Strobe instability

Sam Cogley

Sam Cogley

Last of the Time Lords
Subscriber
I was listening to "Days of Future Passed" on my PS-X70 today and noticed that even after the recap, it still has a hint of warble. I didn't get the same sound on my QL-L2, so I tossed a strobe disc on it and found a similar but slower wobble (apparently slow enough to become inaudible). The PS-X50 has a similar slower-speed wobble. Scratching my head, I dug out the Technics SL-QD33 TT I've never tested and plugged it in - sure enough, same problem. I remember the SL-D2 having a bit of the same issue, too. What's going on here? Frequency instability on my local power main? Surely five different turntables don't have the exact same motor problem. I don't think I have any way to measure the frequency of my wall current...

So, question 1 - has anyone ever figured out a good way to lube these sealed "lifetime lubrication" motors? Question 2 - how do I measure the frequency of my local mains power, and if it's causing problems, how do I eliminate the trouble?
 
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Measuring the AC current frequency is possible using a frequency counter (suitable for 110V signal), but it will simply give you what you expect (60Hz or so) and probably no wobble - even if it is there as brief disruptions are too brief to be shown after the averaging the meter does to count pulses.

However, I would like to guess that it's probably the TTs. Wear and tear on the shaft and bearing, could cause that. A couple of my TTs don't show this brief reduction in speed. Others do.

And as you mentioned, you have observed a brief slower-speed wobble. If it was AC frequency instability, shouldn't it get faster and / or slower at times rather than always slower?

Anyways, just a late night wild guess.
 
SaSi said:
Measuring the AC current frequency is possible using a frequency counter (suitable for 110V signal), but it will simply give you what you expect (60Hz or so) and probably no wobble - even if it is there as brief disruptions are too brief to be shown after the averaging the meter does to count pulses.

However, I would like to guess that it's probably the TTs. Wear and tear on the shaft and bearing, could cause that. A couple of my TTs don't show this brief reduction in speed. Others do.

And as you mentioned, you have observed a brief slower-speed wobble. If it was AC frequency instability, shouldn't it get faster and / or slower at times rather than always slower?

Anyways, just a late night wild guess.
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It's more like an oscillation than a brief slowdown. For whatever reason the X70 does it a bit faster than the other four, but I can't figure out why four turntables would have the exact same problem, even down to a matched rate of speed (with the one noted exception). That seems to be pushing the boundaries of "coincidence" too far.
 
Hi Sam

The thing that jumped to mind when you said all motors slowed (never went over speed) occaisionally (randomly?) were voltage sags in your mains. Could this be a possibility? Maybe a good multimeter will shed some light on the problem?

--Jeff
 
I didn't say they never went over speed - the strobe oscillates on all five TTs. The X70 just does it twice as fast. The blocks aren't moving forward or back continuously, they just wobble.

Power is pretty steady at 124V. I'm not sure when it jumped up, it used to sit right around 114.

I'm going to drag a Variac inside and see what being driven by a transformer does to the problem.
 
Drug a Variac (with a VAC meter on the side) over to the Sony TTs and plugged them in. With the variac right at 120 and the needle steady as a rock...nothing changed on the TTs. Either I have some sort of weird fluctuation in the stability of my power supply or I have five TTs with an identical problem.

Next step, I'm going to plug one into a UPS that's disconnected from the wall.

That didn't help, either. Looks like I have a bunch of weird issues, though only the X70's is audible. Anyone managed to lubricate one of those Sony BSL motors?
 
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OK - I'm a bit confused. When you say that you tossed a strobe disc on 'em, does that mean that the fluctuation was only visible under an external incandescent/florescent light? If so, I'd guess that the turntables that aren't exhibiting an audible wow are spot on, & you're just seeing an oscillation in the AC frequency with those quartz locked tables. Sounds like your PS-X70 may the only one with a real problem.
 
scotthal said:
OK - I'm a bit confused. When you say that you tossed a strobe disc on 'em, does that mean that the fluctuation was only visible under an external incandescent/florescent light? If so, I'd guess that the turntables that aren't exhibiting an audible wow are spot on, & you're just seeing an oscillation in the AC frequency with those quartz locked tables. Sounds like your PS-X70 may the only one with a real problem.
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The only one of the five without a built-in strobe is the QL-L2. On the other four I've used both the internal strobe and the strobe disc, in all cases the results are the same between the disc and the platter edge.

