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Putting Humpty Dumpty back together ( repairing LX200GPS mount)

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#1 DAVIDG

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Posted 20 February 2022 - 01:43 PM

  My club was donated 2 LX200GPS scopes. One a 10" and one  8". Both had issues and we were able to repair both. In the process one of  my club members donated an LX200GPS mount that he bought for parts to keep his own LX200GPS running.  We only needed one gear from the mount so the rest  he gave to me since I wanted to see if I could get it  running. 

    All the electronics and the drive were out of the scopes and in a box. So the first job was to put it all back together to see what was working and what wasn't. Well almost every board had issues. 

   First project was to see if the main power board that holds the CPU and memory was alive. I had two boards in the pile of parts to try. You don't have to have them hooked up to  the rest of the boards to see if they will communicate over the RS232 serial ports. Luckily both boards were alive and I updated both of them using StarPatch to latest StarPatch version of the firmware. I highly recommend this over the Meade firmware and loader software. StarPatch is bug  free and offers many options and most off all fixes the GPS rollover problem.

     Next came the motor driver boards. It was easy to  see the problems with them.  The RA board had  one burned up MOSFET  and the DEC board two of them. Off to Ebay and I was able to find replacements for only a few dollars. These are surface mounted parts and easily removed with low melting solder and liquid flux. The solder stays liquid at  a lower temperature then standard solder . You flood the pins of the IC with the low melting solder and flux and get just pick the part off the board, then give the board a shake to remove what is left of the solder. Then clean up the flux with a little isopropyl alcohol .

    Here are a couple of pictures of the boards with the damaged parts and with them removed. It is easy to "drag solder " the new surface mount parts back in place so I had both boards with new parts installed and hoping that is all it would take to get the mount working. So I assembled everything and power up the mount. I hit the buttons on the Autostar II controller to slew the mount and display on the Autostar went blank, the RA motor moved the scope but very slowly . What  now was the problem ?  I hooked the scope up to my lab power supply to see how much current the scope was drawing.  Powered  it back up and when I tried to move the scope it was drawing a HUGE 4 amps !!!

    Were was that huge draw coming from ? I disconnected the motor and no more huge current draw. I attached the RA motor directly to the power supply and it was the RA motor. The DEC motor was doing the same.

     I took the DEC  motor apart and was treated to an  armature that looked like it was  placed on the barbecue !! It was black from being over heated!

 I can only guess that maybe the former owner tried to use the scope with the power supply that was much higher then the 12 volts ? I also understand that these motors do go bad because of  how the Pulse Width Modulation controls them. Who know but in any case I needed two motors.

  Some digging and I found out that they are 12volt at 12000 rpm with  a special armature designed to reducing clogging and custom made by Igarashi for Meade. Meade has an non disclosure agreement with them so   Igarashi won't sell them to you. Meade sells/sold the whole drive assembly but they want close to $400 for it. You can't put any old DC motor in place of these since the servo control algorithm is tuned for the spec's  of these motors  

      A hunt on Ebay and others  places turned up empty,  so what to do ?  You Tube had a couple of videos on how to rewind motors. It looked pretty easy so I measured the wire diameter on the armatures and ordered a spool off of Amazon for $10 which gave me enough to rewind maybe 50 motors. 

 I was able to carefully unwind the coils and count the turns. I cleaned up armature and then spent about 30 minutes carefully rewinding the five coils on the armature in the same pattern that I removed them. 

    I assembled the motor and it ran !  Better yet it was only drawing about 350ma . I attached it the 50:1 gear box  and  checked the RPM with an  optical Tach and was getting around 245 rpm at 12 volts and the spec's called for 240  at 12.0 volts but the tach isn't super accurate and I was reading the 12 volts off the analog gage on my power supply so I felt that the motor was within  spec.

      Here is a  picture of the  cooked armature and the rewound one. I'm now in the process of rewind the other motor. So more to come,  if I can get  Humpty Dumpty back together again. 

   I want to thank  OzAndrewJ for answering  my many PM's to him since he is the real expert on how  these things work. 

 

                        - Dave 

 

Damaged RA board close up.jpg

Dec Board Qs removed v3.jpg

meade armature.jpg

rewond armature v2.jpg

 


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#2 DAVIDG

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Posted 01 March 2022 - 10:01 AM

 I rewound the armature from the RA motor which was also fried.  The resistance of each coil measured  like the first one I did that ran at the correct RPM and current draw so I'm hopeful when I assemble this motor it will be back to 100%.  So more progress on putting Humpty Dumpty back together again.

 

                               - Dave 

 

armarture 2.jpg


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#3 MikeBY

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Posted 11 March 2022 - 10:28 AM

Wow..

Great work on those motors. 

Wiring on these scopes is particularly touchy because of the connectors. 

Hope you are able to complete it. 

M



#4 DAVIDG

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Posted 11 March 2022 - 11:18 AM

 The Autostar II controller had a dead display and I found one on Ebay that works. The ribbon cable is the same size as the original so it just plugs in. Not a perfect replacement but it brought this controller back to useable. 

   The RA assembly is all put  together and the scope slews at all speeds smoothly  and only draws around 350ma vs the 4 Amps it was with the damaged motors ! I'm working on assembling the DEC axis now.  The total cost in parts has been  about $20. 

 

                    - Dave 

 

Autostar LCD.jpg


Edited by DAVIDG, 11 March 2022 - 11:19 AM.

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#5 DAVIDG

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Posted 27 March 2022 - 08:45 PM

Humpty Dumpty lives !  It just replaced a damaged encoder wheel on the Dec/Alt axis and  the movement of the Dec/Alt on the hand controller  now matches the angle on the analog circles over an angle of movement of 90 degrees. It slews smoothly at all slew speeds. I did a  "fake" alignment in my house, the GPS receiver found the satellites, the time, date, Lat. and Long was correct and the position  the mount was pointing at for  the selected alignments star looks to by correct. 

   To recap here is what it took to get the mount back from the dead

    1) Replaced the burned out SI4947 MOSFET on the RA/AZ motor controller board

    2) Replaced the burned out SI4947, 4936A MOSFET and the 7408 AND GATE on the DEC/ALT motor control board

    3) Replaced damaged encoder wheel on the DEC/ALT motor

    4) Rewound  the armatures on both motors because they were fried from  being overheated. 

    5) Replace the LCD on the   Autostar II controller

    6) Update the firmware using StarPatch to the latest firmware to fixes many problems including the GPS rollover

     Total cost for parts was about $50.

 

       Here are pictures of the  Dec/Alt motor control board with  the replaced parts  outlined in red and a picture of the damaged encoder wheel on the left with the replacement on the right.

 

              - Dave 

 

dec board repaired.jpg

encoder wheels.jpg


Edited by DAVIDG, 28 March 2022 - 08:19 AM.

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#6 kg74

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Posted 30 March 2022 - 11:34 PM

Nice work. Love your spirit.  Keeping notes in the back of my mind in the event a dead one pops up. 

 

I did a similar repair on my hand controller, but built a shell for the screen. Pm me if you want a picture. 



#7 DAVIDG

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Posted 26 April 2022 - 09:01 AM

 One of the companies that repairs Meade scope  asked me if I would try to rewind two motor form a LX200GPS in which the armatures were over heated and the coils shorted since these motor are not available.

  Here is  a picture showing one that I rewound on the bottom and the damaged one on the top. You can see the difference in the color of the wire insulation that has been damaged by the motor over heating which causes the insulation to fail and the winding to short.

 

              - Dave 

Attached Thumbnails

  • LX200 motor armatures.jpg

Edited by DAVIDG, 26 April 2022 - 11:38 AM.

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