ETX TECHNICAL TIPS Last updated: 7 November 2003

DS Drive errors

Sent:	Friday, November 7, 2003 08:27:44
From:	mhutchin@mail.camre.ac.uk (Hutchings, Mike)
I have been troubled by the variable accuracy of the goto features of my
DS90 for some time and have completed all the modifications such as
drive electronics, power supplies and leather washers. While minor
improvements have occurred it is still hit-and-miss, mostly miss!!!

I noticed a problem that may have some relevance while I was taking the
telescope apart for the production of a pier mount. I looked at the worm
gear and clutch assembly and noticed that while there are adjustments
for worm gear backlash, the clutch assembly has no provision. You might
say that if the clutch is working there should be no backlash but this
is not the case even if you over-tighten the lock knobs; Meade suggest
that you just gently tighten the knobs. With the Meade recommendation
there appeared to be 2 degrees of Alt and Az movement due to the clutch!
Further investigation showed that the two three-toothed washers either
side of the worm gear forming the clutch are very loose in the three
grooves of the threaded stud, about 0.2mm gap, and this allows clutch
assembly movement. It appears that the leather washer modification can
affect this if the washer is tight on the thread such that the leather
extrudes into the gap so reducing the play; mine was not!

My solution was to modify the steel washers such that they are a tight
fit in the grooves and this was done using a small hammer to carefully
spread the three tongues on the washers, being sure to check for fit. If
there was too much spread I used a small triangular needle file to
adjust it. I made sure that the washers retained their flatness by
rubbing them on fine grit paper on a flat surface. Applying this
modification to all four washers (Alt and Az), I have reduced the play
to less than 0.25 degrees Az and 0 degrees Alt. The drives now lock with
a light touch on the knobs as recommended. This is not an ideal
modification as the clutches need redesigning but it does the job.

I am disappointed with the Meade mechanics, which contradicts their
excellent optical quality as I have had to make other modifications to
poor mechanisms such as the finder scope adjustment and the focussing
drive that had so much play in it that fine focussing was near
impossible without having to then adjust the tracking. My solution for
the latter was to pack the rack groves with shims to take out lateral
movement and use sticky tape to ensure a tighter fit of the focussing
tube in the main tube thus making focussing possible. The finder scope
mount was modified to two three-screw adjustments rather than the
two-screw adjustments on the original to overcome the problems there.
There is no excuse for poor engineering, even in mass-produced items.

Unfortunately I have not finished the new pier to try this latest
modification on the heavens but I am confident that this has removed
another major obstacle to accurate tracking. I will keep you posted.

Good observing.

Mike Hutchings - mhutchings@orange.net
52 25"N 0 32E

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