ETX TECHNICAL TIPS Last updated: 25 July 2004

ETX 105 Alt/Az gearbox replacement

Sent:	Saturday, July 24, 2004 11:03:11
From:	Jim Beston (james.beston@btinternet.com)
You may remember I mailed you on June 25th concerning a broken ETX 105
alt/az gearbox. At the time I was able to keep the 'scope functioning
well enough (a bit of "slack" in the movement though!) by using a cable
clip to keep the final drive shaft located in it's "ring cradle" which
had broken.

My Meade supplier, Luigi Papagno. of www.f1cameras.com kindly obtained a
new gearbox for me (under the warranty) and I would now like to install
it. I looks quite straight forward but I just thought I'd ask if you
knew of any pitfalls that I might encounter?

I realise that I will have to calibrate & train the drives and check the
persentage settings. Will there be any need to do a full reset?
 
Thanks & Regards
 
Jim

And:

No Mike, no instructions - I guess I'll see if it is as straight forward
as I think to replace it! I've spent quite a bit of money on my 105 -
accessories and the like. So I reckon I deserve a bit of luck on the
warranty.

Regards

Jim

And:

I have already picked up some useful info. from "Tech. Tips" I'll let
you know how I get on!

Jim

I replaced my ETX 105 alt/az gearbox and motor today - not entirely
without incident but now all is working O.K. -so far!!

I thought I would give an account of the replacement just in case it is
of benifit to others:

Removal:

Really no problem at all 3 screws hold the gearbox to the base 3 more
hold the worm drive assembly to the base - tight in its housing but
"wiggled" out O.K. Two smaller screws hold the circuit board to the
gearbox - although I removed these before withdrawing the gearbox, it
would be better to leave them as the board and box can be withdrawn as a
unit and the board more easily separated when the unit is out of the
case. I noted the way round the two electrical connectors were, although
it may not be possible to reverse them on reassembly. The unit was out
in about 5 minutes and I thought I'd have the whole job done in half an
hour.....

Replacement

.......However, I noted that the actual worm section on the worm drive
on the new box (see photo) occupied more of the brass shaft than the old
worm. No real consequence you may think but it meant that the screws
that held the old worm assembly would not now squeeze past the shaft and
into the two holes in its  yoke because the "extended worm" was in the
way. The obvious action was to remove the worm from the yoke, put the
screws in and replace the worm. Easier said than done. I reckon a
Mexican Assembler with the muscles of Superman must have tightened up
the nut at the end of the worm drive. I hadn't access to a pair of
self-locking pointed pliers, to hold the shaft, and I could not shift
this nut without risk of injury to myself or (worse) the new unit!





Plan B swung smoothly into action - two flats were ground into opposite
sides of the two screw-heads which just enabled them to slip passed the
worm and into the holes on the yoke. Straight-forward after that -
except that when I had fixed the whole assembly into the ETX base I
noticed two "O" rings of abouut 3-4 mm i.d.sitting in the ETX base. (By
the way it is a good idea to put some tissue into the bottom of the base
as it lies on its side to prevent little screws dissapearing into the
nether regions of the case - a pair of tweasers is useful in retreving
such screws). I could not remember seeing these "O" rings during
dissassembly, but they were the size of the two screws that hold the
circuit board to the gearbox. Since I had not noted them between the
screw head and the board, I reasoned that they must go between the board
and the base. So off came the box and board to insert them.

On replaceing the unit and powering up I was dismayed to find that the
whole alt/az gear train was very stiff compared to how the old unit was.
I rembered advice from Dr Sherord that the nut at the end of the worm
drive was somtimes tightened too much at the Factory - of course this
was the nut I couldn't shift!! I took the unit out (again) and held the
shaft of the worm in a pair of pointed pliers and clamped the handles of
the pliers in a large vise (vice). By tightening the vise I was able to
get enough of a grip on the shaft to loosen the nut - about 1/4 turn
seemed appropriate!

Back went the unit and when powering up I found that the gears wouln't
move at all! This is where I learnt the importance of tightening the
gearbox assembly down evenly and (presumably) flush with the base. If
you don't the gears lock or make a racket as though (probably the large
thin one) is catching on something - ther must be a very small clearance
somewhere. Having secured the box properly, I proceeded to do the usual
calibrate and train routine. In addition it was necessary to check the
percentage settings using the method of Dr Sherrod as described, in his
mail to your site, of Dec. 12 2001.

The Dec/RA, could be left at zero percent but the ALT/AZ required
setting at 80% to remove backlash and allow a smooth movement.

Although all now seems O.K., I have noted that to manually rotate the
scope in ALT/AZ with the clutch disengaged, requires considerably more
effort than before - although not excessively so. In addition I find
that on slewing right or left in ALT/AZ at moderate speeds (3) there is
a definite, but again not excessive, acceleration as the object moves
across the field of view. Maybe the new box/motor requires "running in".

Anyway I'm a Happy Bunny - I just hope I remain that way!!

Kindest Regards

Jim

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