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ETX125 RA Disassembly / reassembly

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#1 q192

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Posted 18 April 2016 - 07:52 PM

Has anyone here replaced the RA drive on their ETX125?  Mine has a black aluminum disk approx 2" in diameter covering the main gear that I have been unable to remove.

 

I think the only purpose of this thing is to keep the cables from getting caught in the worm gear but I could be wrong.

 

It looks like it should unthread off the central shaft but it doesn't come off easily!   I made a custom wrench out of a hose clamp and a strip of rubber and torqued it pretty hard and it wouldn't budge.

 

Has anyone taken one of these apart?  I'm about to cut this off so that I can get my new worm gear in there if I can't find a way to get it apart! 

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#2 slipstrm

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Posted 18 April 2016 - 09:19 PM

Taking this off is way difficult.  The black disk connects to the RA clutch lever.  When you torque the RA clutch lever this allows the black disk to engage the motor and turn the entire RA base assembly.  Removal is semi difficult and and prone to shearing the Declination wires.  Reassembly is very difficult with 2-3 hours of  trial and error.
One has to be extremely careful to not twist (sever) the wire bundle entering the the middle of the shaft.  The shaft with a few threads showing in the center of the black disk connects to the RA torque lever.  ( other side of the plastic base).   If the silver threads ( middle of the black disk) are rotating you are about to shear the dec control cables!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Here is a helpful link from the great Weasner site with pictures: http://www.weasner.c.../inside125.html  

Before disassembly occurs:  

1.  Note how many aluminum threads are showing from the middle of the black disk.  Usually 1 -3 are showing.  When reassembling this is important as it allows the RA lever to clamp  down and engage the entire aluminum assembly.  Otherwise it is a painful trial and error process.

2.  Lock and unlock the silver RA clutch lever and note that the screw threads in the middle of the black disk rotates about 1/16 turn.  Remove the sliver RA locking lever on the other side of the base.  Use an appropriate hex wrench.

3.  Underneath the silver RA lever is a hex head nut.   The nut/bolt is hollow, the four wires in the middle of the black disk go through the bolt and exit about 1/2 inch from the top of the bolt.  These wires connect to the declination motor in one arm of the mount.  

4.  Remove the blue electrical connector from the PC board.  Mark the position of the connectors.  In your case, yellow goes toward the edge of the PC board.  Red towards the middle.

5  Use a 13mm socket wrench and unscrew the hex head nut, no more than one half turn.  Remember you can shear off the 4 cables if you turn any further.  As you turn the nut ( no more than one half turn) you will hopefully note the black aluminum disk will loosen up enough to hand loosen the disk.  Push down on the hex head nut with one hand and try and unscrew the black disk with your other hand.  You are trying to move the black disk away from the worm gear assembly.

Never turn the silver bolt!! Only unscrew the black disk..

6.  Unscrew the black disk slowly.  Only the black disk should rotate not the silver threads.  If the silver threads move this will shear the wire assembly.

7.  If the black disk refuses to hand loosen you will have to take apart the base.  

7.  Try your luck.  I can send further instructions for the base disassembly.  This is a 2-3 hour procedure.  If you shear the Dec wires then you can fix them but will need replacement wires, heat shield, and a punch down tool.

Good luck.



#3 OzAndrewJ

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Posted 19 April 2016 - 03:37 AM

Gday q192

 

Dont cut anything as it will kill the clutch mechanism

Ref

http://www.weasner.c...f_ETX125PE.html

for some more info on what the guts looks like.

You need to get access to the top of the clutch bolt and unscrew it

a turn or two to free up the mechanism.

This involves major disassy.

It is not designed for easy access.

 

Andrew Johansen Melbourne Australia


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#4 q192

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Posted 19 April 2016 - 10:29 AM

Thanks, guys.  Looks like I need to research this a lot further before doing this! 

 

I appreciate your insight! 

 

Thanks! 



#5 slipstrm

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Posted 19 April 2016 - 05:42 PM

Quick question.  What is wrong with the worm gear??



