns/LX200Analysis/Declination Drive Analysis. Note: should open new browser window over this one.


Subject: Dec Adjustment Adhesive  Top

From: Michael Hart

George S. Anderson wrote:

> I gotta problem. My LX200's DEC motor stopped working right. It would
> drawn so much power that it would burn out fuse after fuse. After opening
> it up and playing with it a while, I found the black stuff that seals the
> manual adjustment mechanism to the DEC motor had broken loose. So the
> motor just wheerrrrs, and wheerrrs, .... until it finally burns out a fuse.
> Does anyone know what the black stuff is? I believe I can reseal it, but
> I would like to use the original sealer.

The "black stuff" you are referring to is not a form of epoxy cement as I reported earlier. John Downs has correctly identified this adhesive as Locktite Black Max. I can provide the manufacturer's number. It is Locktite #38050.

It is very hard and a somewhat brittle and is used to hold a shouldered bronze bearing in position in the outer gearbox cover. This bearing must be adjusted to allow a bit of axial play but not so much that the gear attached to the manual Dec knob moves forward (towards the knob) and disengages the other gears.

If the original adhesive is intact (not cracked), you may want to remove the old adhesive from the case and bearing as there may have been a simple bonding failure. Roughen both surfaces before applying fresh cement. It is quite possible there was no adhesive bonding failure, but rather the adhesive cracked as it is usually quite thin. You may will want to consider using a two part epoxy a bit thicker to prevent similar failures and or use a stronger and a bit more flexible two part cement such as Devcon Plastic Welder.


Subject: Description, Analysis, and Repair of Classic Dec & RA Drives URL   Top

From: Bruce Johnson

I've prepared a detailed description, analysis, and repair procedures for the Classic Dec & RA drives at:

   <> Note: should open a new browser window over this one.


Subject: LX200 Dec Motor Mod   Top

From: R. Wallace <> Date: Nov 2000

The modification I'm talking about is the replacement of the 2 pots on the Dec. motor board. I change the low 'rez', high noise, single turn pots with high 'rez', low noise, 10 turn trim pots as shown in the image. It works well as a fix for Dec. motor problems like hesitant operation or run-away motor operation. I'm sure the operation of my 12 inch LX 200 Dec. motor is what Meade had in mind umpteen years ago.

The 2 replacement pots are clearly shown in the following pic. The pots are Bourns brand pc mount, 10 turn, trim pots. The 203 designation that can be seen in the image indicates they are 20K ohm pots. The exact resistance value of the pots they replaced. They can be obtained at any electronic parts supply store, but maybe NOT Radio Shack. The problem I've always had was intermittent jerky, unreliable slewing and slow motion control of the Dec. motor and gear assembly. The problem with the original single turn pots is they are naturally difficult to set a precise resistance value and hold that value. And, their quality degrades with age. Add a little bit of vibration from a motor and gear train, and you could have problems. According to the schematics from Doc. G's part of the MAPUG site, the pots set a bias voltage level on the LEDs. If the voltage is interrupted or has noise the LEDs don't work properly and the controller can sense false signals and try to correct when no correction is required. Since replacing the pots, the Dec. slewing and slow motion have been smooth and sure. No Dec. motor run away problems or jerky intermittent operation has been encountered.

dec pots


Subject: LX200: DEC Runaway Solved   Top

From: Robert Soubie <> Date: Oct., 2000

The dreaded DEC runaway happened to me while switching the telescope on.

I just solved the problem, after careful unmounting of the DEC subassembly and of the control panel, some testing at my job where I have a lab power supply and an oscilloscope, and some continuity checks on supposed equipotential lines with a digital multimeter.

All those electric tests were done with help of the MAPUG Topical Archives and of the schematics that are available from Doc Greiner's site.

Let it be said that contrarily to previous identified causes for the DEC runaway, the cause for this failure has, I feel, a pretty low probability figure. But it did happen, and here is the crescendo:

1) Opened the DEC motor's cover, checked connections (all cables and connectors), checked for broken wires on all diodes, switched the scope on and moved various wires in hope of finding a bad/broken connection, to no avail.

