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Meade ETX-90 M replace motor/controller options?

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#1 foxshark

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Posted 27 October 2020 - 10:03 PM

I picked up a Meade ETX-90 M in beautiful condition... until I got it home and opened the battery compartment; leaked battery mess everywhere. I cleaned it as well as I could with various ways, still no luck, and then flipped over the circuit board and saw that it was corroded beyond repair.

 

My background is more in software than hardware, so of course I'm thinking replacing the innards with an Arduino and a stepper motor. However, it does not seem to be all that complicated - is it possible to re-make the hardware as a hobbyist these days? Is it simply a voltage regulator that applies the correct voltage to the DC brushed motor? It would be ideal to keep the motor and reduction gearbox for simplicity and keeping to the original fitment.



#2 scottsdalejohn

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Posted 27 October 2020 - 11:24 PM

take a look at OnStep ... Arduino based and all (well most) of the work is already done for you



#3 GreyDay

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Posted 28 October 2020 - 04:33 AM

No end of ETX info here http://www.weasner.c.../techtips.html



#4 John Higbee

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Posted 28 October 2020 - 05:35 AM

Hate to say this, but you may be better off deforking the tube and putting it on a good German equatorial mount.  The ETX-90 mount and its internal drive works (cheaply made and plastic throughout) are as bad as the OTA optics are superb.

 

That manufacturing decision also hobbled the ETX-105 and 125 lines...the only attempt to mitigate this problem (very late in the game) was to upgrade the mount forks to aluminum in the 125PE model; even there, the cheap plastic drive components remained.

 

How Meade could make a mistake of that magnitude has always mystified me.  Just making the existing mount and drive components out of metal would have hugely upgraded the whole product, and the resultant increased ETX price would still have clearly beaten the Questar price.  

 

Good example of how being "penny wise" in producing a product can lead to "pound foolish" results, and customer frustration/disappointment.

 

John


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#5 chthonic65

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Posted 28 October 2020 - 01:47 PM

The short answer is yes, the controller is indeed a simple voltage regulator.  I built one for my own model M without too much trouble.  My own ETX-90M moves at around sidereal rate with 1.71V to the motor.  I'd also claim that a stepper motor would be overkill.  Of course, overkill is sometimes the point.

 

The long answer is all about if it's worth doing.

 

The RA drives on the original ETX-90 were very cheaply made, and out of the box they would typically not track well until you made certain modifications to them.  Weasner's site mentioned above has all the details, pictures, circuit diagrams, etc, on those mods.  You may or may not want to go to all that trouble.  I did, and I got the drive to work pretty well.  The two big problems are:

  1. When you clutch the drive in, there's a LONG delay before the RA axis starts to move.  As in, much more than one field-of-view's worth.
  2. Once you get movement, there can be an intermittent slop, like a pause-and-jerk movement on about a 10 second cycle.  Not everyone gets this problem, but I did.

Both of these issues come from the same cause - there's a big clearance between the main drive gear and its bearing surface.  Again, if you're interested, Weasner's site has the details, but in short, there are two common fixes (I had to do both):

  1. Build a voltage regulator that puts out either 1.7V for tracking, or a higher voltage that speeds up the motor to take up the initial slack.  My own regulator gives either 1.7V or 5.0V.  That solves problem 1.
  2. Shim up and grease that bearing surface.  That solves problem 2, and also helps with problem 1.

After all that, my ETX tracks quite well.  Doing the mods was worth it to me, at least "in the hobbyist sense", in that I got to tinker around, and learn about voltage regulators. 

 

But deforking it would have been FAR easier.

 

- Charlie



#6 foxshark

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Posted 28 October 2020 - 02:32 PM

take a look at OnStep ... Arduino based and all (well most) of the work is already done for you

Yup! I have been following that for a while, and already have the parts for a different mount/project.

 

I was reading up on the metal OTA base replacement for mounting a proper 90ΒΊ diagonal out the back - I should have read further. Thanks!

 

Hate to say this, but you may be better off deforking the tube and putting it on a good German equatorial mount.  The ETX-90 mount and its internal drive works (cheaply made and plastic throughout) are as bad as the OTA optics are superb.

Yeah.. that was my backup plan - I have two Celestron GEM's and tripods, but was drawn by the allure of a grab-n-go / camping setup; something that could fit into a single reasonably sized Pelican case. Yes, I am aware that Pelican and others have a myriad of ways to exchange your money for their storage solutions - even unreasonably sizes ones! lol.gif

 

The short answer is yes, the controller is indeed a simple voltage regulator.  I built one for my own model M without too much trouble.  My own ETX-90M moves at around sidereal rate with 1.71V to the motor.  I'd also claim that a stepper motor would be overkill.  Of course, overkill is sometimes the point.

 

The long answer is all about if it's worth doing.

Fantastic news, this the heart of the matter to me - in both time and money, can it be reasonably restored to a condition where it reasonably tracks things well. I noticed the slop just in hand-advancing the RA knob after unlocking and rough positioning, so my expectations are ... shall we say tempered. I am more thinking of the ability to find a planet, watch it for a few minutes, then get up and tell a friend/family member to come and check it out without it falling totally out of frame. Beyond this, I tend to think that some sort of proper GEM makes much more sense. Of course sunk cost fallacy will set in and say "if you are lugging out a big heavy GEM... why limit yourself to a 90mm scope?"

 

Thank you all for the advice!



#7 GreyDay

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Posted 28 October 2020 - 04:25 PM

The ETX-90 mount and its internal drive works (cheaply made and plastic throughout) are as bad as the OTA optics are superb.

 

That manufacturing decision also hobbled the ETX-105 and 125 lines...the only attempt to mitigate this problem (very late in the game) was to upgrade the mount forks to aluminum in the 125PE model; even there, the cheap plastic drive components remained.

 

How Meade could make a mistake of that magnitude has always mystified me.  Just making the existing mount and drive components out of metal would have hugely upgraded the whole product, and the resultant increased ETX price would still have clearly beaten the Questar price. 

I bought a 125EC with nonfunctioning AZ drive for a very reasonable price, or at least i thought so! I have a friend who's an electrical engineer who designs CNC systems and had agreed to take a look before i bought the scope. I'll spare the details but his opinion was that the mount had been "designed from the outside in" ie form over function using the cheapest manufacturing processes and parts possible. There were problems with the drive motor not running true and worn plastic gears (replaced), Issues on the drive mainboard and hand controller (fixed) and nearly all moving parts needed shimming. 

 

When it was "fixed"  it still lagged and to top it all the OTA which was untouched was out of collimation. After a lot of messing around getting it collimated i sold it a week later and broke even! i was advised by my engineer friend not to buy another one. luckily i only bought replacement parts and the work cost a case of beer but i've been put off the ETX line from that experience.

 

I know a lot of people are happy with their etx's and have trouble free experiences, for me the views were good and the scope looks great but the drive and focuser slop were taking the fun out of it. Since then I borrowed a 90M for a week and had only minor backlash problems with the smaller scope. Maybe the 125 was undermounted or under powered? guess i'll never know.
 


Edited by GreyDay, 28 October 2020 - 04:56 PM.


#8 jgraham

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Posted 28 October 2020 - 07:19 PM

Hopefully you'll be able to fix your ETX-90M, but in the mean time you might try just using it without the motor in altaz mode. One of the attractions of the 90M for me is its simplicity and mine spends nearly all of its time unpowered, configured as a fully manual altaz scope. The addition of a real 30mm RACI finder makes it very comfortable to use as a star-hopper. However, it is nice to switch over to the equatorial mode with the motor drive when viewing the planets at high magnification. The #884 field tripod with its tilt plate makes it easy to switch back'n forth. While you are working on a repair, adding an easily accessible power switch would be a nice addition to make it easy to turn the motor drive on and off.

 

My baby...

 

ETX-90m (5-18-2019)-2.jpg

 




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