Audioclinic (Q and A) (Jul. 1970)

Rise Time

Q. Would you please explain what rise time is?

-Michael Speyer, Seattle, Washington

A. You have heard the term "transient response" used in discussions of high fidelity equipment.

This term is very closely interrelated with the term "rise time." If a sudden impulse is fed into the input of a piece of equipment, that same impulse should be delivered to the output of this equipment without waveform distortion. In other words, if the input waveform has a square waveform, the output waveform should also be a square wave.

If the output signal is not a square wave, this is an indication that the piece of equipment under discussion does not have good transient response, at least above a certain frequency. Because of a deficiency in the equipment, the waveform does not build up as quickly as is demanded by the input signal. To put this in another way, the waveform rise time at the output of the equipment is slower than the waveform rise time at its input.

Resetting Turntable Speed

Q. I just checked the speed of my turntable and found that it was running at 35 rpm rather than 33-34 rpm. Is there any way to adjust the speed?

G. V. V., Omaha, Nebraska

A. About all you can do to decrease the speed of most turntables is to grind clown the motor shaft slightly. Of course, there are tables which are driven from built-in oscillators. If your table is one of these, you should determine whether it is oscillating at the correct frequency.

It will be impossible for you to make such a frequency check without the aid of a frequency counter. If none is available, I suggest that the table be returned to the factory for adjustment.

Before doing anything to your table, check it with a strobe disc. This disc will immediately indicate the correctness of its rotational speed. If you timed the speed of the turntable with a stopwatch, it just might be that the stopwatch was running slowly.

Once you know that your table is running fast, you can then think about grinding clown the motor shaft in order to bring its speed to its proper value.

Remove the turntable. Hold the drive puck away from the motor shaft. A rubber band slipped over the drive puck assembly and anchored to some part of the turntable assembly will serve nicely.

Where the turntable was designed to operate at more than one speed, the motor shaft is arranged in a series of steps. The larger the diameter of the step, the higher the speed of turntable rotation.

Before attempting to make speed adjustments, be sure you know which step corresponds to the speed on which you are working.

You are now ready to begin grinding the step. Use a piece of emery cloth for this purpose. Turn on the motor and, while it is rotating, apply light pressure against the step to be ground down.

Don't try to rush; it is much better to take a little longer and do a good job than it is to rush the work. You could grind off too much of the shaft, and the turntable would then rotate too slowly.

In that event you would have to machine a bushing to fit over the step. Even at that, the bushing might be eccentric or it might not even be possible to attach it.

In that case, you would have to get a new motor.

From time to time you must check the turntable speed when the table is running ever so slightly fast, you should use crocus cloth as the final grinding agent. This will polish the shaft.

The grinding action produces particles which can easily find their way into the motor bearings. Therefore, take the motor apart and clean the bearings very thoroughly and then re-lubricate. Some motors do not normally require lubrication. However, the lubrication in such motors can dry out, especially if you use a solvent to flush out dust particles. It is best to soak the bearings of such motors over night in oil. These bearings are designed to take up oil and retain it.

Remove excess oil and reassemble the motor. Your turntable should now be capable of delivering many hours of reliable service.

by JOSEPH GIOVANELLI

(Audio magazine, Jul. 1970)

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