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slipping Questar clock drive

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#1 Jjones

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Posted 22 September 2019 - 11:20 PM

Greetings, oh wise Questar owners:

 

The clock drive on my 1957 Q has begun to slip. It won't track, even though I can hear the AC motor running and the fine RA control operates smoothly.  Does anyone have any suggestions as to what I can do to fix this problem?



#2 Optics Patent

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Posted 23 September 2019 - 07:22 AM

Some tests: When does it slip?:

  • When flat on a table?
  • Set at your latitude?
  • With a camera attached?

How does it slew (when you forcefully turn the fork relative to the base without turning the RA slow motion knob)? That should be firm and consistent.

Is the fork solidly wiggle free on the base?

Does the RA knob ever fail to move the fork? This is the expected wear point and an occasional service. Unlikely to have wear here cause clock drive slippage but possible.


Edited by Optics Patent, 23 September 2019 - 08:52 AM.


#3 Jjones

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Posted 23 September 2019 - 12:35 PM

Ben, et al,

 

-the fork is solid on the base

-the fork feels smooth and consistent when force slewing (although the RA knob does NOT rotate when forcefully slewing).

-the RA knob moves the fork smoothly all along the perimeter

-the drive (while inside the house) at 0º seems to function fine (this morning).  The drive is quite quiet.

-the drive (while inside the house) at 45º  seems to function fine (this morning).  The drive is quite quiet.

 

The problem seems intermittent.  Is there something inside the base that might need cleaning?  Could it be temperature or pressure sensitive?

 

Sometimes I think I hear the drive at times more loudly than at other times.  I think, perhaps, the last time the drive was not functioning, it seemed louder than it sounds right now.   Could that have any bearing on the situation?

 

Has anyone else had this problem?



#4 Optics Patent

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Posted 23 September 2019 - 05:37 PM

Let's determine if the slow motion drive is slipping. 

 

Look under the turntable and see where the RA pinion engages the edges of the drive discs.  You need to determine whether the pinion is ever slipping with respect to those disc edges.  There should never be any slipping under any circumstances, no matter which is trying to push what.  Usually, it will be the discs that stay still as the knob is turned.  Try this while holding the fork stationary with the base.  You will turn the knob dozens of times until the disc makes a full circle.  A Sharpie mark at the edge will help tell when you have come full circle.  Do this both directions. 

 

If that operates perfectly (which would be a pleasant surprise given the age - and only a ~$200 factory fix if not - maybe half if only the RA needs to be done) then there may be something slipping in the "stack:

 

  1. Motor pinion turns the main gear.
  2. Main gear (resting on Teflon sheets to avoid friction with base housing) has a synthetic disc atop it.  In later models this is pinned to the gear, but yours may rely on friction. 
  3. You'll know the disc is turning because the RA ring is frictionally engaged to the disc.  The Ring, fork, and RA indicator all turn together when the drive runs properly.  The RA reading remains the same as the scope remains tracking an object.  Be sure this is operating right.  The RA ring should move with respect to everything under deliberate force.
  4. The metal drive discs stack on the synthetic disc, and are engaged only by friction.
  5. The turntable is engaged to the drive discs by the RA pinion, and the turntable slides with respect to the discs only when the knob is turned.  I've never seen a failed one slide at the pinion interface.

So, if the RA ring turns when the fork doesn't, there is probably slipping between the synthetic disc and the drive discs.  I've never seen this.

If the motor turns without ring or fork turning, then you'll want to pin the disc to the main gear.  Easy factory fix.  And might be avoidable by tightening the RA axis nut (under the red disc in the turntable).  But if you had stiff slewing already that would be unappealing.

 

Note that if the RA slow motion is slipping, you might get a little temporary help by tightening the knob tension screw at the edge of the turntable under one of the arms. 

 

If it were mine, I'd take off the red disc, take apart the stack, and clean what's dirty, remove all lubricant, and then reassemble and test.  Consider pinning the disc to the gear if that is suspicious.

 

My videos #7,11, and 12 will help with all this. 



#5 Jjones

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Posted 27 September 2019 - 02:35 PM

This is great info.  Thanks much.

 

Here's what I've noticed:

 

1.  The RA pinion never slips against the discs when holding the fork and allowing the base to move while turning the RA pinion knob - either direction. 

 

2.  When I hold the fork and rotate the base, the synthetic disc stays with the base and not with the 'stack'.  This movement feels 'sticky' and not smooth.

 

3.  When moving the fork with the base stationary, the 'stack' moves with the pinion (no slippage) and the synthetic disc stays with the base.  The knob never moves but stays with the 'stack'

 

4.  The RA/degrees circle stays with the base through all of these procedures.   BUT:

 

 

 

As I looked at one of your videos, you clearly spell out the disassembly of the base once the OTA has been removed from the fork.  Is it possible to remove the red disc and remove the entire fork without having to remove the OTA from the fork first?  If so, what is the easiest way of removing the red disc?  It seems quite indented into the base.



#6 Jjones

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Posted 27 September 2019 - 02:42 PM

Accidentally posted an incomplete post above.  My apologies.

 

Here's what I was adding;

 

BUT:

 

5.  When the pinion DOES SLIP when the base is stationary and I am rotating the pinion counterclockwise.  It does not seem to slip rotating it clockwise.  I turned the screw under the fork clockwise a full rotation or so - that seems to have helped with counterclockwise slippage.



#7 Optics Patent

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Posted 27 September 2019 - 05:04 PM

  • ANY pinion slipping in any circumstance says a drive service is needed.
  • The synth disc is (frictionally) engaged to the RA ring and should stay with the base (actually with the main drive gear) while the fork and disc stack rotate together.
  • How you are supporting the mount may affect loads and frictions that cause slightly different behaviors.
  • The inexpensive factory drive service will get any grittiness in the RA slewing cleaned up.

1.  The RA pinion never slips against the discs when holding the fork and allowing the base to move while turning the RA pinion knob - either direction.

 

Good.

 

2.  When I hold the fork and rotate the base, the synthetic disc stays with the base and not with the 'stack'.  This movement feels 'sticky' and not smooth.

 

Correct motion.  Needs cleaning and maybe hub nut tension adjustment.

 

3.  When moving the fork with the base stationary, the 'stack' moves with the pinion (no slippage) and the synthetic disc stays with the base.  The knob never moves but stays with the 'stack'

 

Correct.

 

4.  The RA/degrees circle stays with the base through all of these procedures.

 

Correct.

 

5.  When the pinion DOES SLIP when the base is stationary and I am rotating the pinion counterclockwise.  It does not seem to slip rotating it clockwise.  I turned the screw under the fork clockwise a full rotation or so - that seems to have helped with counterclockwise slippage.

 

If tensioning the screw helps then it can buy you time.  I don't remember the numbers, but you may have 2" of screw length to work with and 50-80 threads per inch.  One rotation will give a placebo effect at best.  Depending where is was set to start, you can turn it dozens of times, maybe more than a hundred.  It will resist safely at the end, but be attentive.  Unscrewing can get you in trouble and require a take-down if it detaches, but there is never any reason to do so.


Edited by Optics Patent, 27 September 2019 - 05:34 PM.



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