ETX-90RA DECLINATION AXIS REPAIR Last updated: 6 October 2010

Sent:	Friday, October 1, 2010 11:54:11
From:	Paul Bonneau (pjb1@tctwest.net)
I have the plain ETX-90 with the manual dec adjustment. On mine, the dec
"slow-motion" adjuster was extremely stiff; so stiff that I worried I
would not be able to detect the end of its movement and thus break
something.

The first photo shows what the mechanism looks like after taking things
apart (the procedure is the same as shown in other tech tips). The two
aluminum knobs turn the screw, and an aluminum button moves on the
screw, and the plastic "yoke" (I guess you'd call it) that moves the
declination has a forked end that slides in a slot on the button.



Essentially all of the friction is where the knobs contact the plastic
arm at the end. In my case the yolk also gripped the button too firmly,
I thought, so I widened the slot in the yolk a tiny bit with sandpaper
(one must be very careful not to remove too much material, thus
introducing slop into the mechanism).

Photo 2 shows where I have flipped the button over and I'm checking the
grip of the slot on the button. Once I got it where it slides, but
without gripping the button too hard, I stopped. I cleaned it and put a
little grease there so it could slide on the button while still gripping
it.



Photo 3 just shows the slot on the end of the yolk that I was (slightly)
widening.



I noticed the button had some slop on the screw; the tap used on the
button must have been too large. I attempted to squeeze that end of the
button with a vice-grip, hoping to deform it enough to grasp the screw
without slop, but I was unsuccessful in that. I wanted to take the whole
mechanism apart but never saw how to get the knobs off. I thought about
greasing the screw with very heavy grease to fill up the slop, but
didn't do that.

Photo 4 shows my hand squeezing the plastic arm. When this is done, the
knobs rotate freely. I watched the action of the screw while rotating
the knobs and detected a bend in the screw. I put the bend at the top of
its arc, and then pressed down on the center of the screw, straightening
it somewhat, at least as much as I could detect. This straightening
removed some of the on-and-off friction in the rotation of the screw
knobs, making it more even.



I had some teflon-based lubricating oil, and put a drop into each point
where the knobs contacted the arm, while squeezing the arm to make room
for the oil to flow into the joint. After doing this, and after
reassembling, the knobs rotated just about perfectly - with enough
friction to hold, but not so much that it was difficult to turn. I
believe there is supposed to be some friction in this mechanism, because
there is no lock for it (the "declination lock" does not lock the
mechanism).

While the thing was still open, I moved the button to the center of its
travel, then with a knife made a little notch where the declination lock
was. This allows me to find the center of travel without having to run
the thing to the end and then going back 50 turns as the manual
suggests!

When reassembling I added some plumber's teflon tape, as suggested
elsewhere here, around the declination pivot pins. I used 3 turns.
Seemed to help reduce slop in that joint.

In the 4 screws holding the OTA to the fork mount, some ham-handed
assembler had broken through the plastic on one screw and cracked a
couple of others. I don't believe these screws should be very tight. I
suppose I will have to see if I can pry a couple of these plastic pieces
out of Meade. The design is very poor here.

That's it!

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