2) Took a DMM and began to measure various parameters, such as voltage across the two emitting diodes in series (should have read something like three volts, read millivolts). Ha-ha, said I, so I measured the output (millivolts again), then the input of the 78L05 voltage regulator: read 2.95 volts, not really enough: should have read at least 8 volts for the regulator to operate properly . This is power coming from the control panel (which I know has a V. Reg. in it) and may be from the motherboard. Of course, no voltage on the receiving diodes side either). So I unmounted the DEC subassembly and took it to my job.

3) Tested the DEC system under 12 volts, using a scope and having a second power supply rotate the motor: all was well and the circuit's output were two nice sinus waveforms in phase quadrature, as per the book.

My conclusion at that point was that there was a problem with the generation of the voltage either in the control panel or in the motherboard.

4) Back home, used the DMM to check connections between the control panel DEC connector pins and the 34-pins flat cable connector or, if any, the control panel itself. I found (or confirmed) that out of the 8 wires, two are for the motor, two are for the ground polarity, two are for quadrature signals and... one is for the 12 volts. Only one? Just read on.

5) Wanting to take voltage measurements on this 12 volts, I connected everything but unplugged the DEC chord at the DEC subassembly level and switched the telescope on. The DMM, at the control panel's connector read 12 volts. The same measurement at the DEC system's connector read... nothing. Having tested the cables and connectors for continuity, I was puzzled. I switched the scope off and retested for continuity, and sure thing the test was ok, but, guess what... on the neighboring pin...

I had found what the problem was: the actual lack of continuity for the 12 volts supply was at those pins' level, inside the control panel's printed circuit (not sure whether it is simply a double sided or a multilayered PCB). Since previously my scope worked, I have to conclude that for whatever reason this connection failed and caused the problem. This is highly unlikely, all the more when you consider that the current through this connection is maybe a couple hundred milliamperes. A mere solder bridge on the back of the control panel between those two pins solved the problem.

Is that all? Well, no. Think of it: the designer decided to devote two pins to the ground connection, and, as I just explained, he most probably decided to also devote two pins to the 12 volts polarity. That would have made sense. So even if that connection "burned", and it did, the 12 volts should have followed the other wire, right?

Wrong: there is *no* other wire. Of course, the coiled DEC chord, being a standard cable, has all 8 wires wired. But the cable in the fork arm only has *7* wires, and the small DEC PCB tracks are in coherence with this wiring.

I am very glad of having fixed this telescope, especially since it was bought in the USA and the people here in France may be reluctant to repair it just because of that (unlegal wrt European laws on trade but very real, this is MEADE's policy outside the US). I think that such a failure would have been very difficult and long to identify and fix, and of course very expensive for the poor victim...


Subject: Emergency LX200 DEC Cable Solution Top

From: Greg Pyros <> Date: Mar 2002

I just found out something that might save someone a hassle at an inopportune moment, and thought I'd pass it on. We all know that the LX200 DEC cable is wired exactly backwards from a standard CAT-5 cable, requiring some custom wiring or ordering a special cable. What I did just find out, though, is that the connector that is built into the fork arm also reverses the cable (again). So reversing the reversed cable gives us a 'normal' CAT-5 cable.

Inside the fork arm the upper (flat black) cable that goes to the DEC motor also has a standard 8-pin modular jack. What this means is that if you have a generic 8-pin inline coupler (female/female) you can use a generic CAT-5 cable, and just use the inline coupler instead of the connector inside the fork arm.

I've been using it for a while with no problems.


Subject: Dec Mod Dilemma (Classic) --part 1 of 11

From: Kevin Macey, Date: Mar 2001

I too am in the process of performing a Dec mod and have run into several issues regarding aligning the fork arms. However, your message reads that you ran the 1" shaft through one arm only. The looseness of the bearing rollers with respect to the bearing cage and the shaft will allow the shaft to pivot around the centerline of the bearing with the section that overhangs the bearing the most being lower. The only way to do the alignment would be to pass the shaft through both bearings leaving one arm loose to adjust the alignment to the stationary fork arm. The loose fork arm needs to be adjusted so that the shaft is level and the bores are in alignment. Alignment being on the same X,Y coordinate and being parallel to each other. The bearing and shaft should be such that when you can get it to spin freely, without binding, and slip from arm to arm without sticking you are close to being aligned.

