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Groove Runner Turntable Speed Control

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Dave Friday View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Friday Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2018 at 10:55am
Hi Graham,what's the distorted waveform ?
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2018 at 11:12am
Hi Barry, it's what comes out of the RP3 circuit. One winding gets voltage via a cap and resistor, the other gets it via the same cap, a different resistor and another capacitor. I did AC theory (for motors and stuff) a long time ago so I haven't a chance of working this one out Wink. For normal 230V use I'm sure a 10k power resistor and a 0.22uF cap would do better Wink.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ernie Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2018 at 7:32pm
Hi Graham,
That’s not a circuit I’ve seen before on one of these little motors. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2018 at 8:48pm
Do you mean the Rega circuit?

I have three instances of the circuit (can only find one right now - typical!) and here is a photo I just took of the board, showing the values indicated in my diagram. I'm quite fussy about getting my facts right in public, otherwise I'd be in dangerous territory, and I was employed in reverse engineering for part of my career, which I have to thank for at least some of my knowledge.



There is also a diagram on the internet which agrees, and that can be found here: https://www.stereo.net.au/forums/applications/core/interface/imageproxy/imageproxy.php?img=http://i204.photobucket.com/albums/bb317/floyd2_01/Rega%2520Motor/RegaMotorDriveSchematics1.jpg&key=f21eafd96e2db0bfa532786a3f2833adf133a6f51aee28b0f99b19932e7f500d

The board image can also be found here: https://www.picclickimg.com/d/w1600/pict/222930888677_/Rega-Planar-2-turntable-motor-may-fit-Moth.jpg

And here: https://en.wikipedia.org/wiki/Rega_Planar_3#/media/File:0081_motor_and_spindle.jpg
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ernie Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2018 at 10:09pm
Hi Graham,
If this was a decent sized motor I’d say you’ve got an odd order harmonic affecting it hence the dip in the waveform. It looks like Rega have added reactance to the circuit to try and control it when powered from the mains. The harmonic could be due to driving the magnetic circuit into saturation.

I may have got my numbers wrong but the standard drive circuit for this motor looks the same as all the others a resistor and capacitor. May be worth trying this circuit rather than the Rega one as the transformer you’ve fitted won’t have the same impedance as the mains supply.

Looking at the data sheet for the motor 9904-111-31104  (Farnell 147876) it just needs a 0.1uF cap between the windings.


Edited by Ernie - 11 Oct 2018 at 10:40pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2018 at 10:44pm
Originally posted by Ernie Ernie wrote:

May be worth trying this circuit rather than the Rega one as the transformer you’ve fitted won’t have the same impedance as the mains supply.


Obviously not, but then again it isn't so high as to cause all of what you see. I have been using the (R and C) circuit you refer to for most of the trials (first circuit previous page, a number of different permutations over close to a year) but have never witnessed any motor having an effect on the output impedance of the transformer or the waveforms. And that included wiring it in series which is higher impedance to obtain 220V, to drive a 3.6 watt motor which stressed the transformer near to its design limits (case gets a bit hot too). Also, the transformer was developed special for the job so doesn't exhibit the same limitations as a mains transformer in reverse. It runs significantly cooler too.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2018 at 10:53pm
Re your edit, the added (series) resistor for higher frequency use is shown here: https://www.mclennan.co.uk/datasheet/1565

As 3/4 of its intended use is at 60 - 81 Hz I considered it a better choice than just having the 0.1uF capacitor. It also runs well at 50Hz, especially as the motor is operating on half intended voltage. Plus it will start and run at minimum output which is just below 90V. It is incredibly quiet in operation and starts up in the same time as a 31813.

Also please understand that the Rega scope waveform was just an observation, and I only included the original circuit as part of the explanation that the Groove-Runner is able to drive the 230V version unmodified (at least the one I have), which might be a plus point to somebody contemplating a purchase.


Edited by Graham Slee - 11 Oct 2018 at 11:02pm
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