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CCD/guide control of LX50 mount produces wacky response

mount Meade
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#1 retroformat

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Posted 21 January 2021 - 11:36 PM

I recently got a Nelson Devices Slo-Mo Genie to help with pointing the Meade LX50 10" SCT I have.  The Genie is a little joystick affair which plugs into the guide/CCD port on the mount.  I don't have a paddle for the mount, so this is a workaround of sorts to getting more comfortable control at high power.  Since the Genie works only with the CCD port, the motions of the scope are (supposed to be) the slowest available - I'd love to find a way to speed those up!  (but not in the way described below)

 

The Genie produces some odd results, which I'm going to guess point at a fault in the mount electronics - but before I open the mount up, I'd like to hear if others have had similar experience.  In DEC, the Genie can make the drive go only one way.  Debugging with a meter, I find that the voltage coming out of the mount and going to the DEC motor comes on only in the one direction (I get 3V for DEC motor operation in the one direction).  The voltage is supposed to reverse for the other direction (I am told), but I get nothing. 

 

In R.A.  I get the "correct" behavior when controlling motion to the East, but not always.  Holding the joystick to go East, the R.A. drive simply stops, and objects drift to the West at the sidereal rate, naturally.  Holding the joystick to go West, I get the correct 2x sidereal rate on the motor, producing an apparent 1x sidereal rate in the field of view, everything drifting to the East.  The odd behavior in R.A. is that every now and then, when I've gone East for a longer while using the joystick, when I let go (i.e. I want the drift to stop, the R.A. drive to come back on) the R.A. drive doesn't just turn back on at 1x sidereal rate - it turns on at something like 8x sidereal rate for a second or two, which puts me back to the West where I started.  This is intermittent, but seems more likely to occur when I've held the joystick East for a longer time, that is, when the R.A. drive has been signaled to stop for a while.  Then it seems to want to make up for lost time, and slew me back to the West!

 

Would anyone have guidance for me?  What should I look for when I open this thing up?  I have a multimeter and a transistor/cap/mosfet tester.

 

Many thanks for any tips!



#2 Geo.

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Posted 23 January 2021 - 11:57 AM

The LX50 ST-4 port looks pretty standard except that the pin assignments are reversed from what you usually see. This is the standard pin numbering for a RJ-12 6P/6C modular jack.

 

RJ-12 P_O.jpg

 

And here's the LX50's pin outs.

 

LX50 AGPort.jpg

 

I won't into the origins of the ST-4 here, but if you want the back story read on: https://www.cloudyni...re-classic-too/

 

Short story, your j-stick was looking for:

 

Pin 1 - NC

Pin 2 - Gnd

Pin 3 - -RA

Pin 4 - -Dec

Pin 5 - +Dec

Pin 6 - +RA

 

Where:

N = DEC+
S = DEC-
W = RA-
E = RA+

 

What it finds in the LX50 is:

 

Pin 1 - -Dec (?)

Pin 2 - -RA

Pin 3 - +RA(?)

Pin 4 - +Dec(?)

Pin 5 - Gnd

Pin 6 - Gnd

 

Pretty sure pin 2 is -RA guessing on the others. RA- or east is usually accomplished not by reversing tracking, but by stopping it and letting the earth's revolution do the correcting. Thus E and W are Stop and Speed Up RA. So E needs no connection to the ┬Ácontroller.

 

How to deal with this? Get a female to female RJ12 connector and make up a short cable for the output to the LX50.

 

RJ cable.jpg

 

The output cable should be a cross over type.

 

 

 

 



#3 retroformat

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Posted 23 January 2021 - 05:22 PM

George,

thank you very much for these diagrams (also for the ones you sent to my other thread about LX50 power supply voltage).

I took the Slo-Mo Genie apart, and observe that for the joystick pushed in each of the cardinal directions, it connects (closes a circuit) from pin 2 (ground) to either pin 3, 4, 5, or 6.  Which I believe it is supposed to do.  It does this with momentary micro-switches, and my metering of the device showed correct function.  Unless the fault is in the cable from the Slo-Mo Genie to the socket on the LX50 mount (which seems unlikely), then the fault must be inside the mount.  Some little component in there is acting up or fried.

I must study the main board diagram you sent, and I guess I can go in there and test whatever components are accessible, except for the ICs.

Boris




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