#6 q192

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Posted 19 April 2016 - 08:02 PM

The gear reduction assemblies on these scopes are weak.  I broke this one in multiple places by putting a dslr on this scope.  Bad idea! 

 

So that affected the gearbox and helical gear.  I was able to get the broken assembly out because it was fractured into pieces.  The old worm block is still there because it won't clear that black part.  And the fully assembled replacement won't go in with that in the way.  Hence that is the current problem. 

 

One other option here may be to cut a hole in the side of the drive base and slide it in that way using the old worm block (should be nothing wrong with it).  I'll study it closer before I attempt anything or cut anything. 

 

Has anyone cut the drive base and reassembled one of these from the side? 



#7 OzAndrewJ

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Posted 19 April 2016 - 08:38 PM

Gday q192

 

You cant cut the side ( but dont need to )

The large white gear that fits on the end of the worm shaft is fitted on a taper.

It can be easily removed.

Soo fit the worm back into rough position without that gear attached.

Fit the motor / gearbox assy

Then slide the worm into the large gear and fix.

Then tension the worm from the other end as reqd

 

Andrew Johansen Melbourne Australia



#8 q192

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Posted 19 April 2016 - 09:41 PM

Thanks, Andrew.  I thought of that and tried to get that gear off of my old unit.  I couldn't get it to separate without putting excessive force on it.  So I had abandoned that idea. 

 

But now that I know it has been done, I'll try it on the new unit and revisit it on the old one.  If I can get one to separate I'll get it back together.  That part of the old one isn't damaged.  It is mainly the gearbox that had issues. 

 

It will be a few days before I have time to work on it again but it is reassuring to know how some people have rebuilt these.  It can be done. 

 

Thanks again! 



#9 Geo.

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Posted 21 April 2016 - 08:43 AM

That's the RA axle nut, Kind'a like the "Jesus nut" on a helo. It is locked in place by a couple slots in the axle housing. To remove it you have to back off the axle bolt.

 

Remove the RA axle clutch knob found between the forks on the top of the mount and using a 13mm (1/2") socket back off the bolt head until you feel resistance from the wires that run up a slot in the side of the bolt. Pull up on the axle nut disk whilst retaining the tension on the bolt head and it turn (CCW to loosen). 



#10 jkd6h

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Posted 09 December 2017 - 11:19 AM

I've just got done repairing my ETX125 using the posts above (thanks!).  I thought I'd add some pictures here for the next person who finds this page and is in the same predicament... the main issue how to unscrew the clutch bolt to disassemble the clutch and replace the assembly.

To remove the optical tube I followed this video of Youtube

    https://www.youtube....wBWB29ow&t=836s

but note his scope is a previous generation and the clutch mechanism is different than mine or whats shown earlier in this thread.

 

At that point, Im looking at the clutch assembly shown in the first picture.   In the second picture you can see a close up of where the black plastic gear holder for the white plastic drive gear has snapped and is now pointing up rather than to the right where it was attached.  (Screwdriver points to it.)  I later tried to move the plastic gear by hand within the support bracket and it would not budge.  They're basically fused together. 

 

I hadn't used the scope in years and my thought is that either the plastic shaft swelled during that time or the lubrication broke down.  Either way when high speed slewing it fused the two together.  A sign that something was broken is an occasional snapping sound when high speed slewing.  I'm pretty sure that was the black moving plastic gear holder (now moving) pushing it's way past the piece of the support bracket it used to be attached to.

 

 

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#11 jkd6h

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Posted 09 December 2017 - 11:33 AM

... continuing...

 

the dilemma is still how to loosen the clutch bolt without twisting and cutting the wires as mentioned before.   I cant see how to remove what I gather are the base mounting screws since they're blocked by the outer base metal ring.  (first picture)  Maybe the entire outer plastic base housing has to be removed to get access.     What I did instead was to remove the plastic covers on the both of the fork arms, flip the base upside down, and then slide the base top cover down a few inches so I could access the clutch bolt from the side with a open wrench (second picture).  

