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Equipment Discussions >> Classic Telescopes

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Atl
professor emeritus
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Reged: 04/13/12

628 Drive Problems
      #5552048 - 12/03/12 01:26 PM

I have a Meade 628 and I have been having an issue with it. During cooler weather the clutch slips. The RA knob is turning from the motor, but I have to tighten it down very tight to make it track from all positions. This makes it hard to adjust the telescope's position. I took the drive apart and cleaned the surfaces with degreaser. It was pretty clean to start, but I did it. It is still slipping. I have tried playing with the backlash screw and adjusting the angle of the worm gear but to no avail. Back during warm weather it was tracking great, but when it dips below 50 the clutch severely slips. I don't see how it could be broken...everything seems solid. I am thinking that I am missing something? It's got me pretty irritated I must admit. I have had thoughts of selling it to get it out of my sight, but it would be nice if I could fix it.

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rdandrea
Carpal Tunnel
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Reged: 06/13/10

Loc: Colorado, USA DM59ra
Re: 628 Drive Problems new [Re: Atl]
      #5552294 - 12/03/12 03:39 PM

The RA knob is turning, but is the worm gear? I don't remember how the worm gear attaches to the shaft, but if there's a pin it might have sheared. Take off the drive cover and see if the worm itself is going around.

If it's turning, try torquing the three clutch pressure screws down a little tighter. Just be careful not to strip them.


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Atl
professor emeritus
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Reged: 04/13/12

Re: 628 Drive Problems new [Re: rdandrea]
      #5553642 - 12/04/12 09:44 AM

The worm gear does turn when the RA knob turns. I verified that definitively. The worm gear also moves the large cog that it meshes with, but the teflon washers seem to require a lot of friction to engage the drive. If I torque the screws down tight enough to get the drive to turn it then becomes hard to move the scope by hand.

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Edward E
scholastic sledgehammer
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Reged: 03/26/06

Loc: Tucson, Arizona
Re: 628 Drive Problems new [Re: Atl]
      #5553672 - 12/04/12 10:07 AM

Is the scope properly balanced in RA, Dec & tube? 80% of the time if a scope does not track it's a balance issue. Next thought would be that you need to tighten the three screws on the worm gear; over time the compression washers wear and you have to tighten the screws more to achieve a non-slip gear. Check to see that the set screws that connect the worm gear assemble to the RA shaft are tight.

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Atl
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Reged: 04/13/12

Re: 628 Drive Problems new [Re: Edward E]
      #5554050 - 12/04/12 01:57 PM

My balance is good. I have been pretty picky about following the instructions in the manual. How tight should those screws actually be. I can engage the drive, but at that point the screws are very tight. Doesn't that put stress on the motor?

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rdandrea
Carpal Tunnel
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Reged: 06/13/10

Loc: Colorado, USA DM59ra
Re: 628 Drive Problems new [Re: Atl]
      #5554257 - 12/04/12 04:41 PM

Quote:

Doesn't that put stress on the motor?




Not if the scope moves. And not if the clutch disengages when you move the scope manually. You can put a lot of torque into a pinion gear from a worm, but because of the mechanical advantage of the worm, not so much the other way around.

You should be able to find a happy medium with the screws. It might change with the temperature (mine does a bit).


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Atl
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Reged: 04/13/12

Re: 628 Drive Problems new [Re: rdandrea]
      #5554281 - 12/04/12 05:04 PM

I guess that is the answer. I just need to tighten it down until it squeaks I guess...lol. I was just worried I would burn up the motor. Back in the summer it didn't require much tightening at all, but fall came along and killed that. It does disengage when the scope is moved.

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Edward E
scholastic sledgehammer
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Reged: 03/26/06

Loc: Tucson, Arizona
Re: 628 Drive Problems new [Re: Atl]
      #5557405 - 12/06/12 12:08 PM

If your compression washers have been nearly flattened with time and use then it would take a lot more tightening of the screws to achieve the same compression on the worm gear assembly than would be required with new, "springy" washers and temp changes would have less of an effect as well. This is what I observed with my 826 mount from the time it was brand new to +10 years. Have a look at the old washers and compare them to a new compression washer of the same size so see the old ones need changing.

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planet earth
Carpal Tunnel
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Reged: 09/07/10

Loc: Ontario Canada
Re: 628 Drive Problems new [Re: Atl]
      #5557460 - 12/06/12 12:35 PM

On my 826 mount I need to tighten the 3 screws fully (not super tight) and then I back off the screws about 1/4 turn.
Make sure your polar shaft moves freely with the 3 clutch screws loosened off, say a full turn or so.
Did you test the scope balance, with the polar shaft horizontal and dec shaft at 45 degrees.If it moves it's not balanced. You'll notice many Cave's and other scopes have a extra weight system on the tube to balance the tube radially to compensate for the weight of focusar eyepiece and finder.
Check to make sure the worm gear turns freely and leave a touch of backlash. Make sure the worm and worm wheel turn freely when the clutch is loosened.

