ETX TECHNICAL TIPS Last updated: 31 May 1999

More ETX drive/base mods

From: James Chambers (ucjttjc@ucl.ac.uk)

Poor bearing First Aid on ETX EC

The drive on my new ETX EC was very poor, with lots of stop/ start motion and slack mainly in the DEC motions. Having spent some time improving the old ETX I had to start again. The main problem is the poor fit of the molded bearings plus the fact they are tapered. When I get time I will machine a new set of bearings in brass or bronze, but in the meantime the following helped.

Take the 4 screws out holding the main tube. Spring or pull the fork apart slightly to release and carefully slide out the tube. Take the 2 arms out. On the driven side there is a white plastic bearing. I degreased it and coated it in Devcon 2 part resin and pushed it home in it's normal seating. That gets rid of 1 source of slack. Don' t use too much epoxy resin and wipe off any surplus. Put some very thin grease or petroleum jelly on both sides of the gear wheel.

Put the driven side back. The keyway had slack in it giving a 1/4 inch rise and fall on the end of the scope tube. I cheated by pushing into the gap a sliver of thin metal ( a coke tin and strong scissors provided it!) Still talking about the driven side I had to tighten the locking knob so much it felt wrong, so I put a steel 3/4 inch washer with a small centre hole under the fake degree plate. Now it locks with less straining on the too small brass thread of the knob. The pressure here is used to squeeze the gear wheel and transmit the rotary motion. What happens is the degree scale is soft alloy and deforms,the washer stops this.

On the undriven side I wrapped plumbers thin Teflon tape around the bearing, 2 turns mine took but experiment and re-assemble with thin grease. This side bearing also had too much side play so I carefully held it horizontal and rubbed it down to shorten how much it protruded, Test by assemble this arm with the degree plate until you get a nice silky action.

Give me a call on ucjttjc@ucl.ac.uk if I can clarify. The optics are super as usual with Meade but the oriental workmanship is very poor, this is typical of this part of the world when it comes to mechanical things, even the Meade tripod had to be machined and re-assembled. The screws are a give away, look at the one to hold the eyepiece, it's far too loose with maybe only 50% thread depth, which does not matter maybe but in other areas it means you dare not tighten anything up properly in fear of stripping the threads. I don't think Meade can control this problem, it's all down to price, It would take me a full working day as an instrument maker to rectify faults with proper metal bearings etc. I,m sorry the above is a bit of a bodge as we say in the UK, but should get you going without machining and will make training the scope on targets more accurate. Of course Meade will wash their hands of you! but if you change for another it will have the same problems so you might as well try to fix it. Poor motion smoothness is a shame and unbearable. Bye the way the RA needs looking at but this is a bigger job, and I will report in due course.

Improving the RA turntable on the Classic ETX

My recent ETX purchase had vertical play and the turntable part of the lower fork could be raised visibly, and the friction seemed high. This was proved by looking at a far off brick wall, boy did it judder or stop/start. A Pentax on top just made it worse It was pointless taking it back, only to get another lot of trouble, so here goes on what to do.

Take the base off as if you were changing the cells. Borrow or buy a 1/4 inch hexagon socket screwdriver and unscrew the one and only screw in the centre. ( Some ETX's may have a slotted screw for an ordinary screwdriver.) Lift off the fork and table, taking care of the thin RA scale. Look at the 3 pads the turntable sits on, mine were certainly not PTFE, but a pale creamy color, a type of nylon or Acetol ( if your ETX has brilliant white pads, just make sure they are all there and snug up against the inner wall.) If you have PTFE pads, forget the following! One of my pads had scuffed away, the other 2 not very happy.

