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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 12th Dec 2018, 5:14 pm   #1
PsychMan
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Default Nice Way To Measure Turntable Speed

Up till now I have used strobe discs when checking turntable speed, I know some have other methods involving frequency counters..

Watching the latest Shango066 video on youtube, I noticed he has a far better way of doing this.

He stuck his android phone on the platter, started the deck, and low and behold, an app using the gyroscope in the phone gave a precise RPM readout

Not sure what the android version is called, but there is an iphone one called "RPM".

Apologies if this is a known thing, it impressed me and my strobe disc shall now be banished.

One word of warning though, make sure you have a decent rubber mat on the turntable. Sitting it on the metal platter your phone will flung away, at least at 78rpm anyway .
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Old 12th Dec 2018, 5:55 pm   #2
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Default Re: Nice Way To Measure Turntable Speed

You might want to question just what acts as the timing reference for the measurement and how the time counting is done. It could be worse than your strobe disc/mains frequency. Some apps don't do real time very well, or suffer from quantisation of start/stop times.

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Old 12th Dec 2018, 6:19 pm   #3
bikerhifinut
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Default Re: Nice Way To Measure Turntable Speed

So if Psychman does an A-B comparison between a strobe disc and his phone gizmo would that give an idea of accuracy?

A.
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Old 12th Dec 2018, 10:51 pm   #4
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Default Re: Nice Way To Measure Turntable Speed

It is not possible to measure the speed without the effect of the tracking weight of the
cartridge and drag caused by the friction of stylus in the groove being considered.
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Old 13th Dec 2018, 2:44 am   #5
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Default Re: Nice Way To Measure Turntable Speed

Well yes, but in turn that friction increases as the amplitude of the groove modulation increases so in essence a true measure of turntable speed is impossible.
However, don't we live with that and measure the speed of the platter when there is no record being played and all tend to agree that this is the speed of the turntable?

Steve.
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Old 13th Dec 2018, 4:52 am   #6
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Default Re: Nice Way To Measure Turntable Speed

Does it really matter? Does a few % of speed inaccuracy ruin the enjoyment of playing records?
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Old 13th Dec 2018, 5:44 am   #7
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Default Re: Nice Way To Measure Turntable Speed

Variations in mains frequency shift both the speeds of synchronous motors and the indications of strobe discs. These are used to command the start-up and shut down of generating capacity. They're well under 0.5% in the UK, and so should be unnoticeable to most people, yet the thought of them has sold a lot of expensive Lingo and similar inverter boxes in the UK, though they have more justification on less developed power grids. The Japanese had their quartz-locked direct drive turntables.

In the non-audiophile world, up-market turntables were expected to have a variable speed feature AND a built-in strobe disc so the user could set the speed using mains frequency as the reference, while the basic models got a synchronous motor and a fixed ratio belt drive. I found this somewhat ironic. Was there really so much inaccuracy in pulley diameters, and belt-slip?

Strobes on the turntable mat could only be seen with no record. Large diameter drop-on strobe discs got in the way of playing the record, but some turntables had strobes around their periphery (underside with mirrors on my B&O 4000) These could be used while playing records and you could check for any effect of stylus drag creating differences on loud patches, or differences between beginning inner and outer tracks if you were interested.

I've had some experience of people using phones to time things that were also timed with calibrated equipment, and had a chat with some friends in the phone development business, so I thought I'd better add a word of caution.

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Old 13th Dec 2018, 8:53 am   #8
Ted Kendall
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Default Re: Nice Way To Measure Turntable Speed

Quote:
Originally Posted by Boater Sam View Post
Does it really matter? Does a few % of speed inaccuracy ruin the enjoyment of playing records?
Depends whether you have absolute pitch, as I do. That's why I use an SP10, or a B790, which reads out to the fourth significant figure, for transfer work. Curiously enough, comic timing can be remarkably speed critical,too - 1% slow can take the edge off it.
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Old 13th Dec 2018, 9:40 am   #9
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Default Re: Nice Way To Measure Turntable Speed

Quote:
Originally Posted by fetteler View Post
Well yes, but in turn that friction increases as the amplitude of the groove modulation increases so in essence a true measure of turntable speed is impossible.
Steve.
But hardly at all. Shure carried out an extensive study of this with both spherical and elliptical stylii, within a comprehensive study of skating compensation (James H Kogen, "The Skating Force Phenomenon", Audio 1967, October 53-56 and November 38-40)

Craig
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Old 13th Dec 2018, 10:50 am   #10
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Default Re: Nice Way To Measure Turntable Speed

I think tracking weight and the effect of the stylus is somewhat irrelevant here, as this is simply a replacement for strobe discs, which do not consider those factors either.

Strobes that work against platters are usually built into turntables and give a visual indication of speed with a record playing. For what we do - which largely concerns auto changers from the 50s - 70s, the most effective method is a strobe disc, or this app.