Sam

Edited by planet earth (12/06/12 12:55 PM)


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planet earth
Carpal Tunnel
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Reged: 09/07/10

Loc: Ontario Canada
Re: 628 Drive Problems new [Re: Edward E]
      #5557485 - 12/06/12 12:48 PM

Good tip on the spring washers Edward, I think I'll do mine now.
Sam


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Atl
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Reged: 04/13/12

Re: 628 Drive Problems new [Re: Edward E]
      #5558058 - 12/06/12 06:45 PM

The big washers are teflon I believe. Where can you buy them?

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Edward E
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Reged: 03/26/06

Loc: Tucson, Arizona
Re: 628 Drive Problems new [Re: Atl]
      #5559665 - 12/07/12 05:40 PM

If we are talking of split (spring) washers, you should be able to get these in a well stocked hardware store such as "ACE" or "LOWES" or a good mom/pop hardware shop.

It's up to you but I would replace the plastic split washers with metal ones and keep the old plastic one in storage to refer to when needed.


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rdandrea
Carpal Tunnel
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Reged: 06/13/10

Loc: Colorado, USA DM59ra
Re: 628 Drive Problems new [Re: Edward E]
      #5560699 - 12/08/12 11:11 AM

I think he's talking about the big teflon washers that are sandwiched around the drive gear in the clutch assembly. Good luck finding those. There are lots of sources on the internet, but you have to buy large quantities.

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Edward E
scholastic sledgehammer
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Reged: 03/26/06

Loc: Tucson, Arizona
Re: 628 Drive Problems new [Re: Atl]
      #5585474 - 12/23/12 12:01 PM

Any improvement or a fix on your slipping clutch?

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Atl
professor emeritus
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Reged: 04/13/12

Re: 628 Drive Problems new [Re: Edward E]
      #5585544 - 12/23/12 12:48 PM

None. The clutch either has to be so tight it won't move or so loose it slips. I am at a loss. I have been thinking of selling it since I bought a 12.5" dob...lol.

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dgreyson
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Reged: 11/06/12

Loc: South Carolina
Re: 628 Drive Problems new [Re: Atl]
      #5586521 - 12/24/12 12:38 AM

Are you sure the clutch is actually slipping? Maybe the clutch assembly it'self is rotating on the shaft, or the equatorial head is slipping on the other end of the shaft.

It sounds like something is contracting when it gets cold enough and is slipping loose then, just my W.A.G.


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Edward E
scholastic sledgehammer
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Reged: 03/26/06

Loc: Tucson, Arizona
Re: 628 Drive Problems new [Re: Atl]
      #5589383 - 12/26/12 09:54 AM

Not good. It is staring to sound like the aluminum ring gear had worn into the plastic grove to the point that it no longer fits snuggly. If so, it's either replace the plastic or find a new ring gear assembly. Check with Don Rothman at Astro Parts Outlet he has a lot parts for these older mounts or you could build a new worm gear/clutch assembly. Edmonds Sci "All About Telescopes" has a good design to use.

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tim53
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Reged: 12/17/04

Loc: Highland Park, CA
Re: 628 Drive Problems new [Re: Edward E]
      #5589459 - 12/26/12 10:43 AM

I haven't worked on one of these in a very long time, but used to build them at Meade 30 years ago.

Pics would help a lot for seeing what you are referring to. There were steps we would take while assembling these to make sure everything was working before be buttoned them up and shipped them out. I might be able to remember the steps if I could see a pic, especially where you think the problem is.

-Tim.


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planet earth
Carpal Tunnel
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Reged: 09/07/10

Loc: Ontario Canada
Re: 628 Drive Problems new [Re: tim53]
      #5589632 - 12/26/12 12:20 PM Attachment (15 downloads)

Here a quick drawing:
Basically C and F are tightened with the 3 screws, with the worm wheel E: in the middle, and D: 2 thin teflon disks on each side. H is a teflon ring that goes into the wormwheel bore.
B and C are setscrewed to A the polar shaft.
Sam

Edited by planet earth (12/26/12 05:21 PM)


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dgreyson
scholastic sledgehammer
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Reged: 11/06/12

Loc: South Carolina
Re: 628 Drive Problems [Re: planet earth]
      #5589696 - 12/26/12 12:55 PM

My drive dosent have "D" but does have "H" between the innder diameter of the RA gear "E" and a concentric step on "C" inside the diamater of "E" which is it's bearing "race"

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