Audioclinic (Q and A) (Sept. 1972)

FM Receiver Overload

Q. I think that I have a problem with my receiver. I receive a number of FM stations at more than one dial location. The extra locations usually appear at some frequency where a weak station is located I receive them both mixed together.

I wrote to the manufacturer. They sent me a new front-end assembly which did not solve the problem. I wrote again; they suggested that my problem was overloading caused by a strong, local station.

I am located about 30 miles north of Boston. I am using a good outdoor FM antenna. I do not have any strong, local stations nearby. All the stations which appear at more than one dial location originate from Boston. I wonder if what I am experiencing is normal or do I have a problem in my receiver?

-Lance Boe; Methuen, Mass.

A. Because you are 30 miles from the stations which are causing your overload problem, I have to think that something is wrong with the receiver. However, before I get into that area too much, it might be worthwhile for you to check to see if perhaps the Boston stations have their transmitters outside the city limits--NORTH of the city. You can see that this would mean that the stations would then be considerably closer to you than you had suspected.

Assuming that the signals are, indeed, 30 miles from you, I would not think you should have overload problems, even with a relatively good rotable antenna. Of course, if you have gone to multiple stacking and have an extremely high mast or tower, then overload is possible.

I suggest that you return your receiver to the manufacturer for a checkup, especially if it is still within the warranty period.

If your antenna is rotable, try swinging it away from the Boston area and note what happens. If the condition clears up, you can at least use the receiver. As an alternative, you can insert specially designed attenuators at the antenna terminals of your receiver.

This will enable you to adjust your antenna for best reception while keeping signal strength below possible overload levels.

When stations come in together even though their frequencies are widely separated from one another, that is a sign of what is called "cross modulation." This generally does suggest some kind of overload condition. Under normal circumstances, I don't think overloading should occur.

I wonder, therefore, if AGC is supposed to be applied to the front-end of your tuner. Lack of AGC could cause this problem because without it, the front-end could be overloaded even when receiving even moderately weak stations. Check to see that proper AGC voltage appears where it's supposed to appear. If your receiver was originally a kit, check for any wiring errors which could lead to this lack of AGC. Check for splashes of solder on the foils. Check for shorted AGC bypass capacitors or resistors of improper value.

Wow and Flutter

Q. What are "wow and flutter"?

-Sgt. Paul Bonney, APO S. F., Cal.

A. "Wow" and "flutter" refer to speed variations which occur in tape recorders and turntables. These pieces of equipment are intended to drive the tape or disc at a constant speed, but they do not, and to the degree that they do not, there will be variations in musical pitch which are in proportion to the speed variation. If the variation is slow, it is called a "wow"; if it is fast, it is called a "flutter." These speed variations are measured in percentage which they deviate from correct speed. We would like to see variations kept to within 1 or 2 tenths of a percent and better if possible. Of course, tape machines designed for speech recording do not need the speed steadiness that is required of machines which are designed to reproduce music.

When the speed variations are extreme, there is an audible wavering of pitch, especially noticeable on sustained tones such as those produced by the piano or the clarinet. However, even when wow and flutter is below really audible levels, it can still add a quality to the sound which decreases transparency.

(Audio magazine, Sept. 1972)

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