Looking back I think it started with: Bad pointing and bad tracking in RA.
The pointing accuracy was always spot on: even with my ATK16IC mounted new objects immediately were on the preview screen, even when the GoTo was over large parts of the skies. I had noticed a reduced pointing accuracy, but as I could easily manually correct I gave it no further thoughts.
For the tracking I had to use frequency 56 Hz to minimize drift, which surprised me.
Yes, I know that drift in RA usually is caused by the Periodic Error of the mount, but in my case I was sure the problem was somewhere else. However selecting a low Frequency was a simple and easy workaround and although I knew something was amiss I postponed any action.
But then - after imaging DSO's with a forgiving focal length of 240mm - I imaged Mars, with much longer focal lengths.
When imaging Mars with my 5xPowermate [focal length 12500mm] I noticed a recurring jump in RA, and a later inspection of the AVI showed that the jump occurred once every 8 seconds
The time for action had come.
I consulted the LX200 Yahoo Group and I received quite a lot of hints where to look for.
The culprit was thought to be close to the motor: broken tooth or dirt in the encoder disk.
|The sole of the scope has been removed.|
|The connectors to the RA motor assembly have been disconnected.|
The RA motor drive assembly.
The RA motor drive assembly.
Please note the black cover of the reduction gearbox.
The RA gearbox: the black cover of the reduction gearbox has been peeled off thus revealing the inside.
All looking OK.
|Here you can see a part of the encoder mask sticking out of the gearbox.|
From BAD to WORSE
In order to detect and actually see the 8 seconds jump happening I wanted to see the gearbox in operation, so I reconnected the RA motor to the scope.
Alas: I made a grave error: I misplaced the wide flat black connector one pin to the right, resulting in a short circuit. Sparks, smoke, a stink and a blown fuses were my penalty and there were consequences too..
Shocked by this experience I consulted my good friend and electronics wizard Matthias Meijer and with his help - by phone - it was concluded that at least 2 components had not survived the short circuit: condensor C1 and voltage regulator VREG1 (see the documents at the bottom of this page).
The 2 dead components: the condensor is the yellow/orange component; the voltage regulator the black one with the flat edge.
Keen eyed Matthias also spotted a crack in the BA10339 chip, so that one need to be replaced as well.
So I went to the electronics shop and for a few Euro I got the replacement parts. I de-soldered the old components, but that turned out to be a tough job. I mounted and soldered the new parts, but was rewarded with a motor run away.
And when I wanted to measure if the IR LEDs were indeed getting the required 5 Volt the motor abruptly stopped turning, but all the fuses are OK, the LED array is burning and the DEC motor reacts when powering up ...
This was the turning point for me: I absolutely need expert on-hand help: electronics is not my cup of tea.
Matthias Meijer came over to help me !
Matthias is repairing the damage on the drive unit: the electronics of the RA motor board.
When this was completed we were disappointed, as the unit did not work when attached to the rest of the LX200.
After a lot of searching and testing we suspected the L2724 RA motor driver [operational amplifier] on the main board [the similar L2724 for the DEC motor was replaced during a previous repair operation].
After installing a new L2724 the RA motor suddenly came to life.
It is unsure whether the original problem - the 8 sec hiccup - was caused by the degrading L2724 or that the L2724 died because of the short circuit.
When the RA motor finally worked Matthias used his oscilloscope to adjust the comparitors, but when this was completed we were not happy with the sound the gear/worm combination produced.
An investigation showed that there was unacceptable play between the gearbox and the worm house: the O-ring was in bad shape.
Matthias first inserted small pieces of hard plastic between the gearbox and the worm house so that there was no more play.
Next the whole connection was fixed with epoxy glue, giving a rigid join.
When testing under the stars the same operation had to be done to the DEC too, and so we did.
Note: make sure that the spring that pushes the worm against the big cogwheel is man enough to do the job.
After the above repairs my LX200 is fully operational again: tracking and pointing works fine, as you can see below !
This 300 seconds UNGUIDED exposure - with my ATK16IC camera and William Optics Zenithstar 80 ED II APO at f/3 [focal length 240mm] piggybacked
on the LX200 - shows nice round stars: a sure sign of nice tracking.
Matthias concluded that the 8 second hiccup problem was not caused by faults in the encoder area, but most likely by the degradation of the L2724 operational amplifier in combination with play between the gearbox and the worm house.
John Mahony of the LX200 group commented: "someone pointed out that if the trim pot adjustment was borderline, it could read some encoder slots but not others (giving results the same as if some disc slots were blocked), and from what I've seen of the way Meade uses these encoder discs, that's entirely possible. So I think it was just Matthias' adjustment of the pots that cured the original problem."
Also see: LX200 DEC problems and repair.
LX200 Motor Assembly Sh11.pdf
LX200 Motor Assembly Sh21.pdf
More LX200 Schematics
If all this is beyond your capabilities [it is beyond mine !!] then you can turn to
Ron Sampson [USA] or Alan Sickling [UK] for help.
On the Yahoo LX200 Forum Ron Sampson writes:
"If you need any assistance, advice or repair service I am happy to assist you!
I repair LX200 Classics in the States (hundreds of them) but I have repaired numerous systems abroad, so international shipping is not an issue.
Please feel free to contact me if you wish"
Of course I would be happy to add more "Techies" here !!!!