JVC QL-7 Turntable; unstable speed

You can't do the calc like that, the published spec is done as an average over a certain number of revolutions. Polar plots are good for seeing if you have a major issue with your mechanical/electrical system or test record but your need a w&f meter to get numbers to compare.

You also need a lab grade test record. @JP ran numbers for me for a few tables and it was obvious that the record itself had problems. All of the tables exhibited the same high frequency variation (similar to your plots actually) and our best guess was that the record itself was faulty as the inertia of the table shouldn't allow that kind of behavior unless something really really wonky is going on.
 
Sounds like I need to invest in a good test record...

Maybe you can sum it up for me, since I'm not sure I know how to correctly interpret the graphs... Are the results good, bad, or ugly?
Does it point us in any new direction regarding the original problem?

And while we're on the subject... Is there a universally recommended test disc around here?
 
MonoMania said:
A way to figure out how much the test record I used is a factor might be for me to re-record the same tracks with different turntables... Not sure how much trouble it is for you to run the analysis though.
Click to expand...

Go for it. Most time consuming part is editing the file. Polar plot is a python script.
 
I use vintage test records as all the new ones I've tried are rather poor. Also, pressings matter as my best and worst are the same Denon test record. Really, most important part is to get to know the record by plotting multiple 'tables that are known to be running right.
 
Yeah, I'd say the results are inconclusive without a second reference point.

My first test record was wildly off center. Made it basically useless.

I've run tests with my second record enough to know that if my app is averaging under 0.06% or so the table is doing good. Either that or every table I've worked on is uniformly bad. :rflmao:
 
JP said:
Go for it. Most time consuming part is editing the file. Polar plot is a python script.
Click to expand...

Will most likely do so tomorrow, with my trusty JVC QL-A2, and then with an idler drive Garrard for fun. Thank you very much for sharing your expertise with me - without the help of this forum I likely would've given up on this turntable already.

Could recording the clips in mono have caused any problem?
 
JP said:
I use vintage test records as all the new ones I've tried are rather poor. Also, pressings matter as my best and worst are the same Denon test record. Really, most important part is to get to know the record by plotting multiple 'tables that are known to be running right.
Click to expand...

I use vintage ones too. I have probably 6 or 7 here and a few at Dad's place. The key is as JP said, getting used to the record. I know that sounds weird but through each W&F track on all the test records, they vary a little, and you can either compound or cancel your errors/readings. Add that to the slight off centre of practically all records and one has to make several tests rotating the record on the platter IME to get meaningful results.
 
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MonoMania said:
On closer inspection, there are several stress marks / cracks on the underside of the platter near the part that meets the spindle. Is this damage, or something left over from manufacturing?

under.png
Click to expand...


I don't think its from the factory and maybe causing the clicking sound?
 
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desertrat748 said:
I don't think its from the factory and maybe causing the clicking sound?
Click to expand...

With the platter on it's actually a very very quiet buzzing sound once per revolution. If I knew how to I'd change the title of the thread [Edit: I figured out how to!]. Doesn't sound like a grinding even... More like an electrical sound, a coil charging or discharging maybe. When I get a stethoscope I'll make a recording of it. I need to do the motor tests which @EngineerNate suggested next time I have it disassembled.

There are a handful of these 'cracks' around the spindle hole. The thing is, this platter is so thick and heavy that it would be difficult to damage that way.... And it doesn't show any signs of being bent - spins true and flat, as far as my eye can tell. The inside of the spindle hole has no apparent cracks.

In the meantime I took Scotch Brite to the spindle hole as advised by @EngineerNate, and also scrubbed the rim of the platter and the inside of the plastic housing around it just in case something was rubbing - made no apparent difference.

I have a bunch of things to check now - transistors, waveforms, voltages, motor... I'd been hoping to narrow it down, but I just have to roll up my sleeves and get the data - then the picture will be clearer :)

Thanks to all who have advised!
 
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@JP After playing that test LP on several turntables I am now absolutely convinced that it is useless, so no point in sending you those clips.

However, digging around in my collection I did manage to come up with some other test tones which should be more reliable:

One set is from a Shure test record (1kHz only)

The others are from a Folkways educational record "The Science of Sound" which has recordings of a tone generator.

You can access both sets of audio clips at the following link:
https://www.dropbox.com/sh/3hwys8o9i0cq7mc/AADimowDy3KXbQ1_qRFXVsQ1a?dl=0

Hopefully this will give a clearer picture the w&f.

Thanks again for all your help!

P.S: I am now an AK subscriber! I wanted to support this website which has been such a great resource over the years. I've only ever started a few threads, but have read probably thousands.
 
Been playing around with a program called wfgui downloadable here: http://www.ant-audio.co.uk/index.php?cat=post&qry=library

It claims to be a software w&f meter.

By recording the Shure disc's 1kHz tone in audacity, changing speed to equal 3150 Hz, and looping the tone... The program is telling me I have a w&f of ~0.06% RMS or ~0.10% peak.

If accurate, that's somewhat decent, isn't it? That is of course just from a single recording; not compensating for the eccentricities of the disc, which I don't know how to do.

Will be interesting to see what @JP 's plots show.
 
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totem said:
Been following this on VE a new program for W&F.
Click to expand...

That app gives me the following readings:

44.73 rpm; WOW +/- 0.04%
33.14 rpm; WOW +/- 0.05%

So this means that the table is running slow too? :(
Confusing, because my speed discs wobble, but they don't drift...

I wonder how accurate it is... My JVC QL-A2 tests very similarly for speed...
(This app says wow is actually worse on the QL-A2 than the QL-7)
 
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Is this with digitally altering the file to have the right frequency or did you find a 3150hz test LP?
 
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Quoting myself from earlier: "By recording the Shure disc's 1kHz tone in audacity, changing speed to equal 3150 Hz, and looping the tone... The program is telling me I have a w&f of ~0.06% RMS or ~0.10% peak."
EngineerNate said:
Is this with digitally altering the file to have the right frequency or did you find a 3150hz test LP?
Click to expand...
It's a 1kHz tone sped up to 3150 Hz.

While you're here.. Do you know much about the neon bulbs for turntable strobes? The SM doesn't seem to give a part number or any specs, but the schematic seems to indicate that it runs on 160V DC... If I'm reading it correctly. What other parameters would I need in order to replace it? The old bulb flickers consistently.
 
Regarding the measurements from the phone app, I'd expect +-1% accuracy on the gyro/accelerometers in the phone with that app. So don't worry too much about the absolute speed measurement from that one.

On neon lights, I'd look into the circuitry driving the neon. I'll take a look at the schematic. I would expect the bulb itself to either work or not.
 
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