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In the Forum: Analog Playback
In the Thread: Copper Mat on a Micro Seiki Gun Metal Platter

Post Subject: Slippage?Posted by Wellington on: 2/2/2016

On the topic of string slippage: I don't think my string, the original one from MS by the way, slips at all once the platter is up to speed. It would slip on the motor pulley when first turned on, as the heavy platter must be gradually brought up to speed, unless you give the platter a spin by hand before turning on power, as I do. Once up to speed, "stiction" (static friction) holds the string in place on the pulley and the platter. This is easy to verify on the platter side because you can watch the platter and string up close and see that they are moving together. It harder to see what's happening on the pulley side, so I ran a test. In the course of rebuilding my motor electronics (mostly replacing all electrolytic caps preventively), I monitored voltages inside the complex drive electronics while putting the table through its paces. The tachometer provides feedback to an error amplifier and then to series-pass power transistor emitter followers that drive the motor windings. I put an oscilloscope probe on those points (and others) while varying the load on the motor. As expected, when the motor encounters more friction, the voltage jumps up to compensate, keeping speed constant. It's a very sensitive system. I watched this voltage on the scope while gently touching the platter. The voltage responded immediately. Even the tiniest touches on the rotating platter, that I dare say nobody would hear as pitch changes, were responded to, immediately. If the string were slipping, this response would not be so immediate nor so proportional. I ran another test. I adjusted the motor speed control to give a platter speed of exactly 33 1/3 rpm using a stroboscope, at my average string tension setting. I then watched the stroboscope as I slightly varied the string tension by pressing a shiny screwdriver shaft against the moving string midway between motor and platter. I also very slightly nudged the RY-5500 motor closer to, and then farther from the platter while monitoring the stroboscope. The speed remained constant. If the string were slipping, the motor's tachometer would not "know" what the platter's speed was and it wouldn't be able to correct it. Tiny friction changes and speed variations caused by motor cogging, stylus drag and external vibrations are simply not enough to break the stiction bond between the string and the motor pulley or the platter, as long as the string tension is properly set over a wide set of tensions. Slippage would make no engineering sense that I can see. It would make speed setting uncertain, and it would result in scraping noise as the stiction is overcome (like a violin bow scrapes on the strings). Plus it would cause excessive wear on the string and pulley! My Kevlar string is probably close to 40 years old and it looks almost new.
Of course there are many ways to set up this turntable and its string. Then most critical factor is string tension. The string has a tensile strength of 24.5kg (54 pounds), so that there is a wide range of string tensions possible between total slippage and the string breaking. MS had little to say about it in the RX-5000 manual. I've set my string's tension so that there is about a half-inch of deflection midway between motor and platter with a light sideways touch of the finger. Not very scientific, but it works. It's not so tight that it's putting a significant side load on the bearings, nor is it so loose that there is any slippage once the system is up to speed. Like many things with this amazing table, it is probably not very critical over a broad range.
Here is what MS had to say:

* Positioning the string>>

First, the installation location of the platter unit is determined and the unit is secured. The motor unit is then provisionally placed between 10 and 15 cm away. The string is then cut to a length enabling it to pass outside the pulley and the platter perimeter, and bound firmly. The end of the string projecting from the join is then cut off with scissors. After checking that the string has been positioned around the platter and pulley, the motor unit is gradually moved out and the string pulled taut. Make sure that you do not make the string too tight. Once the platter rotates, the inertia will have an effect and so there is no need for the string to be pulled too tight because the power is transmitted. However, if the platter is helped to rotate by hand when it first starts up, it will reach the rated speed smoothly.>>

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