I'm leaning towards a combination of a slight oscillation in the mains frequency and not-perfectly-machined/printed strobe markings for the four with only slightly visible and audibly invisible oscillations. I know the strobe lines on both Sonys are a touch out of round. The X70 has something weird going on, and it's been recapped completely along with a total shotgun rebuild of the previously-fried power board. I think I need to either steal the X50's motor or find a donor for it - the only other possible issue is that something is way out of tolerance on the main board and that's causing the trouble. A shotgun rebuild of that board would be a major undertaking, though - I've seen radios with fewer semiconductors. I'd like to keep both Sonys operational if at all possible, so the motor swap between them seems a bit counterproductive, though it would tell me if I'm looking at a motor or board problem. I'm going to look at the motor in the X70 later to see if there is any obvious way to get inside.
 
Nuts. Hate to give up on a perfectly good (tho' discredited) theory. Should point out, however, that the internal strobe on both of the Sonys, and the SL-D2 are neon bulbs running off of the AC main. I don't have a schematic for the SL-QD33, but thought that generation had switched to LED illumination, running off of the internal frequency reference.

Next question - is the audible warble of the PS-X70 synched to the platter rotation?
 
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Actually, the Sonys use neon bulbs running at mains voltage, but the reference signal is generated internally on the power board and the voltage is filtered. It's a cool system designed for stability even in the face of lousy power supplies.

My eyes and ears aren't that sensitive of instruments, though the warble and the strobe wobble seem to be linked.

I did learn something interesting while pulling apart the X70's motor for lubrication. While powered up but not spinning, there are noticeable strong poles where the motor wants to sit. When the heavy platter is installed, it just flywheels through them, which is why I'd never noticed that issue before. It appears one or two of my motor poles has been cooked, though I don't know how to test the innards to figure out which one is out of whack, nor any possible way to fix it. The motor does have a small circuit board inside with two components on it, though I can't figure out what they are. Tiny little blue SMD components with two leads in and two out. The X50 does NOT exhibit that cogging behavior with the platter off and power on - it just spins smoothly.

If anyone has any tests I can do with a multimeter to figure out what's acting up inside the motor and/or ideas on how to fix it, I'm all ears.
 
Bump to see if the daylight crowd has any idea how to test/repair the non-functioning motor pole.
 
Sometimes the motor coil connections are silkscreened on the board. Ohm'm out and look for opens. Next is drive IC. Unless you have the block diagram for the IC, it's tough to understand. You could measure DC ohms from chip Vcc to output node, then chip gound to output node and discover an output device short. That's all if it uses a motor drive IC. From the first part of the thread, it sounded like dirty speed selector switch symptoms, but roasted coils seems more grim. Is the wow, once per revolution? If so, your probably in the right neighborhood (motor).

Sometimes Hall elements get fruity (noisy) and cause these speed instabilities. I don't know if this BSL design uses them, however.
 
Jon_Logan said:
Sometimes the motor coil connections are silkscreened on the board. Ohm'm out and look for opens. Next is drive IC. Unless you have the block diagram for the IC, it's tough to understand. You could measure DC ohms from chip Vcc to output node, then chip gound to output node and discover an output device short. That's all if it uses a motor drive IC. From the first part of the thread, it sounded like dirty speed selector switch symptoms, but roasted coils seems more grim. Is the wow, once per revolution? If so, your probably in the right neighborhood (motor).

Sometimes Hall elements get fruity (noisy) and cause these speed instabilities. I don't know if this BSL design uses them, however.
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It's definitely one (or two) of the four pole(s). When the platter is off and the table powered up (but not set to run), the X50 (with no wow) spins freely by hand. The X70's motor tries to lock in place at certain points when doing the same thing. There's a strong affinity to sit in two different positions, and it takes a fair bit of force to rotate it out of those positions by hand. I'll open it up and check continuity on the little board inside the motor. Nothing appears to be burned or heat-damaged.

There are no ICs in the design, everything is discrete semiconductors. That's why the X70's main board is such a complicated nightmare. I think my KR-720 receiver has fewer capacitors and small-signal transistors.
 
