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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Programmable stepper control help please
I posted the message below in SEB and then though that maybe it should
have been posted here too. So here it is, slightly modified: I recently acquired a programmable stepper motor controller. It was made by Richmill Manufacturing Company Ltd. around 1982. It is designed to drive a unipolar stepper motor. The guy I got it from said it worked fine when he put it away and then upon trying it about a year later it only works intermittently. I did discover one wire in the cable that goes to the motor does not have continuity. This wire is one of the two common wires that are the center taps of the motor windings. I do not have the motor. I do not have a schematic and so far have not been able to find one. I have not applied power to it yet. It has a 3.6 volt ni-cad battery that is used for memory backup and it needs to be replaced. I need advice about replacing electrolytics. Though none are bulging they are old and have not had power running through them for a long time. Should I just replace them as a preventative measure? I have heard about "re-forming" electrolytics. Should I even bother trying this? There are only about 10 of them so they would be cheap to replace. Another question is would it be safe to try to power up and run the control without connecting a stepper motor? Looking at the board I see that 4 MOSFETs are used to switch rectified 50 volts AC to the motor coils. I thought about just trying to turn the thing on and trying to program it and see if it at least thinks it is doing something. Since I don't have any large steppers I thought about connecting a 12 VDC 500 mA power supply in lieu of the rectified 50 VAC to the circuit board and using a small unipolar stepper to test the controller. Finally, what I would like to do is connect the controller to a circuit that would convert the unipolar switching signals to step and direction so that I can use a Geckodrive servo amp and a servo motor. This way I can run whatever size motor I want connected to an indexer that I am building. Thanks, Eric P.S. Since posting the above message I got to wondering how one goes about testing whether an electrolytic capacitor is good or not. I have an Extech DVM that has a capacitor measuring function. Is that a feature that might help? If the measured capacitance is within 25% can I call the cap good? Thanks again, Eric |
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Programmable stepper control help please
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#3
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Programmable stepper control help please
wrote in message news I posted the message below in SEB and then though that maybe it should have been posted here too. So here it is, slightly modified: I recently acquired a programmable stepper motor controller. It was made by Richmill Manufacturing Company Ltd. around 1982. It is designed to drive a unipolar stepper motor. The guy I got it from said it worked fine when he put it away and then upon trying it about a year later it only works intermittently. I did discover one wire in the cable that goes to the motor does not have continuity. This wire is one of the two common wires that are the center taps of the motor windings. I do not have the motor. I do not have a schematic and so far have not been able to find one. I have not applied power to it yet. It has a 3.6 volt ni-cad battery that is used for memory backup and it needs to be replaced. I need advice about replacing electrolytics. Though none are bulging they are old and have not had power running through them for a long time. Should I just replace them as a preventative measure? I have heard about "re-forming" electrolytics. Should I even bother trying this? There are only about 10 of them so they would be cheap to replace. Another question is would it be safe to try to power up and run the control without connecting a stepper motor? Looking at the board I see that 4 MOSFETs are used to switch rectified 50 volts AC to the motor coils. I thought about just trying to turn the thing on and trying to program it and see if it at least thinks it is doing something. Since I don't have any large steppers I thought about connecting a 12 VDC 500 mA power supply in lieu of the rectified 50 VAC to the circuit board and using a small unipolar stepper to test the controller. Finally, what I would like to do is connect the controller to a circuit that would convert the unipolar switching signals to step and direction so that I can use a Geckodrive servo amp and a servo motor. This way I can run whatever size motor I want connected to an indexer that I am building. Thanks, Eric P.S. Since posting the above message I got to wondering how one goes about testing whether an electrolytic capacitor is good or not. I have an Extech DVM that has a capacitor measuring function. Is that a feature that might help? If the measured capacitance is within 25% can I call the cap good? Thanks again, Eric No. You can only properly test electros with a combination of an ESR meter and a capacitance meter. By far, the quickest and most reliable test is with an ESR meter. Mine is used virtually every day in the workshop, and over the years, has paid for itself many many many times over. On lots of occasions, I have seen caps that test fine for value, but having an ESR that has 'gone out of the window'. It does happen the other way around as well, but much more rarely. Many people - including some on here - will recommend that you just go ahead and change all of the caps. If you are experienced, and you are comfortable doing that work to a proper standard, then it's not going to do any harm. Electros have moved on in terms of both size and performance in the intervening 30 years. However, as any on here who know me will attest, I am not a great advocate of just replacing components willy-nilly. In a long career of electronic service work, I have seen far too many examples of equipment that has been wrecked by DIY repairs carried out by people who have a poker for a soldering iron. I have to admit that you don't sound like such a person, but you need to be confident in your abilities and have the right equipment - particularly if the board is double sided. One of the problems if you go ahead and just change the caps without first testing the board, is if it doesn't work after the replacement caps are in, how do you know if it worked before, and it now has a fault that you have inadvertently caused ? I think that if I were you, I would go ahead and supply the circuit with a reduced voltage, as you suggested, and let it just sit there for a while. Reforming of electros is not really an issue with ones working at a few tens of volts such as these. Even so, it's probably not a good idea to hit them with the full supply right away. Once that you are happy that nothing is letting go its magic smoke, it would not be a problem to just load the four drive channels with a resistor each, and hang a 'scope across each in turn. You should be able to see the rotating drive signals for the motor. If they are there, and all look the same, then there's a good chance that it is working ok. You can then make the decision whether or not to replace the caps, based on the knowledge that you at least know that it is working before you start ... Arfa |
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