The main problem I ran into was that the fork arms are toed in towards each other. If I measure the distance between the forks in the area of the Dec. bearings, the measurement gets smaller as I drop. The only way I could achieve alignment is to shim the top part of the fork arms so they did not toe inwards. This increased the spacing of the arms between the tube which I took up with thrust washers.

The fact that my fork arms were not parallel to each other was a major factor in preventing me from aligning my forks. Now my scope freely spins around the Dec axis with zero drag. I have not seen this being discussed before but find it hard to believe that it is isolated.

----- Original Message -----
From: David Olmstead > I'm not sure how to deal with this one. Perhaps Doc G, Jim Slay or others
> could help me out here.
> I'm in the middle of the Dec mod on my 10" LX200. So far things are going
> fine thanks to the procedure as written by Michael Hart and Doc. The bearings
> have been installed and that went pretty much without a hitch, but the
> alignment of the fork arms has run into a snag. I ran a precision ground 1"
> shaft through the new bearings after the one arm was reattached. The scope
> minus OTA was placed on a precision marble table top and I began to indicate
> the ends of the shaft as noted by Jim Slay. Great, everything was adjusted
> and zeroed out on both sides - what a snap this part of the adjustment process
> is! Wrong.
> Just for the fun of it I swung the fork arms around 180 degrees in azimuth and
> indicated the shaft again to check for accuracy. To my frustration I found
> that the shaft was not level at all. So, what this tells me is that the
> bottom base of the scope is not perpendicular to the RA axis. Does this sound
> right? OK, now what? If I am to get the whole thing squared up right it
> seems to me that the base has to be made perpendicular to the RA axis.
> I was thinking of tearing down the whole base and running a shaft through the
> RA bearings. The shaft would then be chucked up in the lathe and then the
> base would get turned and faced thus making it 90 degrees to the RA axis.
> Does this sound like it would work? Is it possible? Are there any "mine
> fields" to be encountered by tearing down the base to this extent?
> I would appreciate any help on this. Doc, I noted that you have photos of
> the RA drive torn down. I would appreciate a low res file of those if you
> would be so kind.


Subject: Dec Mod Dilemma --part 2   Top

From: Doc G

I would not try to machine the base. It does not matter that the RA axis and the base are not perfectly square. They can't be off by much I would think. You can still get the axes square by making the low side and the high side equally off when you rotate by 180 degrees.

The forks should be slightly pinched in at the top without the OTA in place. The idea is that they grip the OTA with some tension when the whole thing is assembled.

Because of the way the RA bearings are built, there is no center shaft hole. The bearings are on a tapered plug.


Subject: Dec Mod Dilemma --part 3

From: Jim Slay <>

Doc G was correct, don't worry about the off perpendicularity of axis, if you mount the base the same as you checked alignment all should be well. It could be corrected but a special machining jig would need to be fabricated and not worth it (I should not say that now I will want to make one!) I also have pictures if interested.

P.S. If your disassembling R.A. this is best time to pin, so as to be able to clean out chips.



Subject: Dec Mod Dilemma --part 4   Top

From: Kevin Macey <>

I too am in the process of performing a Dec mod and have run into several issues regarding aligning the fork arms. However, your message reads that you ran the 1" shaft through one arm only. The looseness of the bearing rollers with respect to the bearing cage and the shaft will allow the shaft to pivot around the centerline of the bearing with the section that overhangs the bearing the most being lower. The only way to do the alignment would be to pass the shaft through both bearings leaving one arm loose to adjust the alignment to the stationary fork arm. The loose fork arm needs to be adjusted so that the shaft is level and the bores are in alignment. Alignment being on the same X,Y coordinate and being parallel to each other. The bearing and shaft should be such that when you can get it to spin freely, without binding, and slip from arm to arm without sticking you are close to being aligned.

