My turntable motor has started humming

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MikeyB456
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My turntable motor has started humming

Post by MikeyB456 » 14 Jan 2020 12:10

I recently changed the arm on my turntable, and during the setup, I noticed a spike in the frequency response of the cartridge at exactly 100Hz. I'm in the UK, so that's 2x mains frequency. I initially suspected a hum issue with the new arm installation, but there's nothing significant at 50Hz, only 100Hz.

On further investigation, I found that it's only there when I'm actually playing a record. In fact, it seems to be physical vibration being picked up when the stylus is in contact with the record. When it's not, the frequency response is clean. So it's not electrical hum pickup.

I also tried disconnecting the drive belt and checking to see if there's anything picked up when the motor is running but the turntable is not. There isn't. So my conclusion is that it's 100Hz 'vibration' being passed via the belt to the turntable/arm/cart.

What should I do? Is the motor going wrong? Or is it the phase-shift cap beginning to leak (it's just a simple synchronous motor setup, no fancy drive electronics).

All help appreciated!

Alec124c41
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Re: My turntable motor has started humming

Post by Alec124c41 » 14 Jan 2020 15:07

You could try a drop of oil to the motor bearing. But you might well be asking the right questions.
I know that there are different, softer, belts available for a Rega, but I don't know of any for other turntables.

Cheers,
Alec

MikeyB456
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Re: My turntable motor has started humming

Post by MikeyB456 » 14 Jan 2020 15:57

Hi Alec,
Thanks for pitching in. It's actually an Acoustic Research. I have a belt that's a bit tighter than the one I normally use (I tried a few when it needed replacing. Surprising - to me - how much they vary). The tighter one makes it run a bit slower (33.0 not 33.33), and shows the same 100Hz spike. Now I know it's there, I'm also sure I can hear it

Oddly, the table runs quietly and smoothly otherwise - no ticking or grinding noises from anything.

The spike definitely wasn't there last year when I fitted the new cart. I kept the Audacity projects of the frequency response and went back and checked. So this is a new phenomenon. I suspect it's been creeping up on me and I've not noticed until the new arm got me into Testing Mode again.

lenjack
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Re: My turntable motor has started humming

Post by lenjack » 14 Jan 2020 18:00

Check underneath to see if anything got loosened.

MikeyB456
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Re: My turntable motor has started humming

Post by MikeyB456 » 14 Jan 2020 20:43

All seems good. I'm afraid it looks very much like some physical issue with the motor.

MikeyB456
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Re: My turntable motor has started humming

Post by MikeyB456 » 14 Jan 2020 21:00

I've decided to try changing the phase-shift capacitor. It's a relatively easy thing to do, and it makes sense to me that the motor would start to vibrate at 100Hz if the two phases it's supplied with get out of balance.
The capacitor is an unusual value (of course!), and will take a while to come, so it'll be a while before I know if I'm right or wrong.

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