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Some play in CGEM Dec axel

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#1 Goran Strand

Goran Strand

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Posted 04 January 2010 - 11:10 AM

I have a CGEM mount and have noticed some play in the DEC axis. If I grab the dove tail plate, I feel some play that also shows in the eyepiece, the stars move a bit. The RA axis is stif. If I grab the counterweight bar I can also feel the play so it's in the axis and not in the dovetailplate.

Anyone else had this problem?

#2 tturtle


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Posted 07 January 2010 - 02:45 AM

We just recently discussed this issue in the Yahoo users group for the CGEM which you may want to join at:


The response is repeated below.

This has become a fairly common issue that is easily rectified. As you have noted, adjusting the backlash setting in the HC does not really resolve this particular problem since it occurs when the scope is at rest. The reason it only occurs with the scope at certain positions is that you are near the balance point for the OTA. The real issue with this is that critical focus is difficult to achieve since whenever you touch the focus knob the view shifts.

I was able to successfully fix this problem in the RA axis using a procedure generally described by others, particularly owners of the EQ6 which has some mechanical similarities to the CGEM. I have posted pictures of the EQ6 in the photos section which identifies the adjustment screws described below. Here is specifically what I did to adjust this problem out in RA:

1. loosen the clutch knobs and remove the OTA and all counterweights.
2. loosen the four large hex bolts that clamp the plate holding the worm shaft to the mount and then tighten the clutch knob.
3. loosen the two small recessed hex screws on either side of the axis, (not the ones on the shiny collar).
4. adjust the recessed screws and put your hand on the end of the counterweight shaft feeling the play and tightening the small hex screws back and forth until the play can just barely not be detected. Some very small amount of play may be desirable.
5. loosen the clutch knob and retighten the four large hex bolts.

The above is for RA axis which would be largely the same for the DEC axis. Note that I am careful in the above not to push on the mount when the clutch knobs are engaged. In my case it took some force to loosen the large hex bolts. Pushing like that on the mount with the clutches engaged can put a lot of stress on the whole drive system and defeat the purpose of what you are trying to do. For the same reason I always loosen the clutches when I am taking the OTA on or off the mount (I think the manual may even state this). You also need to be careful not to overtighten the above adjustment since that will put stress on the worm gear. If you power up the mount and the motor noise sounds "strained" then you have probably gone too far.

Also note that you will need two size hex (allen) wrenches to make this adjustment. I would strongly suggest that you buy a high quality set of wrenches to do this adjustment as you can ruin the bolts (especially the small ones) with a hex wrench that is a poor fit. I purchased a Craftsmans set from Sears that is well machined.
-Tom W

#3 Remi2306


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Posted 17 February 2010 - 02:00 PM

I fixed it according to the following instructions. It'S identical for the CGEM appart from 4 screws instead of 3 to loosen: http://www.astro-bab...ld/heq5-we1.htm and http://timsastroblog...up-for-eq6.html

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