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Equipment Discussions >> Classic Telescopes

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Atl
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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
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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
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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
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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
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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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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
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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
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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
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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
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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
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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
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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
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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
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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 (14 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
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Reged: 11/06/12

Loc: South Carolina
Re: 628 Drive Problems new [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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Geo.
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Reged: 10/01/08

Loc: Upstate NY
Re: 628 Drive Problems new [Re: dgreyson]
      #5589958 - 12/26/12 04:34 PM

The problem I've seen is that the set screws (3 as I recall) that lock B onto A will slip out of the land cut in A and move up A binding against the axle housing. But I remember that clutch pressure was applied by a large handwheel at the end of A.

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tim53
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Re: 628 Drive Problems new [Re: Geo.]
      #5591206 - 12/27/12 01:28 PM

From ancient memory, I recall that the worm wheel is a ring that rides in a step in c and f. The two Teflon disks D are rings that also ride in the step as clutch surfaces between the hubs and the worm wheel. Adjustment should be VERY sensitive, such that minor fractions of a turn of the clutch screws can make a big difference in the amount of friction applied. The lock washers were used to prevent the screws from loose ing on their own. They don't act as springs

When we put the mounts together, we tested them as we assembled them on a bench. First, we made sure the motor was turning by putting a tape "pointer" on the shaft and watching it turn. Then we put the motor on and marked a tooth of the ring gear to see if it moved while we did other things. At this point we might have left the clutch loose, I can't remember. But you could do that test with the whole scope assembled or without the ota or counterweight.

Remember, the worm wheel only turns on e in a day, so you need to wait a while to see the motion. Alternatively, you could set up on the sky and see if its tracking pretty quickly. There will likely be some backlash at first, but so long as the worm and worm wheel are making contact and the motor is running, you should see the scope start tracking your object in under 30 seconds or so (a lot of backlash!)

Tim


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apfever
Carpal Tunnel
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Reged: 05/13/08

Loc: Colorado
Re: 628 Drive Problems new [Re: tim53]
      #5591271 - 12/27/12 02:21 PM Attachment (16 downloads)

Atl,

I'm a sad case for documenting my work, and getting it all posted here where it counts. I've completely done this drive and whole mount from inside out. The string is here:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4966686/page...

There are pictures and process on the drive that I've never posted. Everything was redone and aligned with calipers and micrometers. This is a busy week and I have to jump on an Orion XT8i today. I'll get a few pictures now. I'll get you through this thing dot and tittle. Mine is complete, correct, in good shape, and works excellently. Originally this had an 8" on it. It now goes with a 10" that was offered on this mount from the factory. here's a picture of the basic clutch. The lock ring (far right) is first on the RA shaft and sets the end play in the mount housing.

The clutch starts with the large hub (center). This gets a teflon washer, ring gear, another teflon washer, and cover (upper left). All this is CLEAN, no lube, certainly no grease! I do use a special dry Teflon lube on some of these, but don't remember if I used it on this one. I could use some lube on my brain. Note that gear ring center!!! it has a small teflon strip around the inside. This goes between the gear fing and the hub. It makes the ring tight on the hub. The worm gear ring is thus isolated from everything with a washer top and bottom and a band around the center.


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apfever
Carpal Tunnel
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Reged: 05/13/08

Loc: Colorado
Re: 628 Drive Problems new [Re: apfever]
      #5591275 - 12/27/12 02:29 PM Attachment (14 downloads)

Random picture:

The worm had more endplay than it should. It should be right at zero without binding. I put a socket over the end and used my metal vice to squeeze the brass (porous bronze?) bushing in till the worm rotated freely with just a hint of resistance at the tightest. The loosest position was probably in the tenths for clearance (.0001" = a tenth).


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apfever
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Reged: 05/13/08

Loc: Colorado
Re: 628 Drive Problems new [Re: apfever]
      #5591291 - 12/27/12 02:47 PM Attachment (13 downloads)

After positioning the bushing for end play, I installed the retaining bolt with just a touch of Red Locktite on the threads, and a micro hair amount at the contact point between the bolt head and bushing.

