ETX TECHNICAL TIPS Last updated: 10 November 2003

"Repair" - Do Not Do This

Subject:	ETX-125 RA Drive Inspection and Question
Sent:	Monday, November 10, 2003 14:15:02
From:	drclay@tcworks.net (P. Clay Sherrod)
To:	Randy
Thanks Randy and I totally agree; I did post something similar to this when
the mountings were first released by Meade, letting people know that the
self-repair was not recommended at all and that access was considerably more
limited in the new version than with the old plastic cast models.

A reminder from you would indeed be a good idea.  Thanks again.

Clay
--------------------
Dr. P. Clay Sherrod
Arkansas Sky Observatory
Harvard MPC H41 (Petit Jean Mountain)
Harvard MPC H43 (Conway)
Harvard MPC H44 (Cascade Mt.)
http://www.arksky.org/

----- Original Message ----- 
From: Randy
> Thanks again.  I wasn't planning to post a "How to Do It" message, but
> rather a "Do Not Do It - It Will Not Be of Any Benefit" note, since there
> is mention of taking the old design apart, but nothing that indicates the
> new version is substantially different, i.e. old information still exists.
>
> Given the detailed nature of the "Tech Tips" articles, I think a brief note
> concerning the new internal design of the scope, and the fact that the
> particular RA performance aspects cannot be improved upon by "opening it
> up" or "adjustments" would be in order.
>
> I will leave it to you and/or Mike to make the call on that addition to his
> website.
>
> Thanks again.
>
> Randy
>
> At 12:21 PM 11/10/2003, you wrote:
> >Thank you for your findings; I am very well knowledgeable on the internal
> >design of the ETX 125 "new version" since many of the concepts used inside
> >the system are my designs.
> >
> >That being said, I stand by what I said earlier; the danger of wire shearing
> >is great for the novice and I am urging you to not post "recommended entry"
> >into this system for that very reason; for example, not everyone needs the
> >knowledge to "disconnect" with wire clips in the DEC arms, nor needs to be
> >in that area that deeply anyway; as you have discovered, there is little to
> >be gained by such access and the chance for casual entry damage is very
> >high.
> >
> >I am cc-ing Mike Weasner on this because I believe that 95% of the damage
> >that both Meade and I see in ETX telescopes is done from the curious and
> >those who believe that they should be tinkering around on the inside. The
> >mountings are designed for "assembly line" repair....meaning that they are
> >NOT opened casually to repair one part;  they are, instead, rebuilt just as
> >though they were a new unit from the ground up, regardless of what the
> >repair required might be.
> >
> >Too many times considerable damage to an otherwise suitably operating
> >telescope is done by folks trying to improve on things that sometimes CANNOT
> >BE improved upon because of the design of the system.  Such is the case with
> >reverse backlash and some slack in RA on the "new design."  That is the
> >nature of the beast in the way the system is designed and until Meade
> >changes milling procedures on the plastic castings themselves, this problem
> >cannot be solved by customer intrusion in the system.
> >
> >Thanks for your hard work...I could have saved you a bit of trouble.
> >
> >Clay
> >
> >----- Original Message -----
> >From: Randy
> > > Dr. C,
> > >
> > > Thanks for the reply (again).
> > >
> > > I am not sure how far in to this most recent hardware configuration you
> > > have been doing your "Supercharge" updates.  I spent several hours working
> > > on the system last Thursday evening.  I found the following:
> > >
> > > It is possible to disconnect the RA clutch draw bolt (without cutting the
> > > DEC drive wires) by removing the fork covers, unplugging the DEC drive
> > > connection, removing the lock lever, lifting the base top cover, etc., and
> > > allowing the cable to rotate along with the bolt and the slotted "top hat"
> > > washer it comes out through.  A real pain, and unfortunately for naught,
> > > because...
> > >
> > > The new RA worm drive support casting now has a counterbore in the outer
> > > ends, and the plastic bushings actually ride in an O-ring between the
> > > casting and the plastic bushing in each end.  I guess the idea is to be
> > > able to give some lateral preload to the worm gear without creating a
> > > binding situation due to eccentricity in either the central gear or the
> > > worm gear (i.e. a "spring-loaded" set-up if you will).  However, as can be
> > > imagined, the spring loading is in effect not only on the line between worm
> > > and center gear, but also perpendicular to that.  That is the motion that
> > > was causing the extra play in my unit - the worm drive shaft was being
> > > pushed "sideways" before it actually starts turning.
> > >
> > > Unless the preload nut was tightened excessively, it would not prevent or
> > > reduce this motion.  Also, the preload nut had some serious epoxy-type
> > > yellowish locking material across the end that had to be removed before the
> > > nut could be turned either direction, along with some locking compound
> > > inside the nut threads.  Obviously, all this was a futile effort to adjust
> > > an unfixable problem due to the design.  The end play preload on the worm
