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LXD75 Motor Unit Failure

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#1 Tomyj



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Posted 22 March 2011 - 09:39 AM

At least that is what the message on the Autostar hand-box display says.

This has been a problem since purchasing the mount some years ago.  At that time I was moving into a retirement community and disposed of most all my astronomy related equipment because of the limited space in a much smaller retirement community home.  After moving here I found the light pollution to be so terrific I could not see anything at night.  Some months after moving here I discovered Solar Astronomy and purchased a Coronado MaxScope 60 and accessories to use on the LXD75.

This has been a very interesting hobby!

Anyway, back to the LXD75 Mount purchased new to use on the pier on the back deck of this new home:

From the git-go the mount would not function so to enjoy solar astronomy I removed the motors and installed flexible shafts with knobs to adjust the E/Q mount.  This works for the sun, just a little tweak now and then to track it.

However with solar activity becoming more enjoyable to watch again as we move into these next several years, I'd like to try my hand at imaging as well as just looking.

So would anyone have a clue on what to do to resolve this "Motor Unit Failure" problem?

I don't think the mount is worth the $235 plus shipping (total would probably be close to $300)  Meade's customer support person told me it would cost for them to repair it.  There has got to be some more reasonable solution besides the flex cables installed to adjust its 'aiming' position.

So forum members, please let me know your thoughts on this dilemma, Tommy



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Posted 23 March 2011 - 12:23 PM

Hello Tommy,

Are both motor units bad or just one? Usually the FET's [motor drive transistors] will burn out if the motor has been stalled for a short period. The actual motor itself is usually ok. If you are handy with electronics they can be desoldered and replaced. Meade might be able to give you the part number and you would have to order them from a supplier. The part number should also be on the FET's themselves.

I would also check the cables to the motor controllers. Sometimes these go bad then there is no electrical connection to the motor boards and that will cause an error message.

Also you should be able to manually slew the motors [as a test] with the handbox unless both motor drivers are bad which is unusual. If one motor controller is bad it should not affect the other motor controller.


#3 Tomyj



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Posted 23 March 2011 - 03:41 PM

Good afternoon Brent,

Neither of the motors will cause the mount to slew manually with the Autostar or the original four function hand-box that came with the mount.  Nether motor nor hand boxes have made this thing work since purchased new.  I wish there was some way to check the cables other than comparing the ends to see if the same color wires were in the same place in the connector.

Believe me, Meade would not help me at all, I was told there was no other option but to send it to them along with $235 and shipping and they'd fix it.

I don't have a problem with replacing the FET, just have to find one.  Mouser has been a good source for odd stuff, I'll check there before I rip up the board.

There has to be a way to get something for my money, sure is disappointing to spend that cash and not get a working product.  I really wonder about the integrity of Meade.

Thanks, Tommy



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Posted 23 March 2011 - 04:41 PM

I hope you can get it repaired. Meade has the worst customer service in the business and I would never recommend any of their products for that reason.

I have had many occasions where I needed help or info on a particular issue and never received any satisfaction from Meade. They do have good optics however but most of their mounts including my experience with the mechanical side of the SN10 and LXD200 GPS have been disappointing.

I finally gave up on my LXD75 and bought an Atlas EQ-G which I have had for several years with no problems.

The FET's may or may not be the answer. You can test the motor driver by replacing the motor with a load resistor and seeing if there is any voltage there when you try to slew the motor. Also check for voltage at the FET input.


#5 meadeuser



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Posted 18 July 2015 - 10:33 PM

I'm and electrical engineer, and have been helping a friend with a malfunctioning LXD75 mount.


When I first turned on the drive, the RA motor took off running, and never stopped.  At this time, the motor output gear was not turning.  Turning off the mount, and disassembling the RA motor unit to remove the motor, then removing the two screws that retain the gearbox, the problem was obvious.  Ther were no teeth on a portion of the white nylon gear in the 3-stage reduction geartrain.  Moreover, the motor pinion gear was only mating across about half the width of the nylon gear, i.e., there were intact teeth above (outside) the pinion gear mesh area.


With a very fine tipped soldering iron, I was able to restore (melt) teeth in the stripped part of the gear.  I also pulled the pinion gear further out so it would completely mesh with the nylon gear.  The pinion is a press fit onto the motor shaft.  By heating it with a soldering iron, the press fit preload could be reduced enough to allow a screwdriver to be placed under the pinion and  it out.  The motor and gearbox now worked, although a little noisier.


The motor runaway was caused by blown and shorted FETs.  I ordered mechanically identical, but electrically more robust FETs from Mouser (mouser.com).  Their part numbers are: 78-SI4909DY-T1-GE3, SI4909DY-T1-GE3, 40V 8A DUAL PCH, and 595-CSD88539ND, CSD88539ND, 60-V, 11.7A, Dual N-Channel.  This pair of mosfets cost about $2.00 ea total in quantity 10.


Also the 74HC7048 quad nand gate blows out, which can be ordered from Mouser (595-SN74AC08DR).  This chip is a power buffer driving the gates of the lower fets.  I was also replaced.


I subsequently interchanged the RA and declination motors (they are identical).


Even with these fixes, and FET replacement.  FET failures occurred later.


The mount was being powered on an unregulated power supply labeled "12v, 3400 mA".  I put the specification in quotes because the output of this simple transformer/diode-bridge/capacitor power supply is 16.4 V with no load.  With 3.4 A load, it indeed outputs 12 Vdc.  This power supply can output over 25A into a short circuit.


The last time I replaced the FETs, I powered up the mount from my lab power supply, and not the normal unregulated power supply.  Normal operating current for the mount is about 100 mA, and it increases to 300 - 400 mA when the motors drive under load.


I had the lab power supply current-limit set at 1 ampere.  Everything was running fine, then the current spiked, the power supply went into current limit, the output voltage went very low, and the mount hand controller beeped and did a reboot.


An AHA! moment.  There is a gremlin in the motor controller microcode that causes all the FET's to turn on sometimes.  The FETs are driven from two microprocessor outputs.  There is no logic between the microprocessor and FET's to prevent FET simultaneous conduction.  With the unregulated power supply, large currents flow, the FETs dissipate large power, heat up, and die.  The mount no longer works and burnt phenolic smell is present (the FET plastic encapsulation).


The solution is to current limit the power to the mount.  A very simple method is to place a type 1057 automotive running light bulb in series with the power to the mount (12V/21W).  When the all-on FET anomaly occurs, the mount power is shorted out, and the power supply voltage appears across the light bulb.  The mount then resets due to the low voltage condition, reboots and is functional again.  No blown FETs.


The minor power variations from the series light bulb should be of no consequence.  The hand controller, and both motor controllers contain 5-volt 3-terminal regulators to provide regulated power to the microprocessors and other electronics.


Light bulbs are an ideal current limiter in this application.  Their cold-filament resistance is quite low and dies not interfere with normal mount operation.  The bulb resistance rises markedly when the filament heats up, and provides the needed current limiting function to protect the FETs.


Enjoy!  I hope this solves many mount problems.

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#6 MistrBadgr


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Posted 19 July 2015 - 01:22 PM

Thanks for the post!  That is good information!


Bill Steen

Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

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