Your hardware shop probably sells a little box or wallet containing 4 very small screwdrivers (very handy if your spectacles fall apart) Pick one about one sixteenth wide. Rub the blade at the original angle on what we call an oilstone or knife sharpening stone, until you have a razor sharp cutting edge. Whilst you in the store get a small craft knife, it's rather like a scalpel. Having got the 2 tools, you now need some PTFE. This can be tricky to find, and if you really get stuck I can mail you some from England ( let me have your address). It's odd stuff, it's pure white, does not melt, can be cut easily with a blade or saw, nothing sticks to it (coated fry pans!) It planes well, and can be cold formed. The latter is useful, because if you have a chunk and want to make it thinner, just put it in between mechanics vice jaws, screw up the jaws until you have the right thickness. If you place some smooth metal on the rough vice jaws it will flow smoothly and have a good finish.

I found that some radio frequency plugs and sockets use PTFE (don't confuse with polythene which is waxy and semi clear and no good for us) Take one apart and squeeze the insulator to get a little flat slab which can then be cut into rectangles for the scope. I believe Fluon is also used for gaskets, trouble is we only need such a tiny amount.

Any way we need to cut up the Fluon into blocks about five eighths long by 3/32 square, it sure is a bit tiresome but you only 3, try to get a nice level oblong, it shaves off nicely like hard cheese, you can feel it's nice and slippery on your finger.

Now we are going to make 3 little trenches for 3 little Fluon coffins. Just hold the screwdriver vertical and at each end of the old pads, give a little blow with a small weight or hammer. Then with the knife run along against the wall and 3/32 inch out. You will need a bit of pressure to cut into the black ETX plastic - DO WATCH YOU FINGERS! Then holding the screwdriver low - scoop out into a shallow trench, it does not need to be very deep, say half way up the block. Scoop out from the left and right till you reach the vertical cuts you first made.

With a bit of luck you will make 3 blocks that sink half way into the trenches. Just get them all the same height, you don't need to be perfect because the turntable will allways settle on 3 points, whatever their height. The finish will be nice and smooth from the knife, you can burnish them with a steel blade if you want. Put the table back on (if you can find some VERY thin grease, then put a tiny smear on the central bearing and the rim of the turntable. Rotate it several times to bed it in --- notice how nice it now glides. Put the central screw back and adjust very carefully so the table rotates but does not have any vertical slack. Place the base back on, changing the cells if they have been in use a lot.

You will now have the smoothest rotating ETX, no stops and starts, even with a piggy back camera, and the RA knob will be better at high magnification.

I trust this may encourage you if you have a poor sticky turntable to "have a go" as we say in GB. The optics of the ETX are super, the big let down is the mechanics, in Taiwan they can make the basic parts, but the fit of screw threads, eyepieces, et al leaves a lot to be desired, the Meade tripod is the same, I had to machine the top surface flat to stop the ETX wobbling, and machine the North face to take a compass. A rough tripod allthough it looks fine at first glance. But it's reasonably free from the dreaded tremors!

Cheers, JIm.

(by the way FLUON is the same as TEFLON) Jordan Blessing of Scopetronix adds:

While I agree with most of what you say I think some of the things you
mention are related to each other....

> Put some very thin grease or petroleum jelly on
> both sides of the gear wheel.

JB: Putting grease on here is sure to lead to poor locking action and
eventually most likely to a failed right tube adapter from
overtightening. See 1st symptom below...

> Still talking about the driven side I had to tighten the locking knob so
> much it felt wrong, so I put a steel 3/4 inch washer with a small centre
> hole under the fake degree plate. 

JB:Putting a washer here can cause side to side (endplay) slop in the
OTA assembly. The "fake" dec circle acts as a thrust bearing to keep
this slop down (by riding on that outer ridge). By putting a washer
under it, well....

> This side bearing also had too much side play 

JB:Like you said, the REAL fix is to spend a day out in the machine shop
; ) Take a look at my ETX Tune-UP Page if you have any questions about
what I'm saying. You can link to it from the address below...
--
Jordan Blessing 

       ScopeTronix Astronomy Products
         http://www.scopetronix.com

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