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Perfex / Tasco 10TE / Astro Optical R-74 mount overhaul

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#51 deSitter

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Posted 11 March 2018 - 09:26 AM

Actually the azimuth lock and declination lock have been switched.

 

-drl



#52 Masvingo

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Posted 11 March 2018 - 09:26 AM

Finally, the azimuth circle can be unscrewed (three screws) and then lifted off the mount base, replacing the screws afterwards to keep them safe.

 

azimuth7.png

 

Unusually for me, being the type of person who, armed with a screwdriver, can’t resist undoing any screws in the immediate vicinity, I avoided the temptation to remove the bubble level from the mount base – maybe wisdom does come with age!  If you can’t resist doing this, there is a thread here on the bubble level which also links to another thread detailing its replacement.
 



#53 Masvingo

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Posted 11 March 2018 - 09:28 AM

Having now acquired a pile of bits, I started cleaning up the mount base and yoke.  Having cleaned a lot of dirt off the yoke where the two side arms join the base I made an unwelcome discovery …... there was a prominent crack at the base of one of the yoke side arms running almost the full width of the side arm – the red arrows show the ends of the crack.

 

crack1.png



#54 Masvingo

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Posted 11 March 2018 - 09:30 AM

The crack looked quite well developed at this side of the arm …...

 

crack1.5.png



#55 Masvingo

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Posted 11 March 2018 - 09:31 AM

…… and on closer inspection the start of a crack could also be seen on the other side and a faint trace joining both sides.  Fortunately, the crack had not fully developed and the yoke casting was still in one piece.

 

crack3.png

 

Fortunately, no trace of a crack could be seen on the other side arm. 

 

I’m not sure why one side arm has cracked, neither the mount nor tube show any signs of impact damage indicating any accident such as the mount falling over, maybe it is a casting flaw?


Edited by Masvingo, 11 March 2018 - 09:31 AM.


#56 Masvingo

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Posted 11 March 2018 - 09:33 AM

I was reluctant to reassemble the mount in its current condition, as the thought of what might happen if, or rather, when the side arm gave way completely did not appeal. 

 

Fortunately, both the base and the side arm surfaces are relatively flat, so, after a lot of cursing and a bit of thought, I came up with the idea of using some aluminium angle to make two reinforcing plates, one to go on the outside of the side arm and base and one on the inside.  As the yoke casting was still in one piece, positioning these plates and the necessary fixing holes, would be easier than if the side arm had already broken off.

 

As the local DIY store only had aluminium angle with a relatively thin section I made up two reinforcing plates to go on the outside.  Care was required in drilling the two screw holes in the flat base section to ensure that they came out on the inside of the base section that forms the azimuth bearing surface and that there was sufficient space to get a nut on each screw.  The screw holes in the yoke walls and base were also tapped to take the screws, leaving the nuts to secure the reinforcing plate that was to go on the inside of the yoke.  Due to space constraints on the inside of the base section I did not double up the inner reinforcing plate.

 

crack5.png


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#57 Masvingo

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Posted 11 March 2018 - 09:34 AM

Unfortunately this picture is not very clear but the inner reinforcing plate and one of the securing nuts can be seen with part of a second securing nut just visible.

 

crack4.png


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#58 Masvingo

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Posted 11 March 2018 - 09:39 AM

Having sorted that out, and cleaned up all the other parts (using white spirit to remove the old grease and dirt) I was able to commence the reassembly of the mount, working from the base upwards as the base makes a convenient and stable holder for working on the rest of the mount. 

 

Whilst the reassembly was basically a reverse of the disassembly process I found I needed to make a few additional spacers to ensure freedom of rotation for both azimuth and RA.

 

Firstly, I replaced the azimuth circle onto the mount base, and then I reinstalled the yoke, suitably reinforced, into the mount base section, with the bearing surfaces and the red fibre washer that sits between the top of the base and the flange on the yoke suitably lubricated – I used Super Lube synthetic grease.  However, I found that when the four screws, each with a spring washer, that secure the yoke retaining plate were tightened up, they clamped the mount base too tightly and prevented the yoke from rotating in azimuth.  This explained why the azimuth adjustment was so stiff even though, as noted in an earlier post, the lock knob was not actually pressing the shaped brass rod brake shoe tight against the azimuth bearing surface on the yoke.

 

I made a couple of spacing washers out of thick paper with an outside diameter that was just slightly smaller than the inside diameter of the hole in the mount base that the yoke sits in.  I found that two such washers, with a total thickness of 0.016” was sufficient to enable the four screws for the yoke retaining plate to be fully tightened whilst leaving the yoke free to rotate in azimuth. 

 

The azimuth pointer was then re-attached to the base of the yoke followed by the adjustable support for the RA bearing housing to hold it at the correct angle for the observing latitude.

 

I also made up some spacing washers (total thickness 0.016”) to replace the broken one that was between the RA circle and its seat on the RA bearing casting and reinstalled the RA circle, making sure that when the four securing screws were tightened they held the RA circle securely but still allowed for it to be moved in order to set the RA. 

 

The next stage is to reinstall the RA bearing casting into the yoke, with a large red fibre washer between each yoke side arm and the RA bearing casting, making sure that the pivot bolt for the latitude adjustment is inserted from the side of the yoke that has the cut-out for the key on the pivot bolt that prevents it from rotating when the clamping nut is tightened. 

 

rebuild1.png


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#59 Masvingo

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Posted 11 March 2018 - 09:41 AM

Having reinstalled the RA bearing casting and greased the bearing surface, assembly of the RA mechanism commenced with the three red fibre washers (one wide, two narrow) and the RA worm gear, all suitably greased, being placed in position.

 

rebuild2.png



#60 Masvingo

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Posted 11 March 2018 - 09:42 AM

Actually, the first time I did this, I thought that maybe there should only be two red washers (one wide, one narrow) between the RA bearing casting and the RA worm gear leaving the other narrow one to go between the other side of the worm gear and the large red fibre washer that sits under the RA cover plate, as shown in the picture here. 

 

rebuild3.png

 

However, when reassembled, I notice that when rotated about the RA axis the narrow washer on the cover plate side became displaced.  It seems that the RA gear projects slightly from the other side of the declination bearing casting thereby requiring the two narrow washers.  So, it seems both narrow red fibre washers go on the bottom side of the RA worm gear along with the wider washer.

 

Before placing the declination bearing casting over the RA worm gear, the RA worm, suitably greased was placed in position, but without the eccentric bushings so it could sit up high in its housing in the declination bearing casting, clear of the worm gear, as the RA worm gear was placed in position. Having done so, the RA worm was brought down onto the RA worm gear and the eccentric bushings were slid over the RA worm pinions and located in their seats in the declination bearing casting, rotated as described in the following post so as to bring the RA worm into its correct meshing position with the RA gear.  I found a pair of workshop dividers, with their twin points, were ideal for rotating the eccentric bushings.


Edited by Masvingo, 11 March 2018 - 09:43 AM.


#61 deSitter

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Posted 11 March 2018 - 09:43 AM

There are enough of these mounts around that you may be able to find a replacement for your azimuth pillar. That probably happened when the altitude lock nut was overtightened. The castings are extremely brittle.

 

-drl


Edited by deSitter, 11 March 2018 - 09:43 AM.

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#62 Masvingo

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Posted 11 March 2018 - 09:45 AM

If the mount has not been disassembled before, the indentations (blue arrow) that the grub screws have made on the bushings serve as good markers to initially align the bushings.

 

RA_bushing.jpg

 

If the mount has been disassembled more than once before there will probably be a number of indentations on the bushings and this is where it is handy having pictures of the mount prior to disassembly to give the initial alignment.

 

It is likely that you will have to slightly rotate the bushings just a few degrees from the original position to get the RA worm to mesh nicely with worm gear.  I found I had to have several goes before I got the adjustment right.  Whilst doing this, you also need to ensure that both bushings sit up against the steps on each side of the RA worm to minimise any play in the RA slow motion.  As the grub screws bear on a conical surface of the bushings care should be taken not to tighten the grub screws too much as otherwise they will tend to push the bushings up tight against the steps on the RA worm and cause it to bind.

