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  1. #1
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    Default Power feed buy new or repair?

    Hi all,

    My bridgeport clone came with an x axis power feed that does not work.

    It's a Servo motors type 90 power feed made in the US. It has a cooked circuit board with some cooked rectifier and SCR's.

    Some time ago I problem solved and sourced the replacement discrete components and had a go at a repair but I think someone prior to me has also done a similar thing. I've come to the conclusion that the entire board needs replacing TBH.

    In short I can get a new replacement board from the US. The board seems to be almost a plug and play setup with the plugs etc.

    Question is, at $US230 for the board, is it worth it?

    I see that servo power feeds are very expensive, even S/H old ones like mine. Even sold for parts they are several hundy on occasions. They are also fully rebuildable Even though mine is over 40 years old.

    I love rebuilding stuff and saving stuff from needlessly going into scrap or landfill and apparently these are a very nice power feed unit.

    Should I take a chance or bin it and buy a Chinese POS and be done with it.

    Also, from what I can tell, everything else is in good order. The field windings and armature are good.

    It's totally stripped and cleaned up and been sitting in a box for about a year while I procrastinated over this decision

    Simon




    Sent from my SM-G970F using Tapatalk
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  2. #2
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    Default

    Some pics for anyone interested...

    Sent from my SM-G970F using Tapatalk
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  3. #3
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    Sep 2012
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    Default

    Hi Simon,

    I would stick with it ! Its a simple thyristor motor speed control circuit. Even if you replaced every electronic component you would only be spending a few dollars. I've a circuit diagram kicking about somewhere for a speed control circuit that will provide you with a basic circuit to work with.

    motor-speed-regulator-schematic.gif
    Here you go !
    Best Regards:
    Baron J.

  4. #4
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    Default

    Thanks Baron.

    Couple of things to take into consideration. The motor needs to be able to run in both directions and also the engagement handle has a cam on it which engages one of two micro switches that are mounted on the circuit board which in turn set the current path for the direction.

    If I were to design and build a new circuit similar t o the one you posted then I would need to include these in the correct t place. Not impossible but a bit of mucking around.

    As I mentioned above, a new circuit board is US$230 whic h is a fair amount. I could probably design and build on e for under $100 which would take e some mucking around.

    Stuff it. Maybe I should.

    One thing I forgot to mention,I actually have the schematic of the original circuit. A lovely gentleman from a repair company in the US who quoted me the price for the board also attached it for me in an email to help me repair the original. He was quite passionate about the build quality of these little power feeds.

    Simon

    Sent from my SM-G970F using Tapatalk
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  5. #5
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    Default

    Hi Simon,

    It would help to have a look at the schematic of the original circuit.

    Assuming that the board is in good shape it would be easy to just replace the bad components. The only other thing that I would advise is to check that you can either get or make new brushes for the motor. From the picture you posted the commutator on the armature looks to be OK. No loose segments or any suspicious burning, no nicks in the windings etc.

    If I were to design and build a new circuit similar to the one you posted then I would need to include these in the correct place. Not impossible but a bit of mucking around.
    There are basically only two connections to worry about, and those are the motor. It may look complicated because they might have brought the field and brush gear connections out separately.
    Best Regards:
    Baron J.

  6. #6
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    Default

    Quote Originally Posted by BaronJ View Post
    Hi Simon,

    There are basically only two connections to worry about, and those are the motor. It may look complicated because they might have brought the field and brush gear connections out separately.
    I would anticipate that the field and brush gear connections would be separate at the motor as the only way to reverse it is to reverse relative magnetic polarities between the field windings and commutator/brush gear. Therefore, it needs to have a simple speed control followed by the direction switching associated with the engagement/direction switching mentioned by Simon, or the engagement/direction switching integrated into the control board (hence separate field and commutator outputs from the board).

    Incidentally, if the thing is as simple as Baron suggests, I doubt that it would qualify as a servo system as Baron's circuit does not include anything for positional feedback, generally one of the criteria for a servo system, input tells it where to go, positional feedback tell it where it is, control system compares them and determines decides which way to go and how fast to go to get to the required position ASAP without overshooting.

