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Is this speed variation normal?

justinis

justinis

Active Member
I have an Akai AP-207 that has had speed stability issues in the past. I think I've got them licked but I'm not sure. I made a video to show how the checkmarks on the platter slowly drift right and left. Is this normal?

The turntable is direct-drive. It advertises speed accuracy of 1,000 Hz +/-0.15%, wow and flutter of 0.025% (DIN) 0.025% (JIS).

https://www.youtube.com/watch?v=7lmqbfGRSog
 
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justinis said:
I have an Akai AP-207 that has had speed stability issues in the past. I think I've got them licked but I'm not sure. I made a video to show how the checkmarks on the platter slowly drift right and left. Is this normal?

The turntable is direct-drive. It advertises speed accuracy of 1,000 Hz +/-0.15%, wow and flutter of 0.025% (DIN) 0.025% (JIS).

https://www.youtube.com/watch?v=7lmqbfGRSog
Click to expand...

No, it's not normal.....and because of the amount of error in speed, the posted numbers can be thrown out the window.
 
Measure it with a tachometer of some variety. The strobe markings aren't always 100% accurately printed or machined.
 
It's not the strobe markings.. if you watch it, it's not a constant pattern of slow/fast.. it's random.... If it was the strobe markings it would be a constant pattern...
 
similost said:
It's not the strobe markings.. if you watch it, it's not a constant pattern of slow/fast.. it's random.... If it was the strobe markings it would be a constant pattern...
Click to expand...

No, there's a pattern: 2 progressions, 2 smaller regressions per revolution.

I'd clean the speed pots and see whether it smooths out. It certainly can't hurt but I'm leaning toward an improperly marked platter.

John
 
Thanks for all the responses! The bad news is I have already re-capped the power supply and cleaned the speed pots several times. Sounds like I have bigger problems.

Can someone explain the problems with the platter further? What are "runout" and "concentricity?" How do I correct them? What is the difference in "dancing" and "drifting?"
 
justinis said:
Thanks for all the responses! The bad news is I have already re-capped the power supply and cleaned the speed pots several times. Sounds like I have bigger problems.

Can someone explain the problems with the platter further? What are "runout" and "concentricity?" How do I correct them? What is the difference in "dancing" and "drifting?"
Click to expand...

"Runout" is the wobble that would result from a bent spindle, a loose (worn) spindle bearing or a defective or bent platter. "Concentricity" is the opposite of eccentricity, or in the case of a platter, a spindle hole that is dead center in the platter.

I suppose "dancing" is the oscillation we're seeing in the video. "Drifting" would be a gradual change in rotational speed, either slowing down or speeding up but not oscillating.

Judging from your video, I don't think you have a major problem in either regard. I do see a tiny bit of up and down motion in the platter but just a touch. And, as I said earlier, I do see a repetitive sequence in the movement of your dots. That suggests to me a problem with the dots, not the motor.

It has been suggested that you get a tachometer to measure the precise rotational speed of the platter. You can do that or you can get yourself a plain old strobe disc. Vinyl engine has printable ones for free. If the hash marks on the strobe disc seem to advance and retreat in the same way that the marks on the platter do, then you have a speed problem. If they don't, you don't.

Quick question: did you recap the PS board and clean the pots because of what you observed with the platter or did you notice the problem only after you'd done the service?

John
 
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Your table is probably okay as is. I would focus on platter runout and the speed strobe disc.

I measure my platter runouts using these tools from Harbor Freight (nothing fancy here):
http://www.harborfreight.com/1-inch-travel-machinists-dial-indicator-623.html
http://www.harborfreight.com/multipositional-magnetic-base-with-fine-adjustment-5645.html
KAB specs theirs at +/- 5 mils, and one of the VPI tables has a +/- 3 mil spec. 1 mil is equal to 0.001 inches.
http://en.wikipedia.org/wiki/Run-out

As mentioned the speed strobe disc is freely available online. One rub about speed strobe discs is discussed in the following thread:
http://mail.audiokarma.net/forums/showthread.php?t=499264
 
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Thanks for the suggestions! I will print a strobe disc and see what I can find out!

boreas; said:
Quick question: did you recap the PS board and clean the pots because of what you observed with the platter or did you notice the problem only after you'd done the service?
Click to expand...

The problem was there before the service, but the speed variations were worse. I was trying to fix them.
 
If you can post a 10s recorded sample of a 3kHz or so test tone pref in .wav or .flac format, I'll run analysis and post a plot that might well help you diagnose the problem(s) :pawprint:
 
I recommend you download the platterspeed app to your iTouch or iPhone.
Then you can use a test record with a 3150Hz or 1000Hz test tone to provide you a speed graph.
Thus will help you identify a pattern or irregularity in the speed.



I find thus app quite helpful to diagnose speed issues.

graham
 
Decided to post an update. Sorry I missed the last couple posts -- thanks for those suggestions! I printed out a strobe disc and I could see the speed fluctuations on it as well (though I'm not 100% positive -- it was hard to see with my halogen lamp).

I stared probing under the hood with a scope. A lot of things that look weird. The service manual has a description of how the table works and a schematic. There's a coil in the motor that generates a signal as the motor turns. This signal is used in a feedback loop to stabilize the speed of the motor. The signal is supposed to be 20 Hz at 33rpm, but here is what my signal looks like at 33 rpm:

ttscope.png


You can see the frequency widening and narrowing over time. The problem is since everything in the circuit is dependent on everything else, I can't find the source of the problem. Everything seems to act normally given the signals going into it. I spent hours today probing around and haven't much to show for it. :sigh:
 
I see what you mean. Looks like it is hunting. I was looking at the service manual you put in Drop Box. Have you gone through the adjustment procedures yet? In reading the manual their are a number of signals that are dependent on capacitors to change pulsed inputs into DC outputs. It very well may be that old capacitors are no longer able to do this and the instability you see in the feedback pulse is the result. If the adjustment process does not work then you may just need to re-cap your ckt. boards.

For the strobe disk It is easiest to use a florescent light with the old magnetic ballast. Halogen bulbs use tungsten filaments like standard incandescent bulbs and are hard to use because the light is a result of the heated element and in the 60HZ system the element does not cool down sufficiently between cycle peaks to produce the pronounced on/off effect needed for a usable stroboscopic effect.
 
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Yes, I've done all of the adjustment procedures, most of them multiple times. I even tried doing them while scoping the signal to see if it was altered; it wasn't.

I have replaced all of the electrolytic caps on the board. There are a few ceramics but they are just used for decoupling and I don't think they could be the issue. There are 4 that I think are metal film. I didn't replace them initially because these usually don't go bad, but now I'm definitely considering it. They're used in the feedback loops of a couple opamps so I think you're right: they could be contributing to the problem.

Thanks for the tip about the halogen lamp. I'll probably stick with the scope for now since I don't have a florescent.
 
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