runaway error of my LX200 and troubleshooting (in German)
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,
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
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.
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.
When I finally opened the door of the balcony, I saw pale with
fear the OTA running with full speed in direction North.
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.
Finally Alan Sickling of UK came in contact with me and
helped to solve all the problems via email.
Although I tried to collect the troubleshooting as precise as possible, read or use the following text very carefully under your own responsibility!
Indeed the scope had more than one error caused by the runaway
situation. Both the front panel board and the motherboard were affected too.
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.
The marked pins of Pot 1 and Pot 2 are connected with the
potentiometers 1 and 2.
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
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)
Another method to check the potentiometers is the use of an
analog oscilloscope in DC mode.
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
the lines at the bottom, but shifted one step.
The runaway error was announced many months before the final
problem happened. Sometimes I had a runaway situation of the Dec motor,
On this links you can find Alan's method to repair the runaway problem: 12" LX200 Classic RA Problem and Resolution by Mike Lecza
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.
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.
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.
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..
After getting power back to the Dec motor, we remarked, that it
was still impossible to adjust the potentiometers on the Dec motor pcb.
This is the motherboard of my LX200.
The L2724 chip on the right (U18) was damaged because of the
stressed situation when the OTA hit the mounting.
I bought some L2724 chips from a German internet discounter and
asked an electronic shop in my hometown to change the defect chip.
After repairing all the components which were affected or
damaged by the unobserved runaway situation, it was very easy to adjust the
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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