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ETX-90 UPGRADE

DIY equipment Meade observing star party tripod
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#1 LU1AR

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Posted 14 November 2020 - 05:05 PM

After restoring a second hand ETX-90 RA telescope (The model that only has Right Ascension tracking), I noticed that many modified the power regulator to stabilize its movement.
Also, like mine; many are spoiled by leaking batteries left behind inside.
So I decided to go for a unique Lithium element; It does not spoil by draining electrolyte and can be stored -for years- maintaining a charge.
As the Lithium element has to be charged to a very exact voltage regime and prevent its level from falling below a certain point; I opted for a 1 element charging pad that is used in robotics kits. It uses Micro USB input and gives protected output.
The battery output is between 3.2 and 4.7 Volts. So, for precise regulation I turned it up to 5 Volts using a small 5 volt upconverter source, which gives me a regulated voltage. There are lots of power supplies like this, for Arduino environment.
I make this voltage adjustable for the motor by means of a PWM regulator (By Pulse Width Modulation), which supplies the motor with a voltage of 5 Volts pulsed at 30 Khz, which are integrated into the motor to equal a voltage of 1, 9 volts; where the motor has proper Right Ascension tracking.
The interesting thing is that the motor always receives pulses of 5 volts, which is more than double its working voltage; but trimmed in such a way that it does not exceed the turns at which it must turn. However, this way of use, makes the motor have a high torque from the start, which is able to drag the telescope mechanism with greater force. This allows us to adjust the mechanisms with less play, without danger that the motor cannot move it. All of which makes the use of the telescope more precise and comfortable.
The complete system, drain less than 20 mA. and have a tracking error of les than 4 minutes a day, without any sort of precise adjustment.
I hope this will be useful for you.
Regards.
Edgardo

Attached Thumbnails

  • ETX 90 EM Motorizacion AR b.jpg
  • ETX 90 EM Motorizacion AR.jpg

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#2 LU1AR

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Posted 14 November 2020 - 05:08 PM

More updating for the ETX-90 RA.
Another detail that I want to emphasize is that I enlarged the holes where the side legs enter that allow the telescope to be used in Polar form on a table.
The legs go in loosely as they only thread one turn onto the plastic, rather than the anchors below.
It was enough to enlarge the holes in the lower cup, so that the legs screw three turns and abut against the metal.
The difference in stiffness is very noticeable and I recommend it.
Clear skies.

Edgardo

Attached Thumbnails

  • ETX 90 Patas Mesa 01.jpg
  • ETX 90 Patas Mesa 02.jpg
  • ETX 90 Patas Mesa 03.jpg


#3 sg6

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Posted 14 November 2020 - 05:27 PM

I could be utterly wrong but doesn't the ETX90 take 3 AA's so making 4.5v and then isn't there a 4.8v lithium battery?

I know I have few at 3.7v and thought there is at least one up in the 4.x voltage range. Actually thought I had one.

 

Just thinking you may not have needed to step the voltage up.

 

Maybe the higher rated one is 4.2v but half think possibly higher. Finding one could be the problem as 3.7v seems to have become the standard.

 

And I have only half looked in the rear of one ETX-90 RA once so could be wrong on 3AA's being the power source.



#4 LU1AR

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Posted 15 November 2020 - 08:43 AM

I could be utterly wrong but doesn't the ETX90 take 3 AA's so making 4.5v and then isn't there a 4.8v lithium battery?

I know I have few at 3.7v and thought there is at least one up in the 4.x voltage range. Actually thought I had one.

 

Just thinking you may not have needed to step the voltage up.

 

Maybe the higher rated one is 4.2v but half think possibly higher. Finding one could be the problem as 3.7v seems to have become the standard.

 

And I have only half looked in the rear of one ETX-90 RA once so could be wrong on 3AA's being the power source.

Dear Friend: This model use 4 AA cells, which were ruined inside. The motor needs 1.9 Volts, but lowering the voltage of a Lithium cell (which is simple) to 1.9 volts; drag is very poor. In addition, it is difficult for him to move on cold days and the tracking has an error of near 1 hour every 24.
By raising the voltage to 5 Volts, we can use a pulse width modulator, which gives 5 Volts pulsed in such a way that its average equals 1.9 Volts, but the torque is quadrupled.
Thus, the motor grinds very regularly (4 minutes apart every 24 hours) and has the strength to move the mechanism firmly even using heavy eyepieces or pigiback cameras.
Regards.
Edgardo


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#5 foxshark

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Posted 31 January 2021 - 12:51 AM

I know this post is a few months old now, but I just wanted to say that I liked the solution. I have been working on some H-bridge/Arduino options for my 90M that also had battery leak. Before that, I was looking at some no-programming solutions using something like a voltage regulation, power distribution, and brushed motor speed control (ESC) combo board for a mini quadcopter. This, however, is much more intuitive. How has it been holding up for you? I hear some of those PWM motor controllers can be short lived if you get a bad unit.