 

Note: you will also need a very small allen/hex key (maybe 1.5mm) to loosen the set screw on the RA clutch so you can remove the plastic lever, slide down the top base cover, and access the bolt.

 

Using the open wrench I was able to unscrew the bolt and at the same time flip the DA wires over the top of the bolt head (facing down) at the same time as the bolt was moved.  This moved the wires with the bolt so they didnt get cut.

 

The third picture is a close up of the bolt, slot for the DA wires, and the wire bundle.

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#12 jkd6h

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Posted 09 December 2017 - 11:58 AM

These last two pictures are of the clutch base.  You can see the protrusions in the clutch shaft around the bolt and the slots in the clutch base which, as others have pointed, out makes it impossible to just unscrew the base plate.

 

I found a replacement RA motor/pcb assembly, including the worm gear, on Ebay which I used to replace the broken assembly.   I'm not sure how many of those are available even at the time of this post.   Apparently others have repaired the black support bracket for the white gear shaft by gluing it together and maybe adding a thin splint but, as I said previously, in this case I found the two pieces totally fused.  If you're in the same situation you might try harder than me to free them up.

 

The replacement RA assembly looks to be a different revision of the circuit board but otherwise identical - so no picture.

 

After replacing the motor assembly I pretty much just reversed all my steps to reassemble.   I will say it it did take a few tries to get the clutch bolt tightened to the right amount that with the operating lever it was loose at one setting and tight enough at the other.   Test it out before reassembling.

 

After reassembling everything I reset the Autostar controller, recalibrated the motors,  and redid the drive training.   I was worried about perhaps some electrical compatibility issues since the RA circuit board is a different revision than the original but it all seemed to work.   There might be more backlash when switching directions than before.   After an alignment, it's ability to find and point to an object is "ok".   (Sorry to be vague I cant remember if I had a 20mm or 26mm eyepiece and it would just miss out of view)   I only had one chance to try it so far.   I'm planning on redoing the training and trying again to see if it can be improved.

 

I hope all this helps someone else!

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Edited by jkd6h, 09 December 2017 - 05:26 PM.


#13 OzAndrewJ

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Posted 09 December 2017 - 09:17 PM

Gday Jkd6h

Did you test the preload on the ra axle and worm wheel in its carrier before reassy?

These being too tight are what causes that neck piece to flex and snap.

Ref https://www.cloudyni...-mount-options/

for a current similar thread

and http://www.weasner.c...f_ETX125PE.html

for an analysis of one of the ways that joint gets flexed if the mount isnt free to move.

 

Andrew Johansen Melbourne Australia



#14 jkd6h

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Posted 14 January 2018 - 11:32 AM

A little late on my reply but...

Unfortunately for me I did not deliberately test the preload on the worm wheel in the carrier, but I'm pretty sure I casually spun it in place and did not notice any undue force required.  As far as the RA axle itself, it turned easily when disassembled without the clutch.   I suppose I could have tested it with the reassembled clutch engaged - but the worm gear not installed - and that might have shown some issue but I didnt do that either.

 

Regardless, I can understand your point about any preload causing the neck piece to flex and snap, but would that explain why the white transfer gear is seized in the black support ring?  I cant spin the gear inside the broken holder at all.  I'm still thinking lubrication break down and friction welded the two together but I'm no expert.



#15 OzAndrewJ

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Posted 14 January 2018 - 03:38 PM

Gday Jkd6h

 

I'm still thinking lubrication break down and friction welded the two together

You are probably correct, but if the drive was tight, the torque applied attempts to push the gears up/down relative to each other and this increases the friction load on one side of the collar. The localised rubbing under high load for too long probably caused part of it.

Anyway, good to see its working again

.

Andrew Johansen Melbourne Australia



#16 MaverickGazer

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Posted 12 March 2018 - 10:33 PM

Many thanks to everybody who has contributed to this and related topics.  I had a broken RA drive, and band-aid approaches to fixing it (reinforcement straps and superglue repairs of the flex arm) did not hold.  The pictures and text posted here and in other threads gave me a good idea on how the telescope was put together, and how I could safely twist the central bolt to loosen the clutch plate by gaining visual access to the dec wire and the central bolt by removing the OTA, removing the plastic covers on the forks, removing the clutch lever, and lifting the plastic plate to see what I was doing.

