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Losmandy GM-8 won't move

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#1 NevadaTom

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Posted 09 April 2011 - 07:02 AM

I bought a used GM-8 from a seller here on Cloudy Nights. Said to be in pristine condition, it certainly LOOKS that way. Unfortunately neither one of the motors move in either direction at any speed. The lights on the controller and the handbox do light as expected, but there is not even a hint of any sound or vibration from either motor.

The seller said it has always been kept indoors. I don't doubt that one bit. Under the covers the motors themselves look fine as does the inside of the controller box. There is NO evidence of moisture, corrosion, or insect invasion.

I've switched the cables around and tried different power sources. None of that makes any difference.

Ideas anyone?

#2 skybsd

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Posted 09 April 2011 - 07:42 AM

Hello,
Is this a GoTo GM8 or the model with the Losmandy Model 492 stepper motor drive and control system?

If its the stepper motor model, then your mount probably IS working as expected as the motors will only move at the speed needed to track an object at any of the four quartz rates - that is.., very slowly to the eye.

Turn the mount on, note the positions of the DEC / RA axes and leave the mount alone for about 30 mins.., if the mount is working, you would notice that the RA axis is in a different position.

Here is a nice write up of the GM8 that you may wish to review during that 30 mins..,

Hope that helps..,

Regards,

skybsd

#3 Eddgie

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Posted 09 April 2011 - 08:26 AM

Skybsd is right. Even at MAX speed, these motors turn VERY slowly.

These mounts only use motors for fine positioning and aren't meant to MOVE the telescope to different coordinates like a Go-To mount. If you need to move several degrees across the sky, you push it.

So, even flat out, the movement is too slow to be easily visible.

I fear that you may have had a misunderstanding about how these motors are used in a standard GEM mount.

The good news is that the Push To in the GM8 is far faster than using motors to slew, dead quiet, and uses no additional electricity.

A GM8 will run all night off of a very small battery pack.

Embrace all that is good of the design.

#4 Don Allen

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Posted 09 April 2011 - 08:49 AM

I did not know this about the GM-8. I do hope the OP is okay with this type of set up. It probably takes some getting used to.

#5 jmiele

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Posted 09 April 2011 - 09:04 AM

You can test it during the day. Just point it at a lamp post or house in the distance. Focus up and turn on. It will slowly move out of the EP. If it does, you are tracking is siderial. If it has a hand control, you can use it to (slowly) move the object around the field of view. If nothing works, check the battery. As I recal it was in the base on the bottom of the fork mount base. Some also had a 12vdc plug and ac adapter.
Hope this helps.. Joe

#6 NevadaTom

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Posted 09 April 2011 - 10:34 AM

It is the model 492.

Further investigation this morning discovered this:

First I removed the PC board from the controller and inspected it very closely and carefully with a high power magnifier looking for any cracks or evidence of overheating. There was nothing like that but the connectors for external 12 volt DC had excessive flux on the bottom side of the board where they were soldered in. Not a problem, just an observation.

Next (since I don't have an oscilloscope) I used a digital multimeter to examine the pulsed outputs of the integrated circuit that drives the motors. R.A and DEC were showing different peaks. Of course DEC was doing nothing unless the appropriate button on the handbox was pressed - 5 volt peaks in that case.

R.A. on the other hand was peaking around 12 volts all the time. That makes sense since it should be tracking constantly but the difference in voltage concerns me.

Then I plugged the motors back in. With the room very quiet and my ear touching the motor, the DEC motor was pulsing. Looking at the setting circle I could see that it was actually moving very slowly - and it was reversible. So DEC seems to be OK after all.

R.A. is a different matter. Although I can hear the motor sound, there is no movement on the setting circle. Remember it was the R.A. output from the chip that had a higher voltage. It is possible that current is too low to actually turn the motor shaft even though the pulse voltage is higher. Fortunately there is an easy way to check that theory - switch the cables and drive the R.A. motor with the DEC output. Did that. The motor make sound but the arm still doesn't move.

