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Crystal Oscillator replacement

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#1 sdedalus83

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Posted 09 January 2022 - 11:42 AM

I have a Carton CXD-1 controller and motor for a 180 tooth RA gear and I’m thinking about replacing the oscillator so it will work with 144 tooth mounts like my Mizar clone or a Polaris.

 

The controller has a 5.25Mhz oscillator as pictured. I believe that means I need a 4.2Mhz oscillator (180/144=5.25/X, X=5.25/1.25=4.2). I’ve found the right frequency and size but they have different capacitance and resistance ratings. Are those important? The picture shows the only printing on the oscillator and I can’t tell if it’s indicating any specs other than the frequency.

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#2 Michael Covington

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Posted 09 January 2022 - 01:00 PM

Are we talking about crystals or complete oscillators?  What I see in the picture is a crystal that you can unsolder and replace with another one.

The capacitance and resistance ratings probably make no difference.  Using the wrong load capacitance on a crystal changes the frequency a tiny bit.  You are not going to be able to get quite the right frequency in the first place.

Do calculate the exact frequency, though.  I get 4.2059.  Unfortunately, custom crystals are nowadays not generally available (when I started in ham radio in the 1980s, they were widely available and I ordered quite a few).

I'd use this: https://www.digikey....42-12-1X/827464

 

4.194304 is a widely available crystal (used in European color TVs, I think) and is the nearest I can find off the shelf.  It will make your drive run 0.27% slow.  That will introduce a drift of 2.5 arc-seconds per minute of time.   Much better than having no clock drive at all, but not perfect.  The trimmer capacitor will probably not allow you to pull it up to the correct frequency, but it's worth a try.


Edited by Michael Covington, 09 January 2022 - 01:03 PM.

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#3 Michael Covington

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Posted 09 January 2022 - 01:02 PM

There is a chance that bomarcrystal.com can supply a custom-made one.  But I'd recommend trying a cheap off-the-shelf one first, as proof of concept.



#4 sdedalus83

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Posted 09 January 2022 - 01:06 PM

Thank you. Just the crystal 



#5 Michael Covington

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Posted 09 January 2022 - 02:19 PM

I'd say, buy the one I recommended from Digi-Key for about 70 cents (maybe see if you need some other parts too!), solder it in, and see how close that gets you to what you want.  Seeking a custom crystal can come later.

 



#6 jurdea

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Posted 09 January 2022 - 06:04 PM

4.205 MHZ is available at Mouser.com for $1.09 plus shipping and in the same package.

 

However, it seems a shame to modify what is likely a rare controller/motor combination.  Have you considered buying a new motor and controller like the Orion EQ-1 and modifying it.  You might find an eager buyer for someone for your Carton CXD-1 controller and motor.  I modified the EQ-1 to drive my Polaris mount per the attached instructions and am very pleased with the result.

 

https://www.andico.o...e Mount Hacking

 

Also found a 5.185 MHZ crystal at Mouser to do that mod.



#7 DAVIDG

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Posted 09 January 2022 - 08:15 PM

 You to need go from basically 1/8 rpm  (180/1440)  to 1/10 rpm  ( 144/1440)  so you can use a 12  and  15 tooth gear to reduce the 1/8 to 1/10 rpm without any electronic modification. 

 

                    - Dave 




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