ETX TECHNICAL TIPS Last updated: 28 February 2001

Too Tight DEC Axis

Hello Graeme -

Thanks for the great report; sorry the DEC is a problem for you. What you are describing (with OTA off) is severe FLEXURE in the plastic fork arm, where it literally bends at the attachment holding the worm-driving gear. This is a real problem and might be solved by putting a rigid but thin metal plate behind the U-mount holding the worm and the plastic support of the fork. Part of the reason you are getting: 1) the jerking motions and 2) this flexure is undoubtedly because of what you describe: THE DEC. AXIS IS TOO TIGHT.

You should be able to UNCLAMP your Altitude (DEC) clamp and the tube assembly attempts to fall forward toward the front (front heavy). If it does NOT, then your DEC clutch, clamping mechanism, or trunions (most often the case) are too tight! Proper adjustment sees the OTA fall toward the front freely when unclamped!

Take the OTA off and remove the trunion pieces (the OTA support arms); the trunions are conical very slightly and they need to be lightly brushed with medium grit STEEL WOOL (not sandpaper!). Rub them down, like washing a finger on your hand, over and over and then finish with EXTRA FINE steel wool to polish. Afterward, rub down with Lithium white grease and insert back into the fork hole. Work it back and forth time and time again; it will eventually "seat" and loosen up.

If you DO NOT do this, you are at risk to burn out the DEC drive motor; this may be one reason you are getting arcing; merely the resistance from torque could be doing it. It is my experience that MORE voltage than the specified 12v is advantageous to driving all ETX systems; mine works much better at 15.8 v (the Meade AC adapter) and at 14V )my DC power supply).

Try working on your DEC axis trunions to loosen. By the way, most ETX RA axes are very tight; I would not back off too much on the large nut assembly. The tightness does NOT affect the ability to torque-free drive the telescope in any way. Mine is very tight, and I have grown to like it!

Try my suggestions on DEC and let me know! Thanks for being in touch and the kind comments!

Clay Sherrod

P.S. Keep in mind you DO NOT want grease on the FLAT gear or clutch surfaces - only the gear TEETH; the clutch engaged through friction and most of the tracking and GO TO problems experienced are from excessive grease on the large gear face and that of the clutch plates. Read Part 1 of the Enhancement Guide and check on the "degreasing" downloads that describe how important minimal lubrication can be. DEFINITELY you need to get rid of the junk that Meade puts on (it gets sticky in cold weather and probably is affecting some of your stiffness). Lithium grease is the only way to go, and only a dab of that! -----Original Message-----

From: Graeme
>Hi Clay,
>
>Firstly, I must congratulate you on your hard work in pinning down some
>issues with the Autostar. Your tips on Mike's site have been of great use to
>me over the last couple of days.
>
>I just got my ETX125 this January and have spent more time looking in the
>telescope than through it! When I first tried the scope (the very first
>time), goto accuracy was impressive, producing a high "Grin Factor" rating.
>On subsequent attempts, the RA clutch began to slip as did the DEC which
>took most of the fun out of things. Then I "upgraded" from 2.0I to 2.1Ek and
>generally things went down hill the more I trained the drives to try and
>sort it out (without doing the RESET first)
>
>Additionally, the RA axis was far too stiff when unclamped and I came to the
>conclusion (along with our UK distributor) that the large 1.25 inch nut that
>tightens up on the RA bearing was too tight. It took two of us to slacken it
>and an injured finger was an additional bonus when it decided to loosen up..
>
>I don't understand how tightening up a large bolt against a large bearing
>could make it bind, unless there's something on the other side of the
>bearing I can't see that's causing the stiffness. I don't know of anyone
>who's seen what's on the other side of the bearing and our UK distributor
>who tries to repair them can't get a mechanical diagram out of Meade! Mind
>you, he's got an ETX125 mount he tried to take apart but couldn't
>reassemble. I'm tempted to buy it from him for research purposes.
>
>I'm jealous - you're lucky not having any backlash on your DEC axis. I've
>got strange "backlash" properties on mine - whenever you change direction,
>the OTA moves but it starts slow as something torques up, then quickly
>accelerates to the correct slewing speed. This is being observed with the
>Alt % being set to 01. The motors are running at a constant speed throughout
>the cycle. With the OTA removed, I've observed the arm and it seems to cant
>off its rotational axis before moving even if you assist it with your hand.
>I was going to try the Teflon tape idea but the DEC bearing's already so
>tight that a single wrap of tape won't go in! (Actually there is a degree of
>stiffness in the DEC axis - I don't want to remove any plastic form any
>bearing surfaces until I'm sure it's absolutely necessary and safe to do
>so!!). If only Meade had used metal where it counts...
>
>Hopefully I'll get the DEC axis sussed out. If I could find a way to use
>proper metal bearings instead of the plastic ones, I'm sure this would
>transform things. I wonder if someone will ever make aftermarket metal OTA
>arms for the ETX. Actually a complete kit with an improved clamp system
>would probably sell rather well!
>
>On the subject of motors - have you noticed that when the scope is powered
>from a lead acid battery, there is rather more sparking on the motor brushes
>at high slew speeds? This is bound to shorten the life of the motor and
>there is no significant arcing when powered off AA cells. I'm tempted to put
>a diode or two in series with my external power source to drop the voltage
>to 11V or there about to see if this helps. I wonder if PSU voltage would
>have an effect on drive training?
>
>Thanks again for your tips...keep up the good work!
>
>Regards,
>
>Graeme

Return to the top of this page.

Go back to the Tech Tips page.

Go back to the ETX Home Page.