I haven't checked it for accuracy, but will do by comparing it to my SL1210s. I sometimes put a strobe disc on them and adjust the pitch back a few % to get an idea what 2% slower looks like on a strobe disc.

I think actually a few percent is quite significant. In numerical terms a very small amount, but incredibly audible in musical terms. To me anyway!

Precision is not something often found in old turntable motors, but its nice to know just how far out something is. An example being an RC210 Ive just worked on, despite stripping and oiling the motor, bearings etc, it appeared fairly slow on the strobe. Comparing with the SL1210, it looked to be 1-2% slow. I brought it in from the cold workshop, let it get t room temperature, run it for 10 mins, more like 0.5 - 1% now - probably the best it will get.
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Old 13th Dec 2018, 3:35 pm   #11
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Default Re: Nice Way To Measure Turntable Speed

For showing on TV, aren't feature films sped up by 4% in order to match the 25 Hz frame rate? I assume this speeds up the soundtrack as well? I can't say I've ever heard anyone moan about this.
On the other hand I may be talking through my hat.
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Old 13th Dec 2018, 6:17 pm   #12
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Default Re: Nice Way To Measure Turntable Speed

In the 70's you used to be able to get test tone records and with a quick search I see they are still made now. The one I saw had a 300 Hz tone which I dont think will directly align with any notes on an acoustic guitar tuner which was my first thought. You can always put it into a frequency counter though.

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Old 13th Dec 2018, 8:30 pm   #13
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Default Re: Nice Way To Measure Turntable Speed

I've always been amused by the obsessiveness of audiophiles for 'impeccable' turntable-speed. When you're leaping around to something like Cochran's Summertime Blues a couple of percent faster/slower's really not an issue.

Similarly, plenty of classical music has over the decades been performed impressively faster/slower, and equally with A historically floating somewhwere between 375 and 466Hz according to the instrument-makers and 'zeitgeist' of the composers/performers.

Don't obsess over the speed, enjoy the music!
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Old 13th Dec 2018, 8:34 pm   #14
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Default Re: Nice Way To Measure Turntable Speed

Quote:
an app using the gyroscope
I think they use the compass, a gyro wouldn't know at what speed it was rotating, it would know something is different but not what.
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Old 13th Dec 2018, 9:17 pm   #15
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Default Re: Nice Way To Measure Turntable Speed

My Marantz direct drive turntable has a strobe around the periphery of the turntable with a neon light adjacent to it so you can adjust the speed using the 'pitch' control. This allows you to adjust the speed with the record playing normally.
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Old 13th Dec 2018, 11:57 pm   #16
John Caswell
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Default Re: Nice Way To Measure Turntable Speed

Having spent a good many years repairing T/T I always check them with crystal derived strobe to make sure that they are near the required speed. You get the odd off speed job where the customer has been over enthusiastic with emery/glass paper (I kid you not!) on the motor pulley but generally they all fall well within spec.
The main, rare, problem are those people blessed/cursed with perfect pitch for which I am unable to do anything except recommend a variable speed T/T.

John

Last edited by John Caswell; 13th Dec 2018 at 11:58 pm. Reason: typo
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Old 17th Dec 2018, 12:58 am   #17
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Default Re: Nice Way To Measure Turntable Speed

As has already been said, a strobe disk and a line powered neon lamp may be used to check for correct RPM.
The accuracy of this technique may be improved by checking that the mains frequency is at or very close to the nominal value.

The "dynamic demand" website has an almost real time display of UK grid frequency.

http://www.dynamicdemand.co.uk/grid.htm
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Old 17th Dec 2018, 6:07 am   #18
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Default Re: Nice Way To Measure Turntable Speed

I don't trust the accuracy of gadget type phone apps. I have a spirit level one on mine and it's clearly inaccurate. It's the second of two that I've tried, neither of them any good. The torch is ok as is the mirror.
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Old 17th Dec 2018, 6:27 am   #19
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Default Re: Nice Way To Measure Turntable Speed

Purely on a point of interest, I don't actually see the point of ultimate accuracy regarding speed. Very minor inaccuracies affect rhythm not enough to notice, and pitch similarly. Variations in speed are much more of an issue, and audibly detectable by most people, yet even then, in lower values they are somewhat academic. Just my opinion as a 'reasoned' hifi enthusiast and musician.
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Old 17th Dec 2018, 8:18 pm   #20
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Default Re: Nice Way To Measure Turntable Speed

Quote:
Originally Posted by PsychMan View Post
Watching the latest Shango066 video on youtube, I noticed he has a far better way of doing this..
I saw that as well and downloaded the app. Unfortunately my cheapo android phone doesn't have the necessary giro in it, but it looked to have a lot of play value as my wife used to say about things bought for the children.
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