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  1. #11
    Greg Davis's Avatar
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    Quote Originally Posted by Steve Smith View Post
    Unfortunately this is one of those things which many of us could sort out in minutes if we had it in front of us but is nearly impossible to sort out remotely. You need to find someone local who knows about these things.


    Steve.
    Well, I am not going to pay to have them repaired since they are university property, but the university would rather throw them out than have them repaired. I got them from university surplus since they won't buy more. I will spend a couple dollars on parts, though, since I can get that from the department's petty cash. I don't understand why I can't remove the PCBs completely and just wire the motor to a variable speed switch. Is this possible? I see homemade mixers doing this same thing with computer fans. I understand I have to convert the AC to DC, but surely there must be a simple solution rather than tracking down all the broken pieces.
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  2. #12
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    Quote Originally Posted by Greg Davis View Post
    Well, I am not going to pay to have them repaired since they are university property, but the university would rather throw them out than have them repaired. I got them from university surplus since they won't buy more. I will spend a couple dollars on parts, though, since I can get that from the department's petty cash. I don't understand why I can't remove the PCBs completely and just wire the motor to a variable speed switch. Is this possible? I see homemade mixers doing this same thing with computer fans. I understand I have to convert the AC to DC, but surely there must be a simple solution rather than tracking down all the broken pieces.
    That PCB? It's a variable speed motor control. Sure you can replace it, but the replacement is far more complex than a potentiometer due to the power levels involved. The replacement is going to work similarly to the PCB already in there except it won't be designed to fit in the case. The quickest/dirtiest thing you can try is an AC lightbulb dimmer between the mains and the motor, it will probably give you coarse speed control. You might also blow up the dimmer if it's poorly designed (they don't expect inductive loads).

    Computer fans are 12V BLDC motors, this thing is an AC squirrel-cage motor. They have totally different supply and control requirements.

    See the two surface-mount black things above and below the capacitor that's overheating? My guess is that the top one (3 pins) is a triac which is doing phase control on the motor. Replacing that on the stirrer that doesn't work at all might get you somewhere, or it might not.
    Last edited by polyglot; 07-16-2013 at 07:25 PM. Click to view previous post history.

  3. #13
    Hexavalent's Avatar
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    If you choose to use a simple lamp dimmer for speed control, it may have a tendency to wander with varying loads. This is not much of an problem for quick-mixing chems, but for slower, longer-term mixing, wandering speed can become an issue.
    - Ian

  4. #14
    Greg Davis's Avatar
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    I will try to get some better images tomorrow. I may have a guy that an look at it in person.
    www.gregorytdavis.com

    Did millions of people suddenly disappear? This may have an answer.

    "No one knows that day or hour, not even the angels in heaven, nor the Son, but only the Father." -Matthew 24:36

  5. #15

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    Quote Originally Posted by Hexavalent View Post
    If you choose to use a simple lamp dimmer for speed control, it may have a tendency to wander with varying loads. This is not much of an problem for quick-mixing chems, but for slower, longer-term mixing, wandering speed can become an issue.
    Yup. That's the ghetto way out and it might work fine. Certainly worth the $9.99 to give it a try. If it doesn't work on the bench you can go back and reverse engineer the original controller.

  6. #16
    jp498's Avatar
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    If the part has thermal paste, it's not a capacitor, what's the label for that part on the board? like c??, r??, q??. If the part that gets hot has become disconnected from the heat sink it could have been destroyed by the heat.

  7. #17
    Greg Davis's Avatar
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    The part with thermal paste is not the part that gets hot. In the photo I attached earlier, the part I circled is the one that gets hot, it is labelled "C2". The part with thermal paste is at the top of the board, and the paste can be seen as a grey goop on its surface.
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    Did millions of people suddenly disappear? This may have an answer.

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  8. #18
    Steve Smith's Avatar
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    Quote Originally Posted by Greg Davis View Post
    Well, I am not going to pay to have them repaired since they are university property.
    When I said someone local, I meant someone like many of us here who tinker with things and have the knowledge to repair it rather than a professional repairer.

    Is there an electronic engineering department in the university? (look for a lab technician rather than an engineering student - they won't have a clue!).


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  9. #19
    Greg Davis's Avatar
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    There is, but virtually everyone is absent right now since it is the summer. Most people aren't back until late August.
    www.gregorytdavis.com

    Did millions of people suddenly disappear? This may have an answer.

    "No one knows that day or hour, not even the angels in heaven, nor the Son, but only the Father." -Matthew 24:36

  10. #20
    jp498's Avatar
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    C2 is a capacitor. If it's getting hot, something else is causing it to get hot. You'd need a tech to figure that out.

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