Audioclinic (Q and A) (June 1970)

FM Oscillator Problem

Q. I have a stereo receiver. On occasion, and lasting for varying periods of time, we lose our FM sound, but only at the lower frequencies beginning at about 92.5 MHz. Above this frequency the sound is never lost.

It is not a question of the antenna because the FM .radios in the same area do not exhibit this problem. I have taken the set to an authorized service station and to the manufacturer, but the same trouble still occurs. Can you tell me what the problem is?

-Andrew A. Sandor, M.D., Corona Del Mar, