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Repair of stepper motor for color wheel feasible?


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Just wondering if someone can tell me if there's any point in trying to repair a stepper motor for a color wheel in my Martin RoboColor (original version) that appears to be failing. It's a KH42HM2-017 which I believe has been used in some other Martin products as well.


The problem is that it is no longer moving over the entire range, it will only move so far. I've tried to 'excercise' it a bit with a chase going from 0-255 and back but no change so far. It doesn't seem to be stuck or anything, it just won't move outside that limited range but instead 'bounces back' to the beginning of that range. I can fairly easily move it outside that range by hand, even with the power on, and it will then stay pretty much in that new range.


I have no idea what its innards look like so I don't know what that says about the type of failure or if it is even caused by the stepper motor itself. Would opening and cleaning it possibly do any good?


I may have found a source for a second hand specimen for just 5 Euro so if I can still get that I'm not going to muck around with this one too much, unless cleaning it is likely to help.


I'm an EE btw but my practical experience with stepper motors is virtually non-existent.


Thanks in advance for any suggestions.

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Repairing the motor itself - I wouldn't bother trying, just replace it.


A brand new one isn't all that expensive. One of the cheapest suppliers of new ones is Rapid Electronics.

To find a suitable replacement, measure the motor body and find one a similar size with 1.8o step angle, 14v, with the same number of wires on (6 I think).

The specs of the KH42HM2-017 are:

D = 56 x L = 38,

14VDC @ 0.18A

Phase resistance = 78 Ohms

You could use this 12v motor, as the phase resistance is only 3 Ohms difference, so it really won't make any difference.

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Replacement is always the most reliable option. If you are stuck without a spare, you can loosen the four screws and twist the front and back faces of the motor to see if you can realign the motor casing so the rotor spins freely. This should work as there is some distortion to stepper motors with transit bumps, I have done this to some early stepper motors.


here is an earlier thread on stepper motors

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Well, I put the stepper motor in another fixture and it worked perfectly. Then I put it back in the original malfunctioning fixture and it seemed to work fine, at least for a while. Then somehow it got misaligned but after a recalibration (can be done using the DMX dimmer channel) it was back in line with the rest and is working perfectly. So at this point I'm not sure if it was the stepper motor after all (and to be honest the way it moved did suggest that the motor itself was fine and that the problem was with the control).


Maybe the connector had become corroded or something and the simple act of disconnecting and re-connecting it cleaned it up enough to allow proper signals to be sent to it again? I'm not sure if that could explain the kind of movement I saw though, it would move perhaps 20 degrees and then return to its original position and then would start rotating 20 degrees again. Actually it looked exactly as if the controller was simply ignoring one or more higher order bits but I really don't know enough about the design of this fixture to know if that is likely or not.


I guess for know I'll just keep my fingers crossed and hope that it will continue to work. Thanks to all for your suggestions!

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I have part used fully working Martin stepper motors in stock. Please contact me if I can be of any assistance.


Kind regards,





Dennis Willson

Service and Maintenance Engineer

Live Lighting


Tel: 0117 967 2000

Mob: 07813 203 582

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