Dremel 4000 repair

Discussion in 'The Projects Forum' started by wayneh, Apr 16, 2015.

  1. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    Working on my son-in-law's Dremel today. It does nothing when switched on. Anybody have experience with one of these?

    I have the (useless) owner's manual as well as a parts diagram, but it does not provide any details on the electronics. The switch and controller is sold as a replacement board for ~$20. I suspect that would fix it but of course I'd like to 1) be sure, and 2) find a cheaper fix if possible.

    So far I've done a visual inspection and a little probing with the multimeter. Everything looks clean and mechanically in good shape. No burns. Brushes look good with little wear.

    There is continuity across the thermal fuse. I'm not sure how to test the stator coil but I do see a few ohms across opposite terminals, from one brush holder to the opposite pole of the coil. I have not been able to show that the switch works. I get "open". It could be that I haven't made good contact yet, but I'm wondering if that switch could have failed. Is that a reasonable possibility? I thought a switch like that would work "forever".

    Just two more questions: With the rotor out of the way, is it OK to briefly power up the device and look for power at the brush holders? What is that blue wire and clip for? I was unable to see where it was as I pulled the tool open.

    Screen Shot 2015-04-16 at 11.33.26 AM.png
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    745
    if its mains voltage,i would think its a simple dimmer circuit with a triac,
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you re-assemble it, you could place full AC across the motor, it should run full rpm.
    Or at least check the continuity, the armature is in series with the field windings in these motors, not sure what that third wire would be?
    Max.
     
  4. R!f@@

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    Apr 2, 2009
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    is that bulging ?
     
  5. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    The first picture makes it look that way but I looked closely, and I don't think so.
     
  6. wayneh

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    Sep 9, 2010
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    More board details.

    Screen Shot 2015-04-16 at 1.16.08 PM.png Screen Shot 2015-04-16 at 1.15.12 PM.png
     
    Dodgydave likes this.
  7. MaxHeadRoom

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    Looks like they have gone over to PWM since my old one!
    Max.
     
  8. wayneh

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    Sep 9, 2010
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    So I'm still thinking of plugging it in and probing the switch, stator coil and the brushes for voltage. Safe?
    (I mean safe for the Dremel - I know to be very careful with exposed parts.)
     
  9. Dodgydave

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    Jun 22, 2012
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    i guess the Red and Black go to the motor, so whats the blue wire connected to???
     
  10. wayneh

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    Sep 9, 2010
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    I think the blue wire detects when a brush is overly worn, but I can't yet figure out where it goes.
     
  11. wayneh

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    Sep 9, 2010
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    An update: I went ahead and powered it. The switch appears to work and I see ~85V AC between the two brush holders when I press the switch. This voltage was unaffected by the dial setting. The coils of the stator both show <10Ω but not a short, so I think they're fine.

    Not sure where to look next.
     
  12. MaxHeadRoom

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    Being a universal motor, it works on AC or DC, series field, check the continuity once re-assembled, it should run on 120vac direct, full rpm of course.
    Max.
     
  13. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    try powering the motor directly on mains, it should run at full rpm, or use a light dimmer to see if it runs slowly.
     
  14. wayneh

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    Sep 9, 2010
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    Well, score one for dumb luck. I put it all back together and it runs!

    I believe the problem was with a subtle misalignment of the sliding plastic piece that activates the switch. It's kind of a flaky design that requires mm precision from flexible plastic parts. But anyway I'm happy to learn the controller isn't toast. It won't be hard to change the switching mechanism if the problem persists.
     
  15. Alec_t

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    Sep 17, 2013
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    Good. And where did the blue wire go?
     
  16. wayneh

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    Sep 9, 2010
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    The brass cylinders that hold the brushes and springs are held in place by a ridge of plastic in both sides of the plastic exoskeleton. One of those ridges had a small groove onto which that brass clip slid. So, as far as I can tell, the clip and the blue wire is there to contact the brush holder. I'm not sure how it works, but I read somewhere that the tool will shut down when the brushes are worn.
     
  17. MaxHeadRoom

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    If the brush wears to the extent that it does not make contact the motor is going to stop as rotor & stator are in series.
    Max.
     
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