 

IMPORTANT – as noted previously, access to two of the grub screws that hold the bushings in place is prevented by the declination circle when the mount is fully assembled, so it is worth while taking plenty of time now to ensure that the bushings are adjusted correctly – you don’t want to get everything reassembled and then find that further adjustment is required and have to partially dismantle the mount again. 

 

A final check and adjustment should be done once the RA lock ring and cover plate are reinstalled.


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#63 Masvingo

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Posted 11 March 2018 - 09:46 AM

There are enough of these mounts around that you may be able to find a replacement for your azimuth pillar. That probably happened when the altitude lock nut was overtightened. The castings are extremely brittle.

 

-drl

 

Yes, that makes sense, given the indentations found on either side of the yoke from a wrongly fitted pivot bolt.

 

Hopefully one will come up, although they don't seem so common this side of the pond.


Edited by Masvingo, 11 March 2018 - 09:48 AM.


#64 Masvingo

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Posted 11 March 2018 - 09:50 AM

The large red fibre washer can now be placed in position and the chisel tipped plunger placed through the hole in the washer and into the gap in the RA brake ring.

 

rebuild4.png

 

The cover plate can then be reinstalled and the large screw that retains the split brake ring installed.  Next the four screws that hold the cover plate in position can be installed – you made need to rotate the cover plate slightly to line everything up – and tightened.  Next check whether there is free rotation about the RA axis with out too much play up and down.  If there is too much play, remove some of the spacing washers in the well in the centre of cover plate, if the RA rotation is stiff, as it was with my mount, add more spacing washers.  I had to add a couple of spacing washers, again made out of stiff paper, in order to get free rotation once the RA cover plate screws were fully tightened. 

 

Having tightened up the four cover plate screws and checked that there is free rotation, attach a flexible slow motion cable to the RA worm and check that the RA worm gear can be easily rotated through 360 degrees in both directions and if not, readjust the eccentric bushings again until this is achieved – again making sure that they are also sitting up snug against the ends of the RA worm so there is no back lash.



#65 Masvingo

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Posted 11 March 2018 - 09:51 AM

Next up  was to replace the declination slow motion worm block and worm in the housing in the lower cradle casting.  Having cleaned up the declination worm box I noticed that it had been stamped with a tiny Astro Optical symbol.

 

tangent_arm5+6.png

 

Having greased the worm it was then screwed through the seat for the bushing in the near end of the slow motion housing in the lower cradle casting and then through the worm block until the worm threads were clear of the seat for the bushing and far end of the worm gear pinion was located in the hole in the far end of the slow motion housing in the lower cradle casting. 

 

Next the bushing was screwed into its seat in the near end of the slow motion housing in the lower cradle housing until it was up against the step on the worm pinion.  The lock nut was then placed in position and tightened whilst holding the bushing in place using the slots cut in the end of it. Ensure that the bushing is not up too tightly against the step on the worm pinion so that the pinion can turn freely but without any free play to minimise back lash in the declination slow motion.


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#66 Masvingo

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Posted 11 March 2018 - 09:53 AM

Now the declination circle can be replaced in its position on the declination bearing casting.  I decided not to use any glue, partly because I wasn’t sure if the glue mentioned in my earlier post (#21) was original and partly because I didn’t have any suitable glue to hand and it would have been difficult getting hold of some due to the Christmas holidays.  I figured the locating pin would suffice as the circle would also be held in place by the presence of the declination tangent arm above it.

 

After the declination circle came one red washer, then the declination tangent arm and then two more red fibre washers and then the lower cradle casting with the whole lot being secured by the brass retaining ring which threads on to the declination bearing casting.  The brass retaining ring should be tightened, using an optical spanner, until the half hole in the ring for the locking grub screw is aligned with the half whole in the declination bearing casting (as indicated by the screwdriver).

 

rebuild11.png



#67 Masvingo

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Posted 11 March 2018 - 09:54 AM

The locking grub screw can then be screwed in which will stop the brass retaining ring from loosening – the picture shows the grub screw partially inserted.