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apfever
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Reged: 05/13/08

Loc: Colorado
Re: 628 Drive Problems new [Re: apfever]
      #5591362 - 12/27/12 03:27 PM Attachment (15 downloads)

This is a back tracking. One thing you need to be sure of is that the RA shaft is rotating in the mount housing smoothly, freely, 360 degrees, no binding, no strain for the clutch/drive. If the shaft does not have propper rotation, don't expect the drive to run it correctly. I won't get into doing this here, but if it needs to be fixed then your into stuff like this:

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planet earth
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Reged: 09/07/10

Loc: Ontario Canada
Re: 628 Drive Problems new [Re: apfever]
      #5592144 - 12/28/12 04:39 AM

Thanks for posting the pics and the link Neil.
Most helpful.
Sam


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Atl
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Reged: 04/13/12

Re: 628 Drive Problems new [Re: planet earth]
      #5610866 - 01/07/13 08:31 PM

I found the issue and partly resolved the it. The drive motor is gunked up. I soaked it in wd40 while heating it with hand warmers then ran it for an hour with no issues. I am not sure this fix will hold up, but these motors were not made to open up. Any suggestions? However it is now running.

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dgreyson
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Re: 628 Drive Problems new [Re: Atl]
      #5611102 - 01/07/13 10:54 PM

WD 40 will evaporate and the dry film wont keep the old grease in suspension. Soak it in a good quality gun oil instead that wont dry out.

On stuck motors, as a last act of desperation on a sealed clock motor that wont turn I have a procedure I use to get them working again. synchronus motors use a thick grease on the gears and when its sitting for years not in use it eventually hardens into a solid concretion and the motor is stalled dead. To free a stuck sealed synchronus clock motor I take a decimal size drill bit, available at harbor freight, or any wire drill bit just so its very very small and drill a hole in the side or bottom near the edge away from the shaft. Too big a bit and you get metal chips in the works. Drill just enough to make an opening you can insert oil into, dont plunge the bit down thru the gears. next, use an Ink cartridge refill syringe, Insulin syringe or any other sorts of syringes that have a needle to fit in the hole you made. you can use even pen oilers that you could refill, those are readily available, and alternately squirt warm oil in and shake it around and then turn it hole down and squirt air in to blow out the dirty oil and any metal filings. if you use a small enough bit, the filings are small and insignifigant. repeat untill it comes clean. The old grease is like candle wax but it eventually will soften up. After it's clean put some oil back in again.

Plug it in and look at the window thats on most motors to see if the rotor is moving. If its still stopped, pry up the tabs on the plastic window and use a tooth pick or a needle to manually rotate the rotor without bending it. just push it to give it a start and eventually it will take off if the grease is soft enough or washed out. put the window back and bend the tabs to keep it and keep it running and watch it a few hours till you know its ok. Sometimes I have to work it with oil and pushing the rotor for awhile but eventually it will run. Last, clean off the hole and plug it with a smear of silicone.

Anyhow, thats what I do. put a piece of broomstraw on the gearshaft with a dab of wax and you should see it move eventually. If the rotor is moving but the shaft is stopped, you have a gear thats worn down, maybe stripped.
Better to replace it with a new motor.
It's possible to desolder the baseplate with a propane tourch and solder wick and xacto knives to prise it up but usually the motor is too worn out to do anything for it unless you can move a gear up a shaft to bear on an unworn part of the gear. Never had much luck doing that.


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Atl
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Reged: 04/13/12

Re: 628 Drive Problems new [Re: dgreyson]
      #5611481 - 01/08/13 08:16 AM

Great ideas. The drive will turn it is just temperature sensitive. I bet soaking it in Breakfree would do 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: apfever]
      #5612075 - 01/08/13 02:34 PM

Quote:

After positioning the bushing for end play, I installed the retaining bolt with just a touch of Red Locktite on the threads




That "retaining bolt" is actually how you adjust the end play. The oilite bushing ought to move relatively freely in the hole that's bored for it. It's the pressure of the "retaining bolt" that adjusts how tightly it presses on the worm. Red loctite might make future adjustments difficult.


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planet earth
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Reged: 09/07/10

Loc: Ontario Canada
Re: 628 Drive Problems new [Re: Atl]
      #5613126 - 01/09/13 04:04 AM

Quote:

Great ideas. The drive will turn it is just temperature sensitive. I bet soaking it in Breakfree would do it.



I hope it works out for you.
I was fortunate to find a new RA motor for $20.00 and about $20.00 shipping by posting a want ad on Canada Wide Astro.
Surplus shed use to have them and Bill Vorce (Telescope Warehouse), but I believe they are now both sold out. Worth checking out though in case they get any more.Posting want ads can really turn up some fairly rare ATM stuff.
Sam


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