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

On 3/2/2013 11:57 AM, wrote:
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
Hello, again.
That is a really old board. I remember playing with those things about
that time and recall the motor driver chips were really flakey. May I
suggest researching the motor driver chip and see if you can find any
info relating to problems with that particular chip.

Paul

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