runaway error of my LX200 and troubleshooting  (in German

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On this site: Troubleshooting of the runaway error   Measurement with the oscilloscope   Troubleshooting of the errors of the front panel pcb and motherboard

Update: April,2 2011:

Meanwhile my telescope had another bug, this time with the R.A. motor. Without the knowledge and the experiences I made in 2009 with the repair of the Dec. motor,
I would neither recognize the problem, nor repairing the telescope so fast. At this point again my hearty thanks to Alan Sickling from U.K., who taught me so much knowledge how to handle
my LX200 Classic.

Actually an error appeared during the power up process of the LX200.

During the power up I heard slight cracking noises, which I never remarked before. Also the initial movements of the R.A. motor didn't occur as usually.
The R.A. axis covered a too large distance and changed the direction several times with it. After the finish of the initial procedure of the scope I could hear slight cracking noises in the rhythm of seconds.
In addition the light-emmiting diode of the console was showing shortly its full range lights in the step of the strange noises.

At this point something was going wrong. So I brought the scope into the workroom and I began to think about, what was happening. My suspicion was confirmed soon.
To make sure, that the potentiometers of the R.A motor were working properly, I measured them in the same way as described below. Indeed one of them was out of range.
After tuning the potentiometers of the R.A. motor, the fault was corrected after two hours at all.

Below you find the description of the similiar fault and the troubleshooting report of the declination motor, written end of 2009.

Based on the experiences of the last two years, I would like to point out, that it is better to check the potentiometers of both motors in advance, before a runaway problem might occur.


Story written end of 2009:

My first discovery of an asteroid was the last image of my telescope, before it got broken because of a runaway problem of the Dec motor.

What happened? Usually I use my scope remotely, controlling it with my computer in the sitting room.
Over the night at September, 28 2009, the LX200 automatically took images of some comets.
After standing up on the following morning,  I wanted to reposition the scope with the telescope controlling function of TheSky 6.
Being in the sitting room, unfortunately I didn't remark, that I provoked an uncontrolled runaway situation of the dec motor in direction South with full speed.

When I finally opened the door of the balcony, I saw pale with fear the OTA running with full speed in direction North.
After stopping the power I was really confused what happened. Later we could explain the changing direction of the OTA.
It came from the stressed reaction of the LX200 when the OTA hit the mounting.
So some parts both of the front panel pcb and the motherboard were damaged by this event.

From that moment on the OTA was running in direction North everytime when I turned the power on and I had no idea to solve the problem.

After contacting the LX200 mailing list and picturing the problem in message 30550, I got some interesting informations of a well-known "runaway" situation
of older classic LX200 telescopes.

Finally Alan Sickling of UK came in contact with me and helped to solve all the problems via email.
The following text is written by me, but everything is based on informations and instructions of Alan Sickling.
Alan is very experienced on electronics and he exceptionally knows the technical parts of a LX200 Classic.

Although I tried to collect the troubleshooting as precise as possible, read or use the following text very carefully under your own responsibility!

Troubleshooting of the runaway error

Indeed the scope had more than one error caused by the runaway situation. Both the front panel board and the motherboard were affected too.
The initial buck was a very simple justification problem of the potentiometers of the Dec motor pcb.
The runaway error of the Dec motor came from a shifting of these pots.

Here you can see the removed Dec. motor. The R.A. motor is fundamentally equal.


Already adjusted potentiometers (detail of a built-in Dec motor).

While running the Dec motor with any desired speed over the handbox, the pins of Pot1 and Pot 2 should have ideally 2.42 V.
If you can't measure any power on Pin 1, 2 or 4, the error is probably lying on the cables or on the boards.

The marked pins of Pot 1 and Pot 2 are connected with the potentiometers 1 and 2.
Meade fixed the screws of the potentiometers with a red adhesion, but they are easy to release with a screwdriver.
Then you can turn each screw of the potentiometers with a screwdriver as long as both "Pots" have a value of 2.42 Volt.

You will quickly remark, that the overtwisting of  the screws will affect a runaway in both directions. The ideal position of the screws is exactely in the middle
of the runaways positions. This middle position is representing the desired 2.42 Volt.

De facto everybody can correct the potentiometers maybe even without demounting the Dec. motor and by using a voltmeter on DC mode.


design of the measurement with a Voltmeter in DC mode (example)

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Measurement with the oscilloscope

Another method to check the potentiometers is the use of an analog oscilloscope in DC mode.
On the image you can see, how the probe of channel 1 is accomplished. The ground wire of the probe is attached on the ground pin of the PCB.
The 1:1 probe itself is attached here with pot2, representing potentiometer 2.

It's not easy to take a good image of the screen, but basically you can see, what we can expect. The short lines on the top should have a similiar length as the lines at the bottom, but shifted one step.
here you find the screen with descriptions.

The runaway error was announced many months before the final problem happened. Sometimes I had a runaway situation of the Dec motor,
but I ignored it intentionally, because the error disappeared after turning the power off and on again.

On this links you can find Alan's method to repair the runaway problem: 12" LX200 Classic RA Problem and Resolution by Mike Lecza

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Troubleshooting of the pcb and motherboard errors

Unfortunately this very simple problem was not the only one the scope had. On pin 4 of the PCB of the Dec motor was no power.
This lacking of power on Pin 4 was caused by a short-circuit on the front panel board.

I was really confused of the labels on the board. The 34 pin connector of the front panel board is described wrong on the board.
So please use my markings of the pins from 1 to 34. Important: use the voltmeter only in DC mode!

Pin 17 of the 34 pin connector should deliver the 12 V or 18 V power for pin 4 of the dec motor pcb. So I searched for the culprit and found it finally.

Pin 17 of the 34 pin connector is originally connected with pin 5 of the Dec motor socket on the front panel pcb.
The short-circuit was hard to find, because it was hidden under the socket of the handbox.

Front Panel Component Side LX200 Classic

All essays to repair the damaged solding position had no success. So we decided to span a cable between Pin 17 of the 34 pin connector and pin 5 of the dec motor socket..

Front Panel Component Side LX200 Classic    Front panal component side
large image 2 MB
Front Panel Solder Sider LX200 Classic Front panel solder side
large image 2 MB

Peter Lloyd reported a similiar problem on the LX200 forum in messages 26425 referring back to
messages 22362 and 22358 (dated 29 Nov 2006).

After getting power back to the Dec motor, we remarked, that it was still impossible to adjust the potentiometers on the Dec motor pcb.
Alan found out, that the reason is laying on a problem on the mother board itself. He focused the error to the L2724 controlling chip of the Dec motor.

This is the motherboard of my LX200.
Under the heatsink (cooling plate) are two L2724 amplifier chips (datasheet). One of them is controlling the RA motor (U17),
the other is responsible for the Dec motor (U18).

L2724 Group on a Motherboard of a LX200 Classic

Large image 2.8 MB

The L2724 chip on the right (U18) was damaged because of the stressed situation when the OTA hit the mounting.
On the heatsink you can see the old thermal paste. After unsrewing the heatsink,
 remove the old white thermal paste. Then reapply some new fresh thermolube on the heatsink.

Motherboard LX200 Classic Component Side

Large image 4 MB

I bought some L2724 chips from a German internet discounter and asked an electronic shop in my hometown to change the defect chip.
The guy there did a good job and replaced the damaged L2724 chip.

After repairing all the components which were affected or damaged by the unobserved runaway situation, it was very easy to adjust the
potentiometers of the Dec motor. After two months of repairing time my old LX200 was running again.

Thanks so much to Alan Sickling for his wonderful help. Without him, I would lose my old scope for sure and forced to buy a new one.

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