Pioneer PL-12D motor sync problem, with possible solution?

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NT7000
United States of America
Posts: 2
Joined: 05 Aug 2019 03:06

Pioneer PL-12D motor sync problem, with possible solution?

Post by NT7000 » 08 Aug 2019 22:08

Hi, I have recently acquired a Pioneer PL-12D turntable, and am in the process of restoring it. After cleaning and lubing both the platter and motor bushings, I replaced the belt and turned it on. The turntable then turned 33 1/3 RPM when set to 45 RPM , and much less than 33 1/3 when set to 33 1/3. All the wiring is good, both the motor and platter spin freely, and there is no obvious damage to anything. I decided to run the motor without the platter on, and I placed a sharpie dot on the top of the pulley on the motor. Turns out the motor can't sync, even with no load. I did some research, and came across a Youtube video saying the problem is caused by the permanent magnet in the rotor is getting weak. He suggesting adding a capacitor in line with the motor, so I harvested a run capacitor from a dead ceiling fan motor, and connected it in series with the motor via the cord. Waddya know it worked! My question is, is this a permanent fix, or do I need to keep an eye out for a motor? If this is a viable fix, what size capacitor should I use? Currently using a 3.5 MFD capacitor. Thanks in advance.

NT7000
United States of America
Posts: 2
Joined: 05 Aug 2019 03:06

Re: Pioneer PL-12D motor sync problem, with possible solution?

Post by NT7000 » 12 Aug 2019 22:38

I did some more research and testing, and I think I might be able to answer my own questions. So the whole idea behind adding a capacitor in series with the motor is to add reactive power, this is seen when you connect a voltmeter across the windings. Without the capacitor, you see 120V, but with it you see 160V! This adds enough power to increase the magnetic field strength in the motor to the point where it will sync to the weakened rotor magnet. To check if the motor syncs or not, I put a magic marker line across the top of the pulley, then I pointed a LED night light at it. The idea here is the motor should be turning at the same rate as the AC cycle frequency, and the night light will be flashing at the same speed, therefore the mark on the pulley should appear stationary if the motor is in sync, or rotating backwards if it is not. From my understanding of what capacitors do to motors, too big a capacitor won't cause any harm, while too small a capacitor will limit the current going through the motor, making the problem worse, so I just opted to keep the 3.75 MFD cap, since it works so well. The nameplate wattage for this motor is 10 Watts according to the sticker, actual measured value according to my kill-a-watt (I know, I need to invest in a clamp meter) is 5.4 watts without the capacitor, and 7.8 with it. As for the longevity of this fix, only time will tell. I'm no engineer, but my take is it works now since it compensates for the weak permanent magnet, so unless the magnet continues to degrade to the point that it's no longer a magnet anymore, this fix should be permanent. I have been thinking, it may be possible for someone with access to a machine shop to remove the magnet ring and replace it with a new one, or even better would be a handful of NIB magnets. Hopefully this info will help someone with similar issues.

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