The main problem I ran into was that the fork arms are toed in towards each other. If I measure the distance between the forks in the area of the Dec. bearings, the measurement gets smaller as I drop. The only way I could achieve alignment is to shim the top part of the fork arms so they did not toe inwards. This increased the spacing of the arms between the tube which I took up with thrust washers.

The fact that my fork arms were not parallel to each other was a major factor in preventing me from aligning my forks. Now my scope freely spins around the Dec axis with zero drag. I have not seen this being discussed before but find it hard to believe that it is isolated.


Subject: Dec Mod Dilemma --part 5   Top

From: Bruce Johnson

In a message dated 3/01 David Olmstead writes:

<< I did notice something that I have not heard mentioned before regarding the
boring process. The fork arms where the boring takes place have significant
pockets or voids in the castings. These "bubbles" weren't troublesome enough
to prevent pressing the new bearings but the revealing of them, as the hole
was progressively enlarged, was disconcerting.

I wonder if the "toe in" problem could be eliminated by adding sufficient
thrust washers to the OTA shafts during reassembly so as to straighten things
out - but not too many as to overload the bearings. >>

Dave, My fork arms also showed the 'pockets' in them when they were bored out, too. I guess that's the nature of the beast in them.

As for adding additional thrust washers to the OTA assembly, I added one more .030" spacer than the three recommended, and I am definitely fighting the excess tightening of the OTA as it is rotated in the finished arms. I've even gone so far as to shim both fork arms out by .015"... which should correct for the thickness of the extra spacer... and I still do not have a really free swinging OTA as yet. I'm hesitant to add more shims to the forks to space them further, only because I'm concerned about getting to the point of having a nice, free-swinging OTA, but only because there isn't sufficient tension on the spacer/thrust washer combinations, and not know it!


Subject: Dec Mod Dilemma --part 6   Top

From: Doc G

David Olmstead wrote:
> Kevin, I apologize for the confusion. The fork arm, that contains the Dec assembly,
> was never removed from the base prior to boring. I simply left it in place
> and covered the base so the aluminum chips would not migrate into unwanted
> places. Only one arm was removed, reattached and adjusted.

You would probably find that the original lining up was not too good, so you might as well start with both arms off. After you get the arms lined up to your satisfaction. I would pin both sides so that it can be disassembled and reassembled easily in the future.

> I did notice something that I have not heard mentioned before regarding the
> boring process. The fork arms where the boring takes place have significant
> pockets or voids in the castings. These "bubbles" weren't troublesome enough
> to prevent pressing the new bearings but the revealing of them, as the hole
> was progressively enlarged, was disconcerting.

This is not serious defect as long as the bubbles do not mar the surface too much. It is typical of these rather crude aluminum castings. I saw it on all of the scopes I modified.

> I wonder if the "toe in" problem could be eliminated by adding sufficient
> thrust washers to the OTA shafts during reassembly so as to straighten things
> out - but not too many as to overload the bearings.

This will work. Never worry you will overload the thrust bearings. A 1" bearing will take several hundred pounds. The forces speeds and so forth in the telescope are trivial compared to what the bearings will take.


Subject: Dec Mod Dilemma --part 7   Top

From: Kevin Macey <>

Doc, I agree that the thrust bearings may be more than adequate to handle the load of spreading the fork arms, However, if the forks are not shimmed, the load on the thrust bearings caused by the preload of spreading the forks will be directly transferred to the optical tube assembly. This could have adverse effects on the corrector plate, could it not? Keep in mind that a cylindrical shape can take more load when applied in tension, inside to outside, as opposed to compression, outside to inside. Forcing the forks outwards is putting the optical tube in compression. Shimming the forks eliminated any bearing and optical tube loads. The bearings can be shimmed to result in a preload of +0.000 to -0.001 without any problem.


