Which stroboscopic strip is the one for 50Hz? - diyAudio
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Old 27th May 2014, 05:09 AM   #1
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Default Which stroboscopic strip is the one for 50Hz?

Hi: A record player turntable I have has two stroboscopic strips on its rim (excuse the lack of precision in vocabulary, if you please; I'll keep using 'strip' in what follows not knowing the correct term). One for 50Hz and the other for 60 Hz (mains). But I don't know which is which ( there is an indication on the player itself but it seems ambiguous to me). Each strip consists of a sequence of alternating dark zones and
light zones. At first sight, I think of the two strips, the one with less dark zones (equivalently less light zones) will correspond to 50Hz. I could reason like this: if the light blinks 50 times per second it is logical that the number of dark zones be some multiple or submultiple of 50, say a factor of two, and seeing the number of dark zones in the other strip is not very different, I'll have that the factor is also two there, and hence the strip with less zones will correspond to 50Hz. But I could be wrong and reach the erroneous conclusion.

Then, given the fact that the number of dark zones in one band and in the other, let them be n1 and n2, are alike, more exactly n1/n2 does not differ from one in more than 15 to 20 per cent, will some generous soul give me a hint?

NOTE: It seems the logical thing to me 1 - n1/n2 = 0.20 exactly (or -0.20) if n1 < n2, given that (60 - 50)/50 = 0.20. If somebody finds all this is too complicated I'll try to find some simpler way to put things, but I doubt I'll be successful.

Last edited by stf92; 27th May 2014 at 05:19 AM.
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Old 27th May 2014, 07:27 AM   #2
JoeDJ is offline JoeDJ  United States
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In a standard TT one strobe strip is for 45 rpm and the other is for 33 1/3 rpm.

Also a TT is set up for ether 50 or 60 hz at the factory and should have a label on it specifying which it is.

In other words, you will not get proper speed at all operating it at the wrong hz..
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Old 27th May 2014, 08:37 AM   #3
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Thanks for your reply. I shoud have been more explicit, now I see. On the front of the record player, which is direct drive, there is a manual pitch control and, beside it, a drawing. By one side of this, it is written '33 RPM'. By the other side, '50 Hz' and '60 Hz', one below the other.

To be more specific, the turtable is a JVC L-A31. On a plate on the rear, where the model and manufacturer's names are, it says 'AC 110/120/220/240V - 50/60Hz - 9W'. I suspect that so much versatility does not speak highly of its quality.
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Old 27th May 2014, 09:50 AM   #4
JoeDJ is offline JoeDJ  United States
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OK........ on that model it can indeed be use for either 50 or 60 hz. Evidently it can sense which what hz it is.

According to the manual, a strobe disk is supposed to set speed for 45 rpm.
It says, "Use the stroboscope provided for 45 rpm".......... and use the strobes on the edge of the platter for 33 rpm.
So, both strobes on the platter edge are for 33 rpm

Acording to the diagram on the TT, the top strobe is for 50hz and the bottom is for 60hz

Last edited by JoeDJ; 27th May 2014 at 09:55 AM.
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Old 27th May 2014, 09:52 AM   #5
AndrewT is offline AndrewT  Scotland
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The "strobe" usually works off the mains frequency.
The mains frequency is constantly changing.
The usefulness of the strobe becomes worth nought, especially when the motor and the strobe are both locked to the mains frequency.

Drive your TT with 40Hz from a Power Amplifier and see the strobe lock in. What is coming off the recording?
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Old 27th May 2014, 10:00 AM   #6
JoeDJ is offline JoeDJ  United States
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The strobe has to be set for proper speed regardless.
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Old 27th May 2014, 10:16 AM   #7
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Quote:
Originally Posted by JoeDJ View Post
OK........ on that model it can indeed be use for either 50 or 60 hz. Evidently it can sense which what hz it is.

According to the manual, a strobe disk is supposed to set speed for 45 rpm.
It says, "Use the stroboscope provided for 45 rpm".......... and use the strobes on the edge of the platter for 33 rpm.
So, both strobes on the platter edge are for 33 rpm

Acording to the diagram on the TT, the top strobe is for 50hz and the bottom is for 60hz
[Sorry, I posted before you edited.]

Well then, I'll try to find one. What I did before posting, is to play a recording from the 1970's, a German symphonic orchestra, and use my tuning fork, labeled 440. However, when my ear was satisfied, I didn't see any of the belts to get stationary. But pitch at that time was not exactly standardized. While European orchestras used 435, American ones used 440, this being only a general rule, which must have had some exceptions.

Meanwhile I keep looking for the stroboscopic disc that came with a Garrard 400, though the platter's much larger diameter has to result in greater resolution. Of course, the platter's stroboscope is more reliable than the ear. I think I have now, thanks to your post, all the information I need. Again thanks, Joe, and may you have a good day.

Last edited by stf92; 27th May 2014 at 10:20 AM.
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Old 27th May 2014, 10:23 AM   #8
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Quote:
Originally Posted by JoeDJ View Post
Acording to the diagram on the TT, the top strobe is for 50hz and the bottom is for 60hz
That's what I thought by watching that diagram, but I wasn't sure. This puts a definitive end to the whole question, save I'd like to answer AndrewT.
Thanks a lot.

Last edited by stf92; 27th May 2014 at 10:34 AM.
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Old 27th May 2014, 10:32 AM   #9
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Quote:
Originally Posted by AndrewT View Post
The "strobe" usually works off the mains frequency.
The mains frequency is constantly changing.
The usefulness of the strobe becomes worth nought, especially when the motor and the strobe are both locked to the mains frequency.
Oh!, I did not realize that.

Quote:
Drive your TT with 40Hz from a Power Amplifier and see the strobe lock in.
You mean feed the motor with the power amplifier output?

Quote:
What is coming off the recording?
Bach, Magnificat in D major, Vox PL 8890.
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Old 27th May 2014, 11:27 AM   #10
AndrewT is offline AndrewT  Scotland
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The AC motor and the AC strobe both expect an AC signal of either 50Hz or 60Hz, to allow the Platter to turn at close to the specified speed.

If you feed in a sinewave with sufficient voltage you can drive the TT from a Power Amplifier using a sinewave source.

You can vary the frequency of that sinewave source.
Try 55Hz.
The TT will turn at a new speed.
But the strobe is very likely to tell you it is turning at the specified speed, because both the AC motor and the AC strobe are locked to the driving frequency.

A strobe that is lit by an absolute frequency (maybe crystal generated oscillator) can indicate TT platter speed error.
Is your strobe mains frequency reading, or oscillator reading?
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