ETX-125: AN UNLIKELY SOLUTION TO RA DRIVE JERKINESS Last updated: 17 August 2011

Sent:	Monday, August 15, 2011 17:00:05
From:	Yannick Dutertre (yannickd@gmail.com)
I was playing around with an ETX 125 when I noticed that when turning it
360 degrees, there was an angle of about 30 degrees where it became
noticeably more difficult to turn, as if there was some chaffing
somewhere on the base. This caused the slewing at slow rates in these
areas to become VERY jerky: at speeds 1 through 4, the telescope would
move in noticeable increments (causing a phenomenon of it tracking
jerkily ; as one of the user feedback from November 16, 2004 says: "when
it is tracking it will hold a star fairly well, (there is a bit of
drift) but then every 15 to 30 seconds the tracking will jerk the star
back to center. The RA motor seems to make a bit of a straining sound
before the jerk back"

The slewing outside of this 30 degrees danger zone was perfect (besides
the backlash in the nylon gears but one can't beat physics although
training/RA percent does help).

After useless inspections of the drive (it looked absolutely perfect and
I couldn't find an issue there), I noticed that in the 30 degrees danger
zone, the RA setting circle would become very difficult to turn. And
then I noticed that it was somehow getting stuck: it was too wide, and
was actually chaffing on the mount, causing the jerky movement. Removing
the setting circle and carefully making it less wide by 1mm or so using
scissors solved the issue. It is very surprising that the whole ETX
experience could be spoiled by such a problem!

Now one interesting thing is that I also checked on the demo ETX 125 at
a shop (new model from 2011), and it had the exact same problem! Could
this problem (that is not so easy to spot as it affects a 30 degrees
angle and is not *that* bothersome because the scope does catch up
eventually) be under reported? As such I think this could be worth
checking for beginners in case of problems, before opening the base.

Best regards,

Yannick

And:

And I found that out thanks to your advice of turning the scope from
stop to stop to evenly spread the lubrication. So I guess one more
beginner advice is to try to feel chafing when doing that...

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