Turntable Motor Speed Control

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JamesD
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#16 Re: Turntable Motor Speed Control

Post by JamesD »

My experience with the Ampex 1" VTR is that it can be made to run very smoothly and accurately at low rpm - that was a major part of the slo-mo capability that the machine supported as well as accurately ramped up speed to 2x normal speed to keep the heads aligned with the helical path video tracks. They were very low noise w.r.t the signal output from the video heads - not sure how low the mechanical noise from the motor and bearing was as we didn't measure that... Modern motor catalogues have some described as mechanically ultra low noise and high precision but they are not cheap motors and may be excessive for our application... would be good to find someone with up to date experience to talk too about this...

I'm not sure where synchronous motors stop and stepper motors start once micro stepping with sine-cosine drive is considered... the devil is, no doubt, in the details... need someone experienced to talk to again... it seems all the modern motor developments have been based on improving stepper motor design and they allow for more flexibility in the control systems than synchronous motors - that might be good or bad :D

Agree that starting with the smallest step size stepper motor is a good place to start... would you prefer open or closed loop control system - I can't make my mind up on which might be best... it might come down to how stable the open loop system could be made... too many things to test!!! Too many things to think about!!!
vinylnvalves
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#17 Re: Turntable Motor Speed Control

Post by vinylnvalves »

Interesting it was mentioned in the last article I posted a link to, that hard drive motors are 3 phase synchronous motors. I must have an old hard drive knocking about.. intrigued why a hard drive would have a 3 phase motor. This must mean that the hard drive must have a DC to 3 phase stack of electronics on its board. I would imagine it’s about as powerful as the 1.7w permotec motor in the hyperspace. Now if I could find a motor out of something bigger, an old Winchester drive, or even better a 14” laser video player... Unfortunately Winchester drives sell for silly money, as companies want to keep legacy systems running.

The other thing mentioned in the article is you can change the presentation of a turntable by having a bigger heavier pulley.
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Nick
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#18 Re: Turntable Motor Speed Control

Post by Nick »

JamesD wrote: Sun Aug 09, 2020 12:18 pm My experience with the Ampex 1" VTR is that it can be made to run very smoothly and accurately at low rpm
Would be very interesting to see how well it would work, however one point I can think of is that I can imaging that a helical head is not as susceptible to the same sort of problems with stepping as vinyl. Or it may be that the stepping artefacts are easier to filter out with a low pass filter. I can certainly see how stepper motors will allow the head and the traverse tape motion to be synchronised though.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
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shane
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#19 Re: Turntable Motor Speed Control

Post by shane »

vinylnvalves wrote: Sun Aug 09, 2020 4:50 pm Interesting it was mentioned in the last article I posted a link to, that hard drive motors are 3 phase synchronous motors. I must have an old hard drive knocking about.. intrigued why a hard drive would have a 3 phase motor. This must mean that the hard drive must have a DC to 3 phase stack of electronics on its board. I would imagine it’s about as powerful as the 1.7w permotec motor in the hyperspace. Now if I could find a motor out of something bigger, an old Winchester drive, or even better a 14” laser video player... Unfortunately Winchester drives sell for silly money, as companies want to keep legacy systems running.

The other thing mentioned in the article is you can change the presentation of a turntable by having a bigger heavier pulley.
Just found this on Audio Origami. Might be interesting. http://www.audioorigami.co.uk/archive/floppy-project/
JamesD
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#20 Re: Turntable Motor Speed Control

Post by JamesD »

Nick said
Would be very interesting to see how well it would work, however one point I can think of is that I can imaging that a helical head is not as susceptible to the same sort of problems with stepping as vinyl. Or it may be that the stepping artefacts are easier to filter out with a low pass filter. I can certainly see how stepper motors will allow the head and the traverse tape motion to be synchronised though.
Indeed! I also didn't mention that the heads were mounted on flexible strips with a limited range of movement that were driven in position by a servo loop attempting to maximise the rf signal off tape and that the video once demodulated was fed into a limited range "timebase corrector" that effectively re-clocked the signal to line and frame rate...

However the slo mo aspect is relevant as the beast still has to track the linear track path to maintain the head position within the rf helical track path for the above to work.

In the analogue domain the video signal has to maintain a frequency response from about 5Hz to 5MHz so the opportunity for low pass filtering is minimal - this is one area where digital domain filtering has big big advantages...

As an aside - how did I discover the VTR had a mylar tape loop driving the capstan when the brochure describes direct drive capstan circuits? Well I was repairing a new but faulty VTR that the capstan wouldn't work on and instead of reading the manual I just pulled the schematic and worked through the direct drive circuit and couldn't find a fault...but it wouldn't work no capstan turning... so I decided the power bridge circuit driving the motor must be suffering from transistors with low Beta and changed all four for new... 2 hours later it still wasn't working so I called in the maintenance supervisor to see if he had an ideas as he had just finished the Ampex maintenance training course and I hadn't been on it yet. He poked his head into the machine and lifted out the broken drive belt and ask me if I thought that might be to blame... leaving me gob smacked.... took me months to live that one down...

I really want to try this for real now - another project to add to the list...
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Mike H
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#21 Re: Turntable Motor Speed Control

Post by Mike H »

:lol:
 
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vinylnvalves
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#22 Re: Turntable Motor Speed Control

Post by vinylnvalves »

Bit the bullet and ordered the SG4 chipset and PCB. My rationale was if I have the capability to synthesis a 4 phase supply, I can always only use 2, whereas if I got the 2 phase one and then decided I wanted to run more than 2 phases.... :( . Hopefully the electronics won’t be too taxing... otherwise I may end up bartering my skills ( cnc - horn building etc) for electronics help.
JamesD
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#23 Re: Turntable Motor Speed Control

Post by JamesD »

Just came across this site whilst looking for motor controllers for a future project. https://solomotorcontrollers.com/ Looks quite interesting...

ciao

James
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