AUDIOCLINIC (Q and A) (Apr. 1976)

Noisy Antenna Rotator

Q. I use an FM-band antenna, with shielded 300-ohm cable and drain wire, together with a rotator which is noisy only when rotating. The noise is present during rotation whether or not the drain wire is connected to the tuner. The shielded lead-in is new cable and never touches the boom, mast or other metal on its way down to the tuner. It never contacts the line feeding power to the rotator.

Somehow, leakage from the motor must be getting to the tuner's antenna input via the shielded lead-in. I thought at first it might possibly be coming from the rotator control box in the house, but I moved this unit into another room with no change in the noise level. I have also tried bringing the lead-in drain wire to a cold water pipe with no luck.

What do you think is the cause of my problem and do you have any ideas about how I can eliminate the noise?

- Stanley L. Alekman, Newark, Del.

A. I think that the rotator produces the noise and more specifically that the noise is produced by the system which indicates the direction in which the antenna is aimed. I cannot tell whether this noise is an inherent property of your particular rotator, but this may be the case.

Perhaps, if the sensing element of the rotator is a pot, the pot may need cleaning. To do this, the rotator must be taken down and carefully opened. Service notes from the maker will be helpful here.

Another source of noise from such a motor or control unit is arcing, which can be radiated by the motor and therefore be picked up directly by the FM antenna. The cable feeding power to the rotator can also act as a radiating antenna, giving the noise a better chance to interfere. Hence, even if your lead-in cable is well shielded and is grounded both to the mast and the tuner chassis, noise will be picked up.

Try placing 0.1 pf ceramic disc capacitors between each of the rotator terminals and the common terminal. (Do NOT use electrolytic types.) If your rotator has only three connections, use two capacitors; if it has four connections, use three capacitors, etc.

If it happens that you listen to just one station, and if this station has recently begun SCA broadcasts, your tuner may not handle this situation well. Even if your tuner is equipped with the appropriate rejection circuits, they may be misaligned. While you are checking alignment, check detector and MX circuit alignment.

Stereo Interference with Cable Reception

Q. My year-old tuner is connected to cable TV. I get a high-pitched tone when my tuner is in the stereo mode. The filter helps a little, but the sound is still bad. Reception was fine before I got cable TV. What is wrong? What can I do?

-Charles David, Ft. Wayne, Ind.

A. If your FM reception deteriorated immediately upon installing the cable, I would have to think that your stereo interference is caused by the reception of "direct" signals at the same time that you are receiving signals from the "cable." I suggest that your cable company should improve the shielding of any transformers, adaptors, etc., which may be involved with your particular installation. Unfortunately, there are some tuners which are, themselves, not well shielded. In the presence of only moderately strong signals, they might pick up some signal, even when there is no antenna connected to their input terminals. If your tuner is one of these, then there is probably little that can be done to eliminate the problem, assuming that "direct" pickup is the root of the problem.

If you made some modification to the original installation, such as running extension line from the cable installation point into another room, perhaps this is giving rise to the problem. Often such extensions are made with 300-Ohm twin-lead. It is best to have your cable company make the installation properly. (This assumes that the company does put the FM signals on the line.)

If you wish to check to see if this direct pickup is really what is taking place, disconnect the tuner from the cable and connect it to a regular antenna, which may be an indoor diple for this application. Determine whether the stereo is still plauged by the high-pitched whistle. If it is, chances are that there is something within the tuner.

Steps in Tone Controls

Q. I own an integrated amplifier having twin tone controls for both bass and treble which move in stepped increments of ±2 dB. Since the human ear can only detect a change of volume of 3 dB or greater, why are more and more manufacturers coming out with this type of system?

-Gregory E. Gill, Fort Wayne, Ind.

A. Actually, the human ear can ordinarily hear volume changes of about 1 dB and under special conditions can detect changes of as little as 0.25 dB. However, the 2-dB increment is not all that large, so that it represents a good compromise between what is barely noticeable and large steps where the transition is jerky when the control is moved.

An additional factor here is the cost of the various systems. If the steps were made closer together, perhaps 1 dB, the extra cost of added components and wiring would be much greater, significantly adding to the final price of the unit without adding the same relative amount of performance. On the other hand, if the steps are made further apart, a less costly tone control system could be built, but the audible results would be more unpleasant when the controls were adjusted.

(Audio magazine, Apr. 1976, JOSEPH GIOVANELLI)

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