> > > gear was OK to begin with.  Now I will have to hope I don't encounter
> > > further troubles with the unit, since that yellow lock compound being gone
> > > probably would cause warranty invalidation - at least on that assembly. Oh
> > > well, wisdom only comes with experience....
> > >
> > > I re-assembled and put a reasonable side load on the worm gear (just barely
> > > noticable deflection of the bushing against the o-ring in the
> > > casting).  The drive motor sounds pretty smooth during operation around 360
> > > deg.  There does not appear to be excessive drag in the system due to too
> > > much side preload.
> > >
> > > I plan to send in a note to Mike Weasner's page concerning this design
> > > (along with some relevant sketches, and cautioning tinkerer's (like myself)
> > > that going through this process is wasted effort (since this new design is
> > > not alluded to anywhere on his site that I found).
> > > ~~~
> > >
> > > FYI - Now I have some weird behavior of the AutoStar to contend
> > > with.  After 15-20 minutes in the "Tonight's Best" tour (discovered while
> > > in the field Saturday evening looking at the lunar eclipse), you get locked
> > > into the tour, and cannot "Mode" back to a higher level menu. Powering off
> > > is the only solution.  Also, while investigationg that behavior the next
> > > day (Sunday evening) that "Tonight's Best" tour showed a "Lunar Eclipse" on
> > > Sunday, 09 Nov 03, not Saturday, 08 Nov.  Further, selecting "Lunar
> > > Eclipse" from the list causes a "Proc. Trap 2" message (sounds like
> > > left-over debugging code in the last ROM firmware revision) to be
> > > displayed, and the controller locks up.  I sent a note to Richard Seymour
> > > to see if he had any thoughts.  I plan to re-download the ROM code to the
> > > AutoStar, in case it got corrupted somehow on the last Download (3.0Ee +
> > > Seymour's 30e4 patch kit).  I'll drop you an update note once I figure out
> > > the source of these troubles.
> > >
> > > Regards,
> > >
> > > Randy
> > >
> > >
> > > At 11:21 AM 11/6/2003, you wrote:
> > > >Those bushings can be gently secured via the end play nut on the right hand
> > > >side of the worm drive assembly; no need to remove any hardware at all; you
> > > >must find some way to securely HOLD the left hand side of the transfer gear
> > > >assembly to prevent the small reducer gears from turning and hence stripping
> > > >or breaking.  No need at all to take off the clutch plate.
> > > >
> > > >
> > > >----- Original Message -----
> > > >From: Randy 
> > > > > Thanks for the advise.  I am going to have to figure out how to remove the
> > > > > clutch pressure plate.  The worm gear bracket (and connected motor drive)
> > > > > has to come out to be disassembled and fixed, and that wide pressure plate
> > > > > has to come off to get the worm drive out of the base.  I am guessing the
> > > > > top cover has to be loosened, wire contact pins removed from connector, and
> > > > > then fished out (and back through later) the lock bolt central hole.
> > > > >
> > > > > As I showed in the pictures, the worm gear bushing are very loose in the
> > > > > casting.  Even adding some moderate side preload to the worm drive, there
> > > > > is still the order of 30'-40' of mechanical backlash in the RA drive - most
> > > > > of it due to the lateral shifting of the worm axle.  I haven't put in
> > > > > Seymour's patch to be able to see the RA "training" number (backlash
> > > > > encoder count, I bet), but I bet it is big.  Optimal "training" and set-up
> > > > > followed by the "AZ %" setting requires the % to be set at >>>87%<<< to get
> > > > > reasonable performance taking up the reversal slack in RA.  This setting
> > > > > yields a reversal time of about 5-6 sec at 2xSidereal, with no excessive
> > > > > jumpiness at settings "3", "4" or "5."  From what I have seen and read,
> > > > > values of "%" over 25% on either axis are rarely required.
> > > > >
> > > > > I'll continue to fiddle (carefully!).  I work with a lot of delicate
> > > > > electronics and optics at work, so I feel pretty safe in proceeding with
> > > > > due caution.  Boy, would an exploded diagram of the whole scope be conveninet!
> > > > >
> > > > > I am also concerned about focus image shift.  Meade says 1/4 FOV with the
> > > > > SP 26mm EP is the factory tolerance at any time in the scope's life. They
> > > > > also said that it is prone to worsening with usage.  Mine is not that bad,
> > > > > but is more than 10% of FOV with the 26 mm.  That will move objects
> > > > > completely out of the FOV when a 6 mm or shorter is used.  I see a lot of
> > > > > comments about loose secondary baffle tubes, loose mirror retaining rings,
> > > > > and sloppy end connects on the focus rod (not to mention the sloppy bushing
> > > > > fit where the rod goes through the mount plate, causing a lot of backlash
> > > > > in the adjustment).  If all the mechanical attachments are tight, it has to
> > > > > be excessive clearance between the mirror guide sleeve and the baffle
> > > > > tube.  I am tempted to disassemble and check.  If excessive clearance is a
> > > > > problem, would very thin UHMW self adhesive bearing material placed inside
> > > > > the mirror guide tube be a reasonable fix, to bring the total clearance
> > > > > down to less than 25 um or so?  I can get this UHMW strip 0.5 in. wide down
> > > > > to 50 um thickness (I think, definitely as thin as 125 um).  It is very
> > > > > slick, and very tough - probably will not need any lube on the baffle tube
> > > > > at all if it is installed.
> > > > >
> > > > > I'll let you know what I find.  Supposed to be really cold and cloudy/snowy
> > > > > this weekend, so may have a look then.
> > > > >
> > > > > Kindest regards and sincerest thanks for your advise,
> > > > >
> > > > > Randy Lloyd
> > > > >
> > > > > At 07:37 AM 11/6/2003, you wrote:
> > > > > >On the new design you are indeed going to likely break the wires going into
> > > > > >the DEC axis; it is assembled totally differently than the older models and
> > > > > >unscrewing the clutch plate via the lock down bolt will turn the wires until
> > > > > >shearing inside.  Not advised at all....
> > > > > >
> > > > > >Clay
> > > > > >
> > > > > >----- Original Message -----
> > > > > >From: Randy Lloyd
> > > > > > > Dr. Sherrod,
> > > > > > >
> > > > > > > To begin, thanks for all of your great info, especially that found on
> > > > > > > Weasner's ETX web site.
> > > > > > >
> > > > > > > Using that info, I was able to tune up the DEC drive in my new
> > > > > > > scope.  However, the RA drive is another story.  Regardless of the
> > > > > > > anti-backlash settings (up to 55% anyway) in the AutoStar (following the
> > > > > > > procedures given on Mike's web site to make sure they "take", there was/is
> > > > > > > still a long delay to take up backlash when changing directions on RA. I
> > > > > > > opened the base, and here is what I see.
> > > > > > >
> > > > > > > 1) the 2-wire motor connector was almost disconnected from the servo
> > > > > > > control PCB.  Plugged it on all the way.  No big deal - very sloppy assembly.
> > > > > > >
> > > > > > > 2) Appearance of drive clutch is diffferent than in other photos I have
> > > > > > > seen.  Probably a newer design revision.  Photo attached. Anodized
> > > > > > > aluminum disc, threaded center.  Apparently keyed/splined to the drive axle
> > > > > > > (turns with fork mount).  To remove, the center bolt (clutch draw bolt)
> > > > > > > must be unscrewed (as far as I can tell).  However, the DEC drive leads
> > > > > > > must go up through the hollow bolt and exit the side of the bolt to go
> > > > > > > across to and up the fork arm to the DEC drive.  I can onlyt assume Meade
> > > > > > > pre-wraps the wires CCW before inserting this bolt (or the assembly
> > > > > > > procedure order is other than I perceive, or something like that).
> > > > > > >
> > > > > > > My question: Is it safe to loosen the clutch draw bolt multiple revolutions
> > > > > > > to remove the clutch "pressure plate"?
> > > > > > >
> > > > > > > This is necessary for two reasons.
> > > > > > >
> > > > > > > a) To take the drive gear off and degrease and possible flatten. You have
> > > > > > > to really press the lock lever hard to keep it from slipping; and
> > > > > > >
> > > > > > > b) (Here's the important one) I need to remove the drive assembly to fix
> > > > > > > the worm gear bushings!
> > > > > > >
> > > > > > > As I hope you can see (attached pictures), the plastic bushings are a very
> > > > > > > sloppy fit in the worm gear support casting. By my
> > > > > > > estimation, there is at least 0.25 mm clearance between the bushings and
> > > > > > > the holes in the casting.  When the drive is reversed, the whole worm gear
> > > > > > > and bushings move laterally by this amount in the direction the driven gear
> > > > > > > on the end of the worm shaft is being pushed.  (There is also a small
> > > > > > > amount of end-play - the preload nut needs to be tightened a little
> > > > > > > more).  The net result - it appears to cause about 1/4 to 1/2 deg of
> > > > > > > effective backlash in the RA drive.  Training the RA motor accounts for the
> > > > > > > backlash (or so it seems - initial tests of pointing accuracy are fair),
> > > > > > > but the slew controls "RA percent" setting do not make up enough
> > > > > > > speed/time/encoder counts  to compensate.  At "3" it takes about 5 seconds
> > > > > > > for the RA drive to "catch up" on a reversal, over 20 seconds on "2", and I
> > > > > > > am guessing 45-50 seconds on "1" (haven't waited that long).
> > > > > > >
> > > > > > > To fix this, the worm drive must be disassembled, and the pressure plate
> > > > > > > disc has to come off first to lift the drive assembly out (worm drive +
> > > > > > > gear train + drive PCB + motor).  My intent is to remove the assembly, and
> > > > > > > fix the bushing to casting clearance (probably by making bushings out of
> > > > > > > brass shim stock of correct thickness).
> > > > > > >
> > > > > > > But, I am very concerned that the DEC drive wires will get sheared off if I
> > > > > > > start turning that draw bolt to remove the clutch pressure plate. I seem
> > > > > > > to remember mention of someone doing this on Mike's web site.
> > > > > > >
> > > > > > > Any comments appreciated.  I will gladly send you a reasonable "consult"
> > > > > > > payment if needed.  If you feel this would be beneficial, please forward on
> > > > > > > to Mike's website.
> > > > > > >
> > > > > > > Sincerest thanks,
> > > > > > >
> > > > > > > Randy

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