Topics: Platter Speed Stability

Speed stability is critical to sound quality.  What the speed becomes badly unstable, you will get audible effects called wow and flutter.  But lower amounts of instability can affect sound quality more subtly; when I replaced the wide belt on my Empire with a narrower belt, I noticed that the soundstaging had improved markedly.  The sound is now much more three-dimensional.

A customer who recently tried a narrow belt against a wider version said that with the wide belt, the music “just fell apart.”  Turntables whose motor spindles have a convex shape should use narrower belts, because a wider belt will contact different parts of the curved spindle, which are moving at different speeds – and this introduces instability to the speed of, and tension on, the belt.

You can never achieve a perfectly constant platter speed, because friction is  always acting to slow the platter down, while the drive systems always provide accelerating force in pulses.  The speed will average 33-1/3 RPMs, but many variables will determine how far it strays from this speed, and for how long.

Electric Motors

Electric drive motors do not provide constant speed because they don’t make constant power.  Rather, they create pulses of power which speed up the motor shaft.  If a motor’s speed is to be stable, it is because the shaft and the load it turns have a great deal of inertia.  Each individual pulse of power has a relatively small impact on the shaft speed because the mass it’s spinning is so large.  Conversely, each moment it goes without power in between pulses gives a very small opportunity for friction (and magnetic induction) to slow the system down.  The more frequent and small the pulses of power are, the more constant and stable the speed of the motor/belt/platter system will be.

Put a different way, a motor that spins more quickly will provide more constant speed, as will a platter with a greater mass, and whose mass is distributed more to the outer rim.  If you’ve studied physics, you’ll know that this is called a greater Moment of Inertia.

Direct Drive turntables have a motor that is concentric with the platter and therefore must be made to spin at a relatively slow speed, 33-1/3 RPMs.  A belt drive turntable has a small but fast motor and a belt that acts as a reduction gear system.  The inconsistencies in its speed are greatly diminished by the high frequency of the system’s power-speed-up/mo-power-slow-down cycle.  A direct drive provides pulses of power much less frequently and therefore will have greater variations in the final speed of the platter, even with a platter of high inertia.

Motor Drive Types

Belt-driven turntables provide greater speed stability than direct drive or idler wheel systems.  However, belts don’t last forever.  They will stretch, dry out and eventually become slack and fall out of place.  Proper belt size and tension are critical to maintaining platter speed that is correct, constant, and stable.  With a rubber belt drive, the drive belt is elastic.  The elasticity of the belt de-couples the platter and the motor.  When the motor speeds up slightly, instead of transferring the force to the platter in a jerky, jarring fashion, it stretches out slightly and increases the force on the rim of the platter.  When the motor slows down, the belt goes slack and allows the platter to spin free.

Platter Bearing Lubrication

Other factors that can influence speed stability include bearing lubrication and friction in the motor and platter bearings, dirty speed control potentiometers, power supply and power grid issues, and platter mass.

Bearings: most motors and bearings do not have an oil hole for adding fresh lubricant.  However if the motor is old and noisy, you can add some penetrating oil to try to quiet the bearings and reduce the friction.  WD-40 is a tempting choice, however, this product does not provide permanent lubrication.  It is great for other applications, but its lubricant is very light and will evaporate.  It will clean out the dirty old lubricant, but evaporate quickly and leave the bearings dry.  Other spray-can penetrating oils, such as PB Blaster and Deep Creep by Sea Foam, are a much better option, because they will leave some real lubricant behind permanently.  Use very small amounts and use the little red straw that attaches to the spray can nozzle to apply the product to the motor shaft at the spot where it leaves the motor housing.  These are penetrating oils and will find their way into the bearings even if there is no oil hole.  Do this work in a very quiet room and listen for bearing noise before and after applying the lubricant.

If the motor case does have a hole for adding oil, then use a few drops of light machine oil (such as 3-in-1) until any bearing noise goes away.  You can check for bearing friction by removing the drive belt, spinning the platter by hand, and seeing how long it takes to slow down on its own.

Heavier turntables have a shaft attached to the platter which projects downward into a bronze bearing mounted in the turntable base.  These shafts will have a spiral groove that carries lubricant upward to the top of the shaft as the platter spins.  This system is easy to clean out and re-lubricate.  Our 10,000 cSt silicone fluid will lubricate such bearings effectively without the chemical smell of a petroleum based product.  Use a paper towel to clean out the old residue; putting some brake cleaner onto the towel will help dissolve any old oily dirt on the bearing wall and shaft.  You want to add enough oil to fill the bottom of the shaft; adding too much oil will prevent the platter from dropping all the way into the shaft, or leak out the top.  if your bearing has been run without lubrication, then the shaft will have some play within the bearing.  If this is the case, a machine shop can fabricate a new bearing for you on a lathe, from a brass blank.  They will want the original bearing and the platter and shaft to make measurements from.