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#6 LU1AR

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Posted 04 March 2021 - 07:50 PM

Almost all ETX-90s; They have broken the support of that little "fart finder", which they bring from the factory. It is because the bracket touches against the left jamb, when the tube reaches the end of the path. Once it broke and fixed; it breaks again easier.
While deciding how to reposition the finder, it occurred to me to use a green laser as a designator, which works wonders.
Before facing an adjustment system, I found a green laser sight, which brings X and Y adjustments. It uses a rail support, which forced me to make an adapter where the finder used to go.
The sight is black anodized, waterproof and suitable for bad weather. It is adjusted in both axes, with a small Allen key (Provided). Looking at laser forums (Here), I see that it is the only one that does not crouch in low temperatures. Conventional lasers wrinkle when it's cold, but when they work they are a little more powerful.
The battery provided is Lithium, but NOT RECHARGEABLE. But it can be replaced by a rechargeable one, of the same format.
Tonight, I take advantage of the cloudy sky of My Dear Buenos Aires, to point the beam at the clouds and center it on the field of a short eyepiece. My ETX-90 is getting powerful.
PS: The 35mm film tube. It is the cap that covers the eyepiece holder; so that no dirt gets into the mirror. There is a small hole at the bottom and a bag of Silica Gel is kept inside, which removes the moisture from the tube. I had a lot of work; to get the fungus out of that OTA, which is of impressive quality.
Regards.
Edgardo

Attached Thumbnails

  • ETX 90 Laser 01.jpg
  • ETX 90 Laser 02.jpg


#7 LU1AR

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Posted 04 March 2021 - 08:00 PM

I know this post is a few months old now, but I just wanted to say that I liked the solution. I have been working on some H-bridge/Arduino options for my 90M that also had battery leak. Before that, I was looking at some no-programming solutions using something like a voltage regulation, power distribution, and brushed motor speed control (ESC) combo board for a mini quadcopter. This, however, is much more intuitive. How has it been holding up for you? I hear some of those PWM motor controllers can be short lived if you get a bad unit.

Since I installed the PWM control I have enjoyed my ETX90 telescope much more.
The PWM unit is sold for robotics and is used to regulate DC motors -with brushes- and lights. Parts cost less than $ 5 including the miny USB charge regulator and the DC/DC upconverter.
In the photos you can see two 18650 Lithium elements, because they are dummy copies of 600 mA each. I connected a pair in parallel.
Thanks for your comment.
Edgardo
 



#8 MikeBY

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Posted 10 March 2021 - 02:25 AM

Since I installed the PWM control I have enjoyed my ETX90 telescope much more.
The PWM unit is sold for robotics and is used to regulate DC motors -with brushes- and lights. Parts cost less than $ 5 including the miny USB charge regulator and the DC/DC upconverter.
In the photos you can see two 18650 Lithium elements, because they are dummy copies of 600 mA each. I connected a pair in parallel.
Thanks for your comment.
Edgardo
 

I  like your solution to improve motor performance. How did you adjust the PWM motor controller to provide the correct duty cycle? Did you simply measure DC voltage or some other method?  Do you have a source for this part?  Have you had any heat problem with the motor?
18650 Li-Ion cells are available with much higher capacities. A single 2500 mAh cell, much less a pair as you've configured your holder to carry would provide gobs of power. 


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#9 LU1AR

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Posted 13 March 2021 - 10:16 AM

I  like your solution to improve motor performance. How did you adjust the PWM motor controller to provide the correct duty cycle? Did you simply measure DC voltage or some other method?  Do you have a source for this part?  Have you had any heat problem with the motor?
18650 Li-Ion cells are available with much higher capacities. A single 2500 mAh cell, much less a pair as you've configured your holder to carry would provide gobs of power. 

I used two (Fake) elements - in parallel - because I had them on hand. With the consumption of the ETX-90 they are enough to use it for 30 hours. There is no problem in using a single cell of good quality (AND capacity).
Regarding the adjustment, I used the VOM and adjusted the PWM to obtain a voltage in the motor of 1.9 Volts. as provided by the linear regulator.
The potentiometer on the PWM module allows for a very wide excursion, so I added a resistor in parallel to reduce its travel and be able to use a half turn, to get the expanded setting. It is not necessary for the visual use that I give it; but as soon as I calibrated it, it was left with an error of 4 minutes in 24 hours. So I didn't touch it.
I found the value of resistance experimentally. I don't remember it, but it is something very basic.
Regards.
Edgardo
 




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