Now what?

#7 skybsd

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Posted 09 April 2011 - 11:14 AM

Hello,
Did you try the test I suggested?

Regards,

skybsd

#8 Jeff B

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Posted 09 April 2011 - 12:20 PM

For the RA, there should be just a bit of play/backlash with the clutches fully engaged. If it's hard to tell, disconnect the motor and see if the worm will turn easily with just finger tip pressure. If it's tight, the motor may be stalling and the worm gear needs to be backed off a bit. If it's easy to turn, the motor may be shot, and they do wear out, However replacments can be had easily from Losmandy

Jeff

#9 NevadaTom

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Posted 09 April 2011 - 02:37 PM

Yes, I let it run for over an hour and it didn't move at all.

#10 rmollise

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Posted 09 April 2011 - 02:43 PM

Yes, I let it run for over an hour and it didn't move at all.


It will move like the hands of a clock. Focus on a terrestrial object, and use the HC to slew the scope. Does the image move?

#11 jmiele

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Posted 09 April 2011 - 03:27 PM

Were the clutches tightened? Make sure they are locked. You will feel resistance as you tighten them. The telescope won't move if they are lose and the scope swings freely. Joe

#12 NevadaTom

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Posted 09 April 2011 - 04:03 PM

Quote
For the RA, there should be just a bit of play/backlash with the clutches fully engaged. If it's hard to tell, disconnect the motor and see if the worm will turn easily with just finger tip pressure. If it's tight, the motor may be stalling and the worm gear needs to be backed off a bit. If it's easy to turn, the motor may be shot, and they do wear out, However replacments can be had easily from Losmandy

UNquote

I'm not entirely sure how you do this. I released the spring clip that holds the motor to the plastic gearbox. The little gear attached to the motor shaft turns. It seems fairly strong but I CAN stop it with my fingers.

The little plastic gear visible inside the plastic gearbox turns freely with a small probe. I presume that is not the gear you are talking about however. You probably mean I should try to turn the worm directly with my fingers. What do I disassemble to get my fingers on the worm, and how do I back off tension on the worm if that is what it takes?

Is there a parts/assembly diagram somewhere online?

Tom

#13 skybsd

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Posted 09 April 2011 - 04:38 PM

Is there a parts/assembly diagram somewhere online?


A
B
C

Regards,

skybsd

#14 NevadaTom

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Posted 09 April 2011 - 07:41 PM

Problem solved!

Thanks to all of you for your assistance. Your suggestions and encouragement pointed me in the right direction. After swapping motors and realizing that both of them were working properly when driven by either the RA or DEC output of the controller, I finally noticed that the RA setting circle was moving exactly with the arm. This made it appear that RA was stuck. When I stepped back to observe the whole mount from a distance over a period of time it was obviously working properly after all.

I don't yet have a scope mounted on it so I was not able to look through an eyepiece to see that it was in fact moving. Ah well, it was good for me to learn the ins and outs of the mechanical details.

#15 jmiele

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Posted 09 April 2011 - 10:21 PM

The RA moves because objects don't change there coordinates. LST Local Sidereal Time or RA directly over your head changes. So, sounds like you are all set. Go look at stuff :)
Joe

#16 skybsd

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Posted 10 April 2011 - 04:26 AM

Problem solved!

Thanks to all of you for your assistance. Your suggestions and encouragement pointed me in the right direction. After swapping motors and realizing that both of them were working properly when driven by either the RA or DEC output of the controller, I finally noticed that the RA setting circle was moving exactly with the arm. This made it appear that RA was stuck. When I stepped back to observe the whole mount from a distance over a period of time it was obviously working properly after all.

I don't yet have a scope mounted on it so I was not able to look through an eyepiece to see that it was in fact moving. Ah well, it was good for me to learn the ins and outs of the mechanical details.


Glad to hear its all working as your seller suggested, and that you're happy and satisfied with your mount there :waytogo:

Now then.., where's that scope you need to put on the thing ;)

Best wishes!

Regards,

skybsd






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