TECHNICAL TIPS Last updated: 16 May 2002

Going Inside the ETX-90EC, or How to Almost Ruin a Telescope

Sent:	Tuesday, May 14, 2002 21:47:07
From:	bryangm@attbi.com (Bryan)
Well, some background-->  Being very mechanically and electronically
gifted, I decided to do my own "supercharge".  Everything seemed to go
well except when I put it all back together it did some weird stuff. 
First, it was difficult to get the alignment to go successfully. 
Second, when it actually did say successful, everything SEEMED to work
OK.  I could GOTO a planet or two.  But when I would sit there
observing, say, Jupiter, the scope would just randomly move up. 
Sometimes it would move up and then pretend to be "locking on" to
something, even though the autostar said it was still on Jupiter. 
Sometimes the motors would spin up slowly and gradually increase speed
by themselves...much like accelerating in a car!

After fiddling with it for a week or so, I decided to forget about it
and put it in the closet.  It sat there for a few months until I got my
big 6" Celestron Refractor.  Finding the setting circles on that big
beast difficult to use, I decided to give the little ETX another shot to
help me find things to point the bigger scope at.  I ripped the entire
thing apart, and I mean EVERY nut, bolt, and screw came off of that
sucker.  I cleaned and regreased everything.  Then slapped it all back
together again, making sure to be as precise as possible about
everything.  No go.  My RA motor worked fine but my DEC motor now only
had one speed: very fast!  Even when I set the speed to 1, the DEC would
move like it was still on 9!

Being the type of ignorant fool I am, and never willing to accept defeat
(especially from a lousy machine!) I ripped the whole thing apart again,
though this time concentrating on the electronics.  I broke out the
handy multi-tested and had at it.  I noticed the problem fairly quickly
-->  apparantly I had pushed or bumped the capacitor that controls the
voltage applied to the motors.  I noticed because one of the leads was
pulled and you could see the fatter silver part that is supposed to be
INSIDE the cap, not sticking out.  So I desoldiered that sucked and went
hunting in my "pile o' electronics junk" for a replacement capacitor. 
The original cap is a 220uf 25v polarized capacitor.  The closest I had
was a 220uf 35v and a 220uf 16v.  I tried the 35v first.... I had speed
control again but the motors would not move smoothly... very jerky and
inconsistant.  So I tried the 16v.... too slow and also jerky.

NOTE:  I tried the 35v again but failed to notice the polarization.  BAD
BAD BAD!  The thing exploded!  Giant smoke cloud and the foulest smell
ever!  It smelled like a dog had crapped right inside my ETX!  Note that
the polarized caps are labeled for Negative and the circuit board of the
ETX (at least mine) was only labeled for Positive.  Just make sure you
have them opposite!

I realized that it was the voltage that was important and not the farads
(uf).  I rummaged through my electronics salvage and came across a few
big capacitors, but that were 25v.  The closest 25v cap I had was a
1000uf (roughly 10 times as big physically as the 220uf).  I wasn't sure
about this so I asked my friend at work (he has a degree in electronics
engineering, specializing in robotics)  He told me it was no problem at
all for this application, and the bigger cap would probably be better
because its charge won't be drained as quickly as the 220uf (though i
don't know how quickly the ETX motors drain it anyway) -- its fine just
as long as the voltage is correct and the cap isn't too far out of
tolerance.

Well, I used some spare wire and soldered that to where the old
capacitor used to sit on the circuit board, soldered the wires to the
new (big) capacitor, and taped the capacitor out of the way to the
inside of the plastic base.  Careful here because the circuit board has
conductive tracing on BOTH sides, and the cap is attached to that
tracing on both sides!

Verdict?  I slapped it all back together (AGAIN!  UGH!)  and fired it
up.  All of my speed settings worked!  I took the whole thing upstairs
and Reset, calibrated, and Re-Trained on a water tower a few miles away.
 That night I took it out to give it a try.  I got a successful
alignment on my FIRST TRY!  I was skeptical so I did a GOTO to
Jupiter.... it centered right in the middle of the eyepiece.  I did a
GOTO to something else (I forgot, sorry) in the other direction and it
also centered.  Then I put it to Saturn, and it was good to.  I went
back to Jupiter and left it there for a couple of hours.  When I came
back Jupiter was still in the eyepiece, though it had drifted a little
bit.  I did the same thing for the next couple of days and everything
went well.  Before, I could only look at Jupiter (or whatever) for about
2 minutes before it would go nuts on me.  This was all done in Alt/Az
with high precision ON.  I am going to try polar alignment some day (its
not that critical to me, as I have my big scope now to do whatever I
would have done with the ETX in polar mode)

Sorry if this was too long, but I figure I might as well be complete
just incase you had similar problems.  The ETX tune up guides were
extremely helpful in setting up all the mechanical aspects of this
scope!

If you have any questions, fire away and I'll try to answer as best as
possible!

-bryan

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