    Servo action should not be required for feed motor on a manual mill where only requirement is adjustable constant speed rotation is primary requirement, but is required for anything CNC where the control system needs to know the position at all times. Steppers are an exception in CNC as the controller counts steps and direction and keeps a running tally to determine position, which sort of works until something happens to cause a motor to miss one or more steps, and then the whole control system is out of whack until it is re zeroed. Servoes don't have this issue because they are constantly measuring shaft position and supplying that information to the amplifier/driver system.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  7. #7
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    Saskatoon, Saskatchewan, Canada
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    Default

    I'm no sparky but I would get the new one. Reason being the resale of it will be more if it is original and not cobbled together with what might be perceived as junk parts.

    Pete

  8. #8
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    Hi Malb,

    It's not an actual servo motor as such. Servo products is the brand name.

    Pete, a new power feed from servo products is not an option for me as they are about $US800. If I bought new it would be a $300 "cheapie" from china via eBay. The point being that a new Chinese power feed would be inferior to the rebuilt power feed I have now. Hence the dilemma of repair or replace.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  9. #9
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    Default

    I meant to get the new Bridgeport circuit board not a new Bridgeport feed.

    Pete

  10. #10
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    Quote Originally Posted by QC Inspector View Post
    I meant to get the new Bridgeport circuit board not a new Bridgeport feed.

    Pete
    Yea. It's tempting. There also a chance that I can do that and still end up with a power feed that does not work. I guess it's something I have to weigh up. I think I will do some more testing on the armature and the field windings to make sure they are still OK. If I find that they are suspect then it makes the decision a whole lot easier.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  11. #11
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    Hi Simon,

    Those motors will run quite happily on as little as 15 or 20 volts DC, or AC for that matter. Make a note of where the wires go, then simply connect the armature in series with the field coil and put a voltage across the whole lot. Beware of the back emf if you hold the wires with your fingers

    The motor should run smoothly in either direction.
    Best Regards:
    Baron J.

  12. #12
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    Thanks Baron.

    Btw, here is the schematic (in 2 parts) for the type 90 power feed.

    SimonTapatalk Cloud - Downlaoad File Type%2090%20Schematic%20Left%20Side%281%29.pdfTapatalk Cloud - Downlaoad File Type%2090%20Schematic%20Right%20Side%281%29.pdf

    Sent from my SM-G970F using Tapatalk
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  13. #13
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    Default

    Hi Simon,

    Very interesting schematic ! Very clever the way that they have used a transformer to provide the phase switching to drive the thyristors and the way that they have split the motor field winding.

    Whilst component replacement would be absolutely no problem, the motor would be an entirely different matter.
    All the components are old style from looking at your pictures.

    Do you have a multimeter ? Do you want to start to repair it ?
    Best Regards:
    Baron J.

  14. #14
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    Yes I have a multimeter and some basic electronic knowledge.

    I won't have much of a chance over the weekend but yes I'm keen to at least explore the possibility of repair. Bare in mind that I suspect that the actual board ie tracks are damaged. I would imagine I will need to make a new board. I have made boards in the past using a freeware software package and using a laser printer and etching. The most critical part is ensuring the pc mounted micro switches are positioned accurately.

    I suspect the first part is proper testing of the armature and field windings?

    Simon

    Sent from my SM-G970F using Tapatalk
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  15. #15
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    Hi Simon,

    The first thing is to take a couple of good pictures of the drive looking straight down and towards the motor, then we can see exactly where everything goes.

    From an earlier picture two things jump out, the thyristors should have a rubber tube over them. The metal tags are at mains potential, far to easy to touch and get a belt (shock). The other is that the motor is probably going to have to be assembled into the case in order to run it, so initially ohmmeter tests are needed to make sure that the windings have continuity and also the armature doesn't have any opens.

    This probably means unsoldering the wires to the motor circuit board !
    Best Regards:
    Baron J.

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