That cogging you've noticed when there is no platter is exactly the thing that makes belt and idler drive devotees dubious of direct drive. And they have a point -- you can use mass to smooth out those pulses, but that mass and that motor effect are actually like a weight and and spring -- there is a mass where where the motion will be smooth, but less mass or more mass and you get oscillation (of course this is also a problem with belt drives and, at a higher frequency (I think that's what theory suggests) with the limited compliance of idler drives. And the varying drag of the stylus and the record is affecting that motion, but in a constantly changing fashion.
Quartz lock can help smooth out large variations, but its a reactive system, starting after the first variation, and lasting longer than the last variation -- so its a source of error itself. Of course the error may be at a lower order of magnitude and ignoreable, but maybe not.
I have a similar problem with my old Kenwood KD 500 -- I can't hear it, but I sure can see it, and it drives me nuts. The little squares go from standing still to rocking a bit back and forth and than back to still -- Aiieyeeee!
 
Nat said:
That cogging you've noticed when there is no platter is exactly the thing that makes belt and idler drive devotees dubious of direct drive. And they have a point -- you can use mass to smooth out those pulses, but that mass and that motor effect are actually like a weight and and spring -- there is a mass where where the motion will be smooth, but less mass or more mass and you get oscillation (of course this is also a problem with belt drives and, at a higher frequency (I think that's what theory suggests) with the limited compliance of idler drives. And the varying drag of the stylus and the record is affecting that motion, but in a constantly changing fashion.
Quartz lock can help smooth out large variations, but its a reactive system, starting after the first variation, and lasting longer than the last variation -- so its a source of error itself. Of course the error may be at a lower order of magnitude and ignoreable, but maybe not.
I have a similar problem with my old Kenwood KD 500 -- I can't hear it, but I sure can see it, and it drives me nuts. The little squares go from standing still to rocking a bit back and forth and than back to still -- Aiieyeeee!
Click to expand...

Condemning a system based on a broken unit isn't exactly the height of logic. As I stated above, my PS-X50, with the exact same motor, shows no signs of that cogging effect. The X70 was apparently hit by lightning at one point, I had to rebuild the power board from the ground up before it would work at all. It looks like some stray juice got through and zapped part of the motor.

To repeat - with the platters off and the power on (but the motor not engaged), the X50 shows no cogging, it glides smooth as silk. The X70 exhibits extreme cogging to such a degree that it's difficult to rotate the magnet structure/spindle by hand. The fact that I have two near-identical turntables is helping the diagnostics to some degree, but I really don't know much about motor construction.
 
The Strobe on my recently acquired KD 550 wobbles back and forth a bit too - and my Pioneer PL 550 has a bit of a lump in it's strobe as well.

I think with both TTs (one quartz lock, the other not) that the oscillation seems almost timed with the RPMs, which might suggest it's somehow related to the markings machined on the platter (like you suggested S Cogley).

I can't hear the wobble on either model.
 
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Sam, if it is on Nights in White Satin, there is a little warble in the lead tape Pinder used to generate the string section on his Mellatron, so it isn't your TT on that song. Don't go crazy because the "problem" will show up on all your TTs.
 
Dave, thanks for that tip. The noticeable problem is in the organ right at the start of side 1.
 
My comment about direct drives was based not just on the experience of moving platters where you can feel a pulsation, but also on the tendency of the motors without platters to move in very jerky fashion if at all. And if you have taken direct drives apart, you've seen that there are a bunch of coils -- usually six or more, but not an infinite number, and certainly less than in even a low end stepping motor. So I don't have any doubt that the drive is not completely uniform. That doesn't mean its audible. But the wobble in the strobe is visible in mine and I wonder about it.
Specifically my comment was prompted by your mentioning that you had the same problem on all five of your tables and nothing changed when you used the variac. It wasn't solely based on your single apparently damage table. Maybe I misread your post, but it sure sounded like you were wondering about the same thing I'm wondering about.
 
I've never had a wobble in any direct-drive turntable, and I've had some oldies. Belt and idler-driven decks are a different story.

Oh, and there's no way in Hades the AC line will have variable fluctuations in frequency, that would suggest the whole North American net is unstable... regional nets DO slow down even to around 59.92 at times of extreme loads, but that catches up by going over after the surge in demand passes to again balance system phase.
 
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