Back in 2002 when I purchased my first telescope, an 8” LX90, the first month or so of usage brought up a very frustrating problem. When slewing the scope to a new target it would suddenly stop mid-slew, yet the motors continued to whir. The cause was the RA clutch disengaging. Gotos were now ruined and after re-tightening the RA knob to engage the clutch I had to start the alignment process over again. Very frustrating.

I solved the problem after receiving some useful suggestions from the LX90 yahoo group, not a single clutch slip in nearly 7 years usage :) Earlier today I received an email asking about how I fixed the issue so I thought I may as well type it up.

The cause of the clutch slippage is pretty simple, as is the fix. The knob that you tighten on the base of the LX90 to engage the clutch is pushing against not only the clutch but the top of the LX90 base too. As the scope slews, the friction between the base and knob is sufficient to slowly unscrew and disengage the clutch. So the fix is simply to raise the knob off of the base of the LX90.

To do so, use an Allen key to loosen the small screw on the side of the RA clutch knob. Then turn the knob anti-clockwise until it catches and starts to unscrew. It should eventually come fully off as the photo below shows.

Removal of the RA clutch knob

Removal of the RA clutch knob

Notice the circular scratch marks from the RA knob contacting the base.

Inserting a nylon or fibre washer (not metal) over the bolt that protrudes from the base, one that is small enough to fit inside the hole, will raise the RA knob just enough to prevent contact occurring with the telescope base when the clutch is engaged.

If you look at the base of the bolt, you should be able to make out the washer I’ve added.

Nylon washer to raise the clutch knob

Nylon washer to raise the clutch knob

Re-assembly is straight forward, just be sure the clutch is fully engaged before you tighten the tiny screw on the RA knob.

With this simple fix, you only need to apply a little pressure when tightening the RA knob and engaging the clutch. Also, no matter how much you slew the scope, the knob should no longer contact the base and cause the clutch to slip.

More recently, I’ve taken apart the DEC axis to fix a significant (when viewed through eyepiece) amount of slop of 5-10 degrees. When I get a spare moment, I’ll upload photos of the disassembly. They show the amount of excess grease inside the dec axis and the cause of the slop, an over tightened screw on the worm gear!