Last updated: 30 June 2002

Inside the ETX-105

Sent:	Saturday, June 29, 2002 5:46:37
From: (Bruce Dickson)
Great site!

I'm trying to improve the dec axis locking of my ETX 105 - it looks like
I need to remove the optical tube from the forks to get to the clutch.

Has anyone (besides Clay :) ) experience with this? (I tried searching
the archives for any ideas, but the '105 seems to be quite different to
the '90 and '125 models.)

I decided to take the plunge, and dismantled my '105. As suspected, it's
quite a bit different from the other models - the primary difference
being that it is _not_ possible to remove the tube by slipping it out of
the mount with 1/4" Whitworth bracket attached.  This makes using the
'105 as a guide scope a more difficult than the other ETX models.

I found that the declination axis lock is _much_ more positive now that
the clutch is free of grease.

Here's the drill I used for a complete strip down. Photo's to follow
once I've downloaded them from the camera, but for now this might help
someone who's trying to degrease the clutch or mount it on a larger
instrument. LHS refers to the left hand side when the tube is viewed
from the eyepiece end.

Clear, dark skies


1. Make sure the lens cap is on. Remove the batteries, controller,
finder + bracket and  focus knob or electric focusser (if fitted).

2. Remove the 3 screws in the rear plastic molding on the same surface
as the focus knob. (I've called this the EHA - for eyepiece holder
assembly.) These secure the tube into the mount. The optical tube can
now be gently slid out. Note that the flip mirror will be exposed inside
the EHA, so you could clean it if necessary. Set the optical tube aside,
so that it won't roll off your bench ;-)

3. Remove the locking nuts and declination scales from both fork arms.
Remove the brass ring from the RHS shaft. Remove the 3 screws from the
LHS and RHS plastic covers on the fork arms. Remove the outer plastic
covers. Removal of the inner plastic covers can only be done after steps
6 and 8 below.

4. Turn the scope over, and remove the 3 rubber feet from the base.
There are screws under each of these that secure the cover to the RA
drive mechanism. Remove these screws, and carefully lift the cover. The
battery box has two wires which are fairly short, and care is needed to
prevent them being damaged.

5. Inspect the edge of the aluminium casting. There is a notch cut into
it. Rotating the base will allow the notch to be alligned with the
screws that secure the fork arms. Loosen the screws than hold the LHS
fork arm, then remove them one by one. I found it was necessary to use a
little "Prestik" on the end of my Allen key, to prevent the screws
falling into the base. (Prestik is a locally manufactured grey goo,
that's often used to stick posters onto walls - I don't know of a US
equivalent, but there are several in ZA.)

6. The LHS fork arm is now loose, and can be eased out of the base. Take
care not to damage the plastic pin that the scope rotates around, as you
remove it from the bearing. You can leave the 3 screws that secure the
bearing in the LHS fork arm in place.

7. Remove the 4 Allen cap screws from the sides of the EHA that secure
the EHA to the mount. This alows the EHA to be reattached to the optical
tube for guide scope use, or you can proceed with further dismantling to
degrease the declination clutch. As noted by ???, the EHA is "dimpled" -
these dimples are the reason the tube cannot be slipped out of the mount
like the '90 and '125.

8. Loosen (but do _not_ remove) the 3 screws that secure the RHS fork to
the base. If you remove these screws, you may damage the wiring in the
fork arm that runs to the dec motor. Pull gently on the bracket that
attaches the EHA to the mount, and it will pop out, exposing the main
bearing, declination gear and clutch washer. These can be removed and
degreased. As with the other models, I'd recommend an absolute minimum
of lithium grease on the rotating parts, and nothing anywhere else.

9. Don't use paper towel on the declination gear - the corners are
sharp, and you will get a lot of paper fibres jammed between the teeth -
a solvent and an old toothbrush are probably better.

10. Assembly is basically the reverse of this procedure. _I think_ the
order for the declination clutch components is (inside to outside) main
declination bearing, declination gear, clutch washer with flat side
inwards. I'm not sure of this - I didn't know what to expect when I
pulled the bracket out - but it matches the wear patterns, and my scope
seems to be working fine.

11. You will probably need some thin double sided tape or contact glue
to get the rubber feet to stick. Sorry - I just found this out too!


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