Subject: Dec Mod Dilemma -- part 8  Top

From: Doc G

You make an extremely good point about the need to not crush the tube. I really did not mead to imply that one should do more than a few thousandth of an inch. I always worry about shimming the bottom of the forks since the surface is so small and not perfectly machined in the first place.

In the three that I did, I had only to add one 30 mil washer and in the other two only one 10 mil washer to get a good fit.

Unfortunately there are a lot of machining flaws in such a mass produced item.


Subject: Dec Mod Dilemma --part 9

From: Kevin Macey <>

One of my fork arms had excessive and large voids in the casting and I had to oversize the bore and sleeve it. This was not much of a problem.

With regards to the "Toe in" problem, I shimmed my fork arm where they attached to the RA base. I inserted shims as required to each side to achieve parallel forks. The shims are only on the top side of the fork/RA base interface in order to push the upper section, DEC bearing, outwards. I needed to install four thrust washers, one 0.030" and one 0.060" thick washer per side. This eliminated the skew of the bearings with respect to the shaft and resulted in a good alignment. I subsequently drilled and reamed holes for steel dowel pins to prevent loss of alignment. I can now disassemble my forks and reassemble them and not loose my alignment.


Subject: Dec Mod Dilemma --part 10  Top

From: Bruce Johnson

Well, I just found another solution to another problem I've been having during the overhaul of my 10" LX200 classic. This one was related to the Dec bearing mod.

For my bearing replacement, I used the method of using bearings with an insert for the inner race, as described in Doc G's site, one of the addendum. I also took the forks to a machinist to do the boring of the holes and pressing the bearings in place. I made sure to tell him that the bearings must be pressed flush, as was emphasized in the write-up. All well and good so far.

When I assembled everything, I found that my OTA would NOT swing freely, as it was supposed to. Rats! So, I added a couple of shims to the forks, figuring I was somehow pressing the forks against the thrust washers and spacers too hard. No good. Same thing. It would swing, but not really freely.

FINALLY... after playing and playing, I found that the separate inner race 'tubes' for the needle bearings was just a *tad* wider than were the bearings!!! (A couple thousandths). Now, since the machinist had drilled the holes for the bearings, exactly to the depth to make the bearings press flush with the fork, this meant that the cotton picking inner race inserts were actually sticking out BEYOND the bearing surfaces, and when everything was put back together, it was these insets that was actually pressing against the steel spacers and thrust washers! No way could the thrust bearings and spacers do their job when they were being squeezed by these inner races!!!!! DUH!!

Finally, when it eventually dawned on me what was happening, I simply ground about .010" off of the ends of the inserts and put it back together WITHOUT any extra spacers added to the fork arms. EUREKA!! The tube swings like a pendulum now!!

Naturally, anyone that hasn't read the details of the bearing mod will have no idea of what I'm talking about. In fact, considering the terms I've been using.. probably incorrectly.. even those that HAVE read it won't understand!

Okay you machinists. Have a good laugh. Point at the dummy that didn't know the 'obvious.' But for anyone that's planning on doing the Dec bearing mod and using the bearings with an inner race insert in the future, hang on to this email. At least if you make the same mistake as I did, you will know to contact me and ask what in the world I was talking about!


Subject: Dec Mod Dilemma --part 11 of 11  Top

From: Daniel Kell, Date: Mar 2001

Architectural stone doesn't have the flatness required. Lee Valley <> sells a granite surface plate for $29.95 plus shipping. Its part number 88N85.01. This is flat to plus or minus .0001" It's 2"x9"x12" Its even listed as being moisture and corrosion proof. For a true straight edge see Brdige City Toolworks <> part number 1101-03 The SE-24 is a 24" straight edge, flatness is plus/minus .002" over blade length, both sides. Parallelism is plus/minus .004" over the blade length. They run $49.00 plus shipping. The surface plate is listed in stock and the SE-24 is out of stock. I have both tools and have done business for many years with both companies. The are excellent people to work with and stand behind their products


Page 1    MAPUG-Astronomy Topical Archive   AstroDesigns   Top