 

rebuild12.png



#68 Masvingo

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Posted 11 March 2018 - 09:57 AM

Next, the screw and block assembly that links the declination tangent arm to the declination worm block was installed, not forgetting the two thin washers that sit between the declination worm block and the tangent arm.  And now was as good a time as any to attach the declination and RA pointers at their respective locations on the mount.

 

Finally, the upper cradle can be reattached to the lower cradle casting (I also took the opportunity to replace the felt lining with new felt) and the two short shaped brass rods that clamp the declination axis and the azimuth were installed, taking care to orientate them properly followed by the two lock knobs.  As previously noted, the azimuth lock knob was not long enough to push the brass rod up tight against the azimuth bearing surface on the bottom of the yoke to clamp it so I also inserted a further short spacer, cut from a six inch nail, between the brass rod and the lock knob so that the azimuth lock would work properly.

 

And voilà, one overhauled mount…

 

finish2.png


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#69 deSitter

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Posted 11 March 2018 - 09:59 AM

If the mount has not been disassembled before, the indentations (blue arrow) that the grub screws have made on the bushings serve as good markers to initially align the bushings.

 

attachicon.gif RA_bushing.jpg

 

If the mount has been disassembled more than once before there will probably be a number of indentations on the bushings and this is where it is handy having pictures of the mount prior to disassembly to give the initial alignment.

 

It is likely that you will have to slightly rotate the bushings just a few degrees from the original position to get the RA worm to mesh nicely with worm gear.  I found I had to have several goes before I got the adjustment right.  Whilst doing this, you also need to ensure that both bushings sit up against the steps on each side of the RA worm to minimise any play in the RA slow motion.  As the grub screws bear on a conical surface of the bushings care should be taken not to tighten the grub screws too much as otherwise they will tend to push the bushings up tight against the steps on the RA worm and cause it to bind.

 

IMPORTANT – as noted previously, access to two of the grub screws that hold the bushings in place is prevented by the declination circle when the mount is fully assembled, so it is worth while taking plenty of time now to ensure that the bushings are adjusted correctly – you don’t want to get everything reassembled and then find that further adjustment is required and have to partially dismantle the mount again. 

 

A final check and adjustment should be done once the RA lock ring and cover plate are reinstalled.

I found that even putting the bushings back as found was insufficient to avoid a lengthy and frustrating process of seating them correctly. The slightest pressure from the set screws increases the tension on the worm to the point of annoyance and unacceptable stiffness. I still have to redo mine because I'm not satisfied with it (and likely never will be).

 

-drl



#70 Masvingo

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Posted 11 March 2018 - 10:13 AM

Yes, and it's not helped by the fact that one needs to remove the cradle, declination tangent arm and declination circle in order to reach two of the set screws.  I'm wondering if the screws will become loose with vibration over time and require re-adjusting but I was reluctant to use any locking compound on them.



#71 Masvingo

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Posted 11 March 2018 - 10:23 AM

Actually the azimuth lock and declination lock have been switched.
 
-drl


Is this the declination lock?

image1.jpeg

And this the azimuth?

image2.jpeg
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#72 deSitter

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Posted 11 March 2018 - 10:46 AM

Is this the declination lock?

attachicon.gif image1.jpeg

And this the azimuth?

attachicon.gif image2.jpeg

Yes and there's a brass monkey that goes under it as well, different than the one in the azimuth hub.

 

-drl


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#73 AUricle

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Posted 11 March 2018 - 11:22 AM

Once again, as in the past, just a wonderful step-by-step illustration.

It's one thing to have the patience to do the work, quite another to stop at each step to document the process so thoroughly.

Thank You!bow.gif waytogo.gif


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#74 Masvingo

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Posted 11 March 2018 - 12:23 PM

different than the one in the azimuth hub.

 

-drl

Ah, that maybe explains the issues with the azimuth lock on mine and why I needed to insert a spacer behind the azimuth brass rod in order to get the azimuth lock to work as both brass rods were the same on my mount.



#75 Masvingo

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Posted 11 March 2018 - 12:26 PM

Thanks AUricle.

 

I did, however, get a bit impatient on the reassembly (as I was hoping to observe that night) so forgot to take a lot of pictures.  Luckily, reassembly is pretty much the reverse of the disassembly.




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