ETX-60 REPAIR Last updated: 8 June 2008

Sent:	Thursday, June 5, 2008 10:54:55
From:	Chris Erikson (Chris.Erikson@insitu.com)
First off I would like to think you for the invaluable contribution your
site makes to ETX owners. You have created the gold standard gold mine
of ETX/DS info. I've been visiting for months and still haven't read
everything, and have learned a heck of a lot.

Secondly I have a contribution to make below the hash marks you'll find
the contents of a thread from CN concerning my trials and tribulations
with GOTO runaway and uncommanded speed changes during drive training.
Info from your site gave me the confidence to open it up, then I ran
into the fact that many of the articles concern the last revision of the
ETX base, which has smaller gears and an entirely different worm setup
and motor placement than the newer ones.

You do have one link to some pictures of the guts of an late model ETX70
with the plastic gears, here is info on what I ran into on my ETX60BB
with the large gears, my problems and frustrations, then how I resolved
the issue..part of which I had inadvertently caused. It's wordy but
informative I hope. My desire is to contribute to maybe helping some
other poor sod figure out their troubles.
------------------------------------------------------------------------

Part 1) The problem
 
Hi folks 
The 497 handset I picked up was working well but now the system has
developed a couple problems.

The other night my GOTO's started failing big time, it would get close
then during what is usually the 'refinement' phase where it zeros in,
suddenly the scope just takes off at high speed and slews all over the
place.

I pulled the scope apart (1st time dissassembly) to check the encoders
and found grease everywhere in there, they really slop it on. I didn't
see any in the encoder wheels but I carefully used a fine q tip to clean
alt and az encoders anyway. Spun them each again to check that no cotton
strands were left on them, they look clean.

Yet now I have two problems.

I reassemble and problem number one during reassembly is I am not sure
how tight to preload the big plastic nut which retains the RA axis and
is underneath the roller bearing and outer locknut that the RA bolt
screws into.

I suspect I have it too tight as now the base seems 'sticky' in RA when
locked, but if it's any looser I can't find the spot where the RA knob
will lock at one position and allow free smooth motion in RA on the
other. If I set it for smooth free motion, it will not lock. If I
preload enough to get a lock, it's not smooth when I unlock it.

I finally find a spot where it's kinda smooth when the RA is not
tightened, and locks when it is.

Now I go to 'calibrate motor' and it fails. Try again, it works. Try
again, it fails. Try again, it works. I give up when it passes and
proceed to train drives.

I start in RA training, center up the target, it asks for > to place
target back in center...and halfway through the slew to recenter the
target..the speed changes and I get runaway.

What is going on and what is the technique for getting this stuff right?
 
Part 2) The resolution
I believe I solved this problem. It appears that the issue was a
combination of two factors.

First factor...the inner large plastic nut which is the primary retainer
for the aluminum shaft RA axis was too tight. This is the nut with the
castellated (sp?) top (slotted top as used with cotter pins in other
applications). The RA bolt threads into large threaded washer type nut
which traps and retains the slots in the lower plastic nut. Anyhow, I
think this was too tight.

Without the RA/AZ bolt in place, I tightened the plastic nut until the
axis wouldn't move, then slowly backed it off in 1/8 turn increments so
that it held the base on firmly, but didn't stick when rotating and
turned smoothly. NOTE: The plastic nut comes with locktite applied. I
have not reapplied it and if it stays adjusted, I'm going to leave it
off.

Second factor. When I had it apart the first time to check the RA/AZ
encoder wheel, it seemed to me the metal spring clip which serves as a
pressure spring for the worm gear was too loose. I carefully tweaked it
wider (more pressure) a mm or so. I think this was a big mistake.

I returned it to about where it was when I got in there the first time.
Apparently caution is the word when you think you are smarter than the
factory, in some cases at least. I relearn the old saying the hard
way...if it ain't broke, don't fix it.

So after returning the tension 'spring' to it's prior place, and
spending some time experimenting with the plastic nut torque, I was VERY
pleased to get no motor errors on my first Autostar boot while on the
bench after pulling my hair out for a week on this thing.

I think the problem was twofold..motor calibration and motion sensing
probably relies on consistent "ambient" resistive torque due to friction
in the non ball bearing system here, so it's very sensitive to
preloading. In addition, having the preload on the worm gear too high
probably messes up the same kind of thing throughout the gear train.

I think the system was sticking and then breaking loose throughout the
bearings/gear system and this was confusing the processor. A DC servo
motor system relies on rate measurements for position estimation. This
means spikes in rate of velocity change due to high stiction will
totally hose up the math used to figure out position, since they must
use derivatives of velocity in order to calculate position.

