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 Post subject: Getting a modern turntable to run at 16 rpm?
PostPosted: Aug Sat 07, 2021 1:59 am 
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Joined: Dec Sun 07, 2008 7:05 am
Posts: 4836
Location: Meridian, MS 39307
When it comes to the two-speed motors that have the third outboard pot on them to get 78 rpm, as often found in most "Crosley/Victrola"-style record players and in cheaper turntables, such as the Audio Technica AT-LP60, I thought I'd try my hand at getting it to run slow enough for 16 rpm and even the 8 rpm NLS talking book speed.

The schematic is one that I drew from a '90's Kenwood turntable. On this one, some of the motor control circuits are external to the motor; but, the circuit seems basically the same as what's found on the newer "self contained" motors with the control circuit built inside the motor.

To get 33 rpm, they have a variable resistor in series with a fixed resistor connected in the feedback circuit. To get 45 rpm, another variable resistor and fixed resistor combination is placed in parallel with the feedback resistance for 33 rpm, thus lowering the resistance and causing the motor to run fast enough to spin the turntable at 45 rpm. On 78 rpm-equipped turntables, another value of resistance is placed in parallel across the 33 rpm feedback resistance, and this makes the motor run even faster for 78.

Getting the motor to run faster is easy; but, I'm finding out that getting it to run slower is not so easy.

I took my sacrificial "crosley motor" apart, removed the 33 1/3 rpm trim pot, and replaced it with a 10K pot for testing. I was still unable to bring the motor speed down enough for 16 rpm and, even at that, the speed was wavering.

So, unless I'm missing something, it may not be possible to get this motor to run slow enough for 16 rpm.


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 Post subject: Re: Getting a modern turntable to run at 16 rpm?
PostPosted: Aug Sat 07, 2021 4:18 am 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 2027
Location: Great Bend KS
Maybe turn the motor pulley down on a lathe?


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 Post subject: Re: Getting a modern turntable to run at 16 rpm?
PostPosted: Aug Sat 07, 2021 6:43 am 
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Joined: Feb Wed 07, 2018 6:52 pm
Posts: 1126
Location: Stone Mountain, GA
The settings have 300 + (0 to 300) ohm settings. Probably 400 and 500 total or so for 45 and 33 1/3. Looks like 1k would be needed for 16.

You put in a 10k pot. It must have been real close to one end. Probably varied a bit.

Try using a 1k pot or change one of the resistors to 680 ohm.

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 Post subject: Re: Getting a modern turntable to run at 16 rpm?
PostPosted: Aug Sun 08, 2021 12:21 am 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 804
Location: Colorado Springs, CO 80917
Would a variable frequency AC source for the motor do the trick?


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 Post subject: Re: Getting a modern turntable to run at 16 rpm?
PostPosted: Aug Sat 28, 2021 8:44 pm 
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Joined: May Sun 08, 2011 8:52 pm
Posts: 696
Location: Bay City Mi
I think they are referring to modern DC motors in the 9 - 12 volt variety


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