 

My broken ETX 125 is now working.  I regret that I didn't take pictures along the way to document what I was doing.  It would have made for a nice presentation at the local club.  And now that I have it working, I'm not going to jinx it by tearing the scope apart and starting over.

 

Given the number of people having this problem, perhaps this needs to be a sticky thread.

 

MaverickGazer



#17 Handy13

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Posted 11 June 2018 - 04:41 PM

Wow!  I just replaced my RA drive because of a 30 to 60 second backlash.  JDK6H's #10, #11, and #12 posts were absolutely invaluable.  It was, even then, a real struggle.   Multi-day job of maybe 20 hours - so far.  Maybe these scopes come in different versions?  Had a great deal of difficulty getting into area shown in JDK6H's photo where he is using the crescent wrench.  Finally man-handled (carefully!) the inside fork arm cover of the Dec motor side to get it out of the way.  Had to have a friend over to help figure that out...

 

Part of the RA backlash problem was an incorrectly sized plastic spacer washer at one end of the worm gear that allows the gear to slop around.  This was an obvious manufacturing defect!  Other problem was that some klutz who owned this scope before me apparently semi-stripped the screws that hold the worm gear in place.  He was probably trying to solve this same backlash problem. 

 

Looked everywhere for solutions regarding stripped screws then finally applied thin coating of solder to the middle of the screws.  That seems to work well.  Would have used a tap to move from #4 to #5 metric screws but I don't think the bigger screw heads would fit in the space available...  The solder works...  Not great but the screws tighten now...

 

I obtained new board and worm gear from here:

https://www.ebay.com...D-/231879100914

 

Currently I am researching info on how to get the OTA back in place.  Seems to not want to go the last quarter inch.  Still searching for a solution...

 

Thanks,

 

Howard



#18 OzAndrewJ

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Posted 11 June 2018 - 05:36 PM

Gday Howard

If it is like mine, there are 2 small alignment tags on each fork arm sideplate and these need to be aligned to the holes in the OTA section.

Sometimes it just takes a little sideways pressure to get them to engage.

Andrew Johansen Melbourne Australia



#19 Handy13

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Posted 11 June 2018 - 06:30 PM

Do you mean the the "tabs" need to fit into the "slots?"  There is not enough

clearance for this to happen.  Do I need to file the slot edges a bit?

 

DSCN5426.JPG DSCN5427.JPG

 

 

Thanks,

 

Howard



#20 OzAndrewJ

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Posted 11 June 2018 - 06:38 PM

Gday Howard

Yep.

IIRC, the 2 little tabs near the screw holes at the rear slightly spring out the rear end of the saddles.

As such you sometimes need to slightly press it back inwards for the main tabs to engage.

 

Andrew Johansen Melbourne Australia

 

PS i never had to file anything, but just to confirm it needs to go that far

get a ruler and measure the distance from the centre of the rear screw insert to the front of say Tab1

Now move to the saddle and see where this puts the front of the tab relative to the slots.


Edited by OzAndrewJ, 11 June 2018 - 08:00 PM.


#21 Handy13

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Posted 11 June 2018 - 09:54 PM

OK, I used a Dremel tool to reshape the leading edges of the tabs and slots and they final were able to slide together - with difficulty - but the OTA is now properly in place and seems stable.

 

Thank you Andrew for helping me see what needed to be done. 

 

Cursory testing shows backlash greatly diminished.  Will do more testing within the week I hope. 

 

Again, thanks everyone who posted here and particularly JDK6H whose images are essential to understanding disassembly of the RA motor section of the scope!

 

Thanks,

 

Howard



#22 MaverickGazer

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Posted 12 August 2019 - 08:54 PM

Handy13,

 

I'm sorry I didn't see your post earlier.  What I did to reset the OTA into the fork arms was to use some Armor All wipes on the plastic as a dry lubricant.  The OTA snapped into place fairly easily.


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