I think my speed changes were due to the processor thinking it was
driving constant speed or whatever, but since it's math was based on
sensing speed which was being skewed by drive train issues, the real
speed was not what it thought.

One way I was able to tell immediately if these combined adjustments
were doing anything is that once out of time/date mode, your scope
should immediately settle to it's normal beeps and chirps as it does
default position tracking, even if you are not yet aligned. When it was
screwed up, I would get runaway or go straight into motor cal mode
immediately after time/date settings. When I had it right, that did not
occur and I knew something had changed for the better. Lesson here
is...if you're ever getting runaway at any speed or go straight to motor
cal after time/date, something is getting sideways in your drive train
or bearing setup.

Part 3) A couple more things

While I had it on the bench and once I got it tuned right so it would
boot normally without runaway or instant motor cal mode, I did a drive
training on the bench. Normally you are advised to train drives in the
mount style you will be using, and this makes sense. I use polar mode
with my PVC pier (see ghetto ETX pier thread). However, since I wanted
to see if I could get through the entire process before I left the shop,
I did drive training on the bench in ALT/AZ position anyway, indoors.

Since my shop is not big enough to reach close focus on something
inside, I couldn't look through the scope for the training process. I
decided to find a good point target to use in conjunction with the laser
pointer. What worked out *great* was a corner of the room near the
ceiling. It turns out that where two walls and the ceiling come together
makes a pretty accurate target for the green dot. When you're off
center, the laser is smeared and not round, and the small off axis glow
around it isn't round either. When you're aimed smack dead center at the
corner, the dot is a perfect circle centered in the perfect circle of
the off axis glow. Even the tiniest bit of off center, and it's
obviously not right.

So I ran through the process without a hitch (yee ha!), then dragged the
whole works out to the pier for the do or die real world test. Usually I
use a barlowed 9mm and a distant target for drive training, in the
correct mount orientation. here I used the laser and a corner of the
shop at most 8m away, in the wrong scope orientation.

While setting up and getting located at polar home, I was ready to be
dissapointed. After diddling with this problem for over a week I was
getting really cranky and in spite of the obvious behavior changes on
the bench, I was prepared to restrain myself from throwing the whole
works in the dirt when it failed again. When combined with my wrong
orientation drive training indoors, the best I was hoping for at least
no runaway/speed changes and the need for a drive retrain in polar mode.
Well, the results were more than satisfying.

Date, time, two star, check. Polar home, check. Laser On. GOTO Arcturus.
Whine whirr chug, slew, slow slew, beep. A couple degrees off Arcturus,
no motor fault, no runaway! Not bad. Hopes rise. Center Arcturus, enter.

Next, Vega. Whine whirr chug, slew, slow slew, beep. Laser looks danged
close. Peek through the 9mm...there it is in the field! Woohoo! Center
it up. Enter. Alignment Successful!(Pole error 27' ^, 47' <. Ok,
whatever. I'll deal with that later. I love this report function,
though. 497 rocks)

M13...yup. inner 50% of field. Ring Neb...bingo. Inner 50%. User
ecstatic.

I play around and determine I'd be a field off after a GOTO transit of
the whole sky, but GOTO'd to objects within a quandrant or so it's
always in the field. Leave it for an hour and object remains pretty
danged close to where you left it. For now it actually works a little
better than when it came out of the box. Suuuweeeet!

I was pretty late by now and I didn't feel like dragging out the DSI and
associated junk, so I trotted out the Z12 and spent a pleasant hour
using the laser pointer and ETX GOTO in parallel with the Z12 as a
finder system for it. M63, M94, M5, a little of this a little of that.
M56 is sorta underwhelming compared to the other great globs for this
season. M63 and M94 are pretty danged cool in the Z. NGC6543 is danged
bright! Wow in the Z12 and easily visible in the ETX, though dinky.

All in all the most satisfying night out in a while even though viewing
through sucker holes. Actually getting something like this fixed and
working well is a pretty good feeling. The indoors drive training using
the GLP appears to have worked pretty nicely and I will not hesitate to
use it again when necessary.

That's it. Hope it's useful. Thanks for the great site.
 
Best Regards,
 
Chris Erikson

And:

Yup, a general reset was the first thing I tried. Reset, calibrate,
train.

Then I went back to my 494 handset, and it had the same behavior. I'm
still not quite certain how I didn't get the plastic nut torque right
the first time as I was pretty careful, but I think my extra caution on
it the last time around combined with fixing the worm tension did the
trick.

Chris Erikson

Go back to the Tech Tips page.

Go back to the ETX Home Page.