Last updated: 4 January 2013

A DEC problem fix

Sent:	Tuesday, June 8, 2004 10:25:33
From:	Michael Morris (
Further to my recent posting regarding a loss of drive to the DEC axis
on my ETX 90 EC, I have found and cured the problem and I thought you
might want to know what I found. Enclosed picture shows the offending
piece of kit as being the main drive shaft for the DEC axis.

The brass worm gear is mated to the plastic cog wheel by means if a conical end to the brass drive shaft that fits snugly into the shaft of the plastic gear. It appears that this normally provides enough friction to provide a smooth and continuous transfer of power from the horizontal gear to the brass worm gear. The end of the plastic gear shaft is held in place by a very small Allen bolt that screws into the end of the conical brass shaft of the worm gear. It looks as though when the OTA reaches an end stop this arrangement allows the conical brass shaft to turn within the plastic shaft, therefore acting clutch that prevents the motors either burning out or the drive breaking the end stops when the OTA reaches the ends of its vertical travel. In my case some grease that is used to lubricate the system appears to have worked it's way the thread of the hex bolt meaning that every time the DEC axis met any significant downwards resistance it resulted in the hex bolt on screwing itself and all drive being lost as the brass and plastic shafts where no longer being pressed against each other. The fix was straightforward. I cleaned off the grease from inside the thread in the brass shaft and off the thread of the hex bolt. I then applied a tiny amount of Lithium grease to the head of the hex bolt. I then put a dab of 'ThreadLock' inside the thread of the brass shaft and quickly put it back together before the 'Threadlock' set. I all SEEMS to work OK again. It strikes me that you have to be careful not to over tighten the hex bolt as this cause the brass shaft to fail to slip if it is heavily loaded, causing the motor to burn out, the threads on the cogs to get stripped or the end stops get broken. None of this has happened to me (YET!) I hope this is of use. Michael Morris

Subject:	etx-90 Dec fix - further tip [NEW!]
Sent:	Monday, December 31, 2012 22:17:31
From:	ross (
I found I had the same problem as Michael Morris, an allen key that
unscrewed itself on the Dec gearbox tapered gear. It required one or two
re-assemblies and adjustments to get the pre-load just right on the gear
on its taper, (don't forget, the cup washer on the gear is convex. The
open end of the cup faces away from the gear). In the end I did it the
same way that you tighten a spark plug; over tighten a bit to get
everything seated properly, loosen right off again and then gradually
tighten the allen key until it takes a firm movement to get the gear
shaft to move within the taper while holding the nut with an appropriate
7/16" spanner.  A firm movement to me was the amount of force I could
easily exert on the gear by holding it between thumb and forefinger
without using any wrist movement at all. There's probably a newton/metre
measurement for it but I prefer just a good firm poofteenth.

Also, I found the worm gear assembly goes back in more easily if you put
the inner two screws and washers in before you re-assemble the worm
gear; and the whole assembly goes back in the gearbox more easily if you
face the third screw flange towards you while inserting, then turn it
around once it's in place.


Ross Williams


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