Dirty speed controls will also adversely affect speed stability by giving inconsistent voltage to a DC motor.  If the turntable does not hold its speed, or if turning the speed all the way up results in the platter remaining too slow, then a worn potentiometer is the most likely culprit.  The only other parts that will wear with age are electrolytic capacitors, which can dry out and fail after about 25 years.  This type of capacitor is commonly used in power supplies, and when they fail they will produce a hum; this means that the voltage output will be very inconsistent.  However these are easy to replace on a circuit board if you know how to solder.  As potentiometers age, dust can gather on the resistor material that the wiper travels upon.  With use, the wiper will wear away the resistor material, and the spring in the wiper can lose its force, producing inconsistent or weak contact with the resistor material.  Special cleaners for electronic controls are available from electronics shops, but as with motors, the critical issue is getting that liquid compound through the sealed case and onto the critical parts within.  These electronic parts were usually custom-made for the turntable manufacturer, and unless that manufacturer offers replacement parts today, it’s highly unlikely that you will find a new potentiometer that will fit directly in place of the old one without modification.  If the pot control is not marked with its resistance value, you can measure it with an ohmmeter to see what the value of the correct replacement will be.

One possibility for keeping a vintage turntable running properly when a direct replacement for the speed control is not available or will not fit into the existing console is to run an outboard box for the controls, and patch new potentiometers into the existing circuit with an umbilical cord.

Replacing the Drive Belt

Turntable Tuneup stocks the widest selection of turntable belts available, and has had a few models custom-made for certain applications.  All of our belts are newly made, custom molded (never glued) from synthetic rubber compounds known for longevity and proper friction.

We have flat and o-ring belts for platter drive systems, as well as small square-cut belts for driving cueing mechanisms, auto-return and other control mechanisms.  In all, we stock more than 300 different belt models.

To find the correct belt for your turntable, first click on your turntable manufacturer, and from the brand name page, choose your turntable model.  If your model is not listed, don’t worry.  We can easily supply the correct belt based on your measurements of the original belt.  If the belt is missing, it is easy to measure the belt’s path using a piece of string.  Belts will stretch into place, so the correct belt will be about 3 percent shorter than the measured distance of the belt’s path.  Belt lengths are measured from the entire belt circumference.  Each belt model we offer includes the belt length, in inches.

To measure your original belt, mark a point with a paperclip and use a ruler to measure the entire distance.  The belt should be held taut, but not stretched, when you measure it.  Or, you can use two pins, or bent paperclips, to straighten the belt out from the inside, and then double the distance between the pins when the belt is held taut, but not stretched.

Most platter drive belts are installed underneath the platter.

Belt Width

Many turntables have motors whose speed is not adjustable; these are called histeresis motors; their speed is determined by the frequency of the AC current that feeds them.  These motors are typically fitted with convex drive spindles, and the platter speed is adjusted by changing the mounting angle of the motor on the turntable chassis.  If the spindle has a convex shape, then you may do better with a narrower belt.

When you use a wide belt on a convex motor spindle, different portions of the belt are touching the spindle at different points, which have different tangential speeds.  The spindle will be moving different portions of the belt at different speeds, and that will result in chatter.  A narrower belt will be moved at a more consistent speed because it’s contacting the spindle along a narrower contact patch, with a narrower and more consistent tangential speed.

With a DC motor that has a speed adjustment knob and a flat-profile motor spindle, this is not an issue, and a wider belt will probably last longer and withstand greater tension without aging.

Whether your belt is old and stretched out or not, you may be well-served to replace your platter drive belt with a narrower version.  All of the flat, narrow belts carried by Turntable Tuneup will have a part number beginning with FRZ or FRX.  Medium-width belts have prefix FM, and wide belts have the prefix FRM.  The number following the prefix indicates the belt’s circumference (measured while taut but not stretched out) in inches.  Every belt model is listed along with its circumference, width and thickness.

Belt Tension

A belt-drive system utilizes the elasticity of the rubber belt to absorb the variations in speed.  The motor spindle will spin at a slightly inconsistent speed due to the nature of electric motors.  As the motor speeds up, it will pull on the belt, but the platter can’t be sped up very quickly due to its high moment of inertia.  With a belt drive or idler wheel drive system, the speed will be much more jerky, because there isn’t the shock absorber effect provided by the length of rubber belt material between the motor spindle and the platter flange.  Belt drives use elastic coupling to provide better speed stability.  Getting the correct tension in the belt is critical to tuning the speed stability.  A belt that is too tight will not only produce more unstable speed; it will also make the platter run too fast.  A belt that is too loose will slip over the motor spindle because there isn’t enough friction between the belt and the spindle.