Audioclinic (Apr. 1981)

"S" Me No Questions...

Q. My new tuner sounded fine for several weeks. Now it has a strange type of distortion. Music consisting of low fundamentals with few overtones sounds fine. However, on higher pitched musical tones, especially those having many higher harmonics, the tuning indicator shows mistuning. Interstation hiss is present, and there is considerable distortion, particularly with the letter "S" during spoken broadcasts.

Can you make any kind of diagnosis from such a short list of symptoms?

-- Name withheld.

A. Since your problem is one which I have not had the "pleasure" of tackling, I can only conjecture as to its cause.

My first guess is that there is instability in the i.f. amplifier stages. Assume that the i.f. stages oscillate when no signal is present; assume also that when a signal does appear, the gain of the i.f. stages is reduced because of a.g.c. and that the oscillation or tendency to oscillate is minimized. When modulation is low, all is well. When modulation is near 100 per cent, however, the tuner is not always tuned to the center of the channel be cause the signal itself is moving ±75 kHz. This is the same as saying that the tuning dial is moving rapidly over this 150-kHz bandwidth. Since the tuner is on the verge of oscillation, it is obvious that the i.f. stages are regenerative and that the tuner is sharp in terms of selectivity -- sharper than it should be for the i.f. system to accept the full bandwidth of a well-modulated FM signal. The momentary extremes of carrier deviation which bring the carrier 75 kHz to one side or the other are sufficient to result in a weakening of the signal presented to the i.f. stages; as a consequence, they start to oscillate. Further, this oscillatory condition sharpens the i.f. stages still more, with the result that the signal completely disappears from the i.f. bandpass for an instant. This, in turn, causes the hiss you mention. If I am correct, the hiss is not steady and is interrupted by the modulation, but so rapidly as not to be noticed.

The cause of such i.f. instability can result from bad grounding of a component or shield partition or possibly a defective bypass capacitor.

My second guess is that it has some thing to do with the power supply. Assuming that the regulator in the power supply has failed, the power supply volt age will increase, and this increase could result in the situation you describe.

There could, however, be another con sequence of an unregulated power supply. If, for some reason, modulation peaks result in a change of load on the supply, these changes could result in changes of power supply voltage. Such voltage changes would affect the frequency of the tuner's local oscillator, and these momentary oscillator frequency shifts could result in temporary mis-tuning, as shown by your indicator. Obviously, this too would result in a tremendous amount of distortion.

A quick voltage check should reveal whether the power supply is, indeed, supplying a voltage which is considerably higher than that called for in the ser vice manual.

AM I on Your Wavelength?

Q. I read with interest your discussion of FM and AM signals in "Audioclinic, " March, 1980. One point puzzles me, however; you stated that for FM reception, the receiving antenna must be in the "line of sight" of the transmitter, implying that the reverse is true for AM reception.

Why, then, will an AM broadcast received in a car be cut off or severely attenuated while travelling under concrete bridges, through tunnels, etc., while FM signals won't be?

-Alex Schuman, Latexo, Tex.

A. "Line of sight" refers to any signal path which stretches directly be tween the transmitting antenna and the receiving antenna and which does not go over the horizon. This term does not take into account physical obstacles, such as a bridge or tunnel, limiting the radio's visibility of the broadcast antenna.

Higher frequency radio waves (such as broadcast FM) have a way of penetrating obstructed locations where longer wavelengths (such as broadcast AM) can't. This is why FM stations can be heard when driving in an underpass, and AM stations cannot be heard. On the other hand, AM stations can be heard over the horizon, or out of the line of sight, while FM stations cannot.

(Adapted from: Audio magazine, Apr. 1981; Joseph Giovanelli )

= = = =