Steadying motor speed

Discussion in 'General Electronics Chat' started by Veracohr, Apr 6, 2011.

  1. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    I've got a belt-drive turntable which has very slight but regular speed (and thus pitch) fluctuations and was wondering if there's a way to really lock down the motor speed. I checked the voltage the motor is getting and it's pretty darn steady; my scope showed it fluctuating no more than 0.1Hz around 60Hz. Any ideas? Or is this just something you have to deal with when you have a cheap belt-drive turntable?
     
  2. debe

    Well-Known Member

    Sep 21, 2010
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    How old is the belt? do the motor bearings need lube. Asuming this is not new.
     
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    What is the period of the variation in speed, is it once per motor rev or once per table rev? If it matches the motor, I'd check to see if the drive pulley is off-center. If it's once per table rev, likewise for the pulley that drives the table (or the entire table, if the belt goes around the table edge). If it's neither of those, then maybe it's the speed of the motor.
     
  4. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    I think it might be about the period of the table revolution. I'll check when I get home. It's definitely a slow period.

    It's not old, maybe 3 years or so, but IIRC it's done this since it was new, so I kind of doubt the motor needs cleaning or anything like that.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Thye should have a new belt every couple of years.

    Belts are still commonly available through HiFi shops and repair centres.
     
  6. debe

    Well-Known Member

    Sep 21, 2010
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    If its one rev of turntable it will be the belt, it will have afaulty patch in it were it has stood for a long time around the motor shaft, seen this fault before. Replace the belt they arent expensive.
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Second on the belt diagnosis.

    Sitting unused they 'form' into the shape of the pulleys. They also get brittle and cracked with long exposure to atmosphere oxygen and loose gripping power.

    Change the belt.
     
  8. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    OK, I guess I'll try to find a replacement belt, but as I said it's only a couple years old and I'm pretty sure it did this when it was new.
     
  9. John P

    AAC Fanatic!

    Oct 14, 2008
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    It can't possibly be the belt. If it were, the variation would occur once per pass of the belt, not once per revolution of either pulley. Most likely a bulge or kink in the belt would show up most when it goes over the motor pulley, as that's smaller. Spread out over a large circumference, as the output pulley would be, I doubt if you'd notice it.
     
  10. eblc1388

    Senior Member

    Nov 28, 2008
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    Remove the belt and manually turn the turntable platter slowly. Feel with the hand the applied torque/reaction to see if at any position there is a "click" or change in " mechanical resistance".

    If the platter turns very smoothly, then it will be the belt.
     
  11. debe

    Well-Known Member

    Sep 21, 2010
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    In one rev of the turntable a faulty spot on the belt will pass the motor pulley.
     
  12. John P

    AAC Fanatic!

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    In one rev of the turntable a faulty spot on the belt will pass the motor pulley once at most, and possibly not at all.
     
  13. John P

    AAC Fanatic!

    Oct 14, 2008
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    In one rev of the turntable a faulty spot on the belt will pass the motor pulley once at most, and possibly not at all.

    Good idea about checking the bearings for smoothness. But there is the possibility that even if the bearings work correctly, the entire pulley is off-center. A pulley that's not perfectly circular would have a similar effect.
     
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It's the most typical symptom of belt failure; repeating pitch fluctuation.

    Generally the belt circumference is only 10 or 20% larger than the turntable pulley circumference so to the user the pitch fluctuation appears very similar to turntable speed.
     
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