The Turntable Story

 

How Turntables Work

The ordinary shaded-pole motor, which runs your electric drill or power saw, is no good for such precision work because any variation in the voltage of your house current will change its speed. Most turntables use a specially designed "induction" motor which is fairly stable in feed, though extreme changes in line voltage may disturb it (look for a tag stating its requirements: "95-130 volts" means disaster-proof). Even this isn't absolutely steady. The 60-cycle alternation of AC electric supply, however, is invariable (an electric clock practically never goes wrong), and thus a "synchronous" motor, which decides its speed by the frequency of alternating current, can keep a constant rpm unless a complete power failure occurs. It also eliminated the dangers of turntable rumble and extruded "hum." Getting this constant speed of the motor up to the turntable (in three different varieties) takes considerable ingenuity.

Today's best and most expensive turntables use one of five methods to translate motor speed into turntable rotation. On the Rek-O-Kut and the Garrard the power gets to the turntable by means of a "rim drive"; that is, the final agent is a bard-rubber drive wheel which locks into position between the motor's axle spindle and the inside rim of the turntable. This is the most common way of making a turntable spin. Usually the spindle, the upward-protruding end of the motor shaft, is cut in "steps" to three different diameters. The speed-control knob locks the wheel against one of the three steps. When the wheel locks against the part of the shaft with the greatest diameter, the turntable spins most swiftly, and so on. A conical or tapered spindle may be used to give continuously variable speed---anywhere from 15, say, to 100 revolutions per minute.

There are several variations on this procedure. Rek-O-Kut, for example, locks wheels of different diameter against a one-size spindle; the new Weathers uses a ceramic disc instead of a rubber drive wheel, and attaches the disc directly to the motor shaft. The D & R applies to the drive wheel to the outer rather than the inner rim of the turntable. On the Scott the turntable drive is direct: that is, the drive shaft of the motor locks into one of three gears on another drive shaft, which in turn is geared to the center of the turntable. The Components Corporation uses a linen belt which fits directly onto the drive shaft (at one of three diameters) and then fits around the circumference of the turntable. The Fairchild runs the belt inside, to a cast-iron flywheel below the table. There are arguments for and against each of these methods. The Components Corporation gets the motor farthest from the turntable and the pickup, thus minimizing the danger of noise from the motor. For the same reason, though, it is rather bulky and unattractive, and requires the most elaborate mounting. Direct drive uses metal parts only and can thus be machined to the closest tolerances. It also lasts longest, at least in theory---but not necessarily in practice. And when something goes wrong, the repair may be expensive. Rim drive requires occasional replacement of the rubber-tired idler wheels and drive spindle-tops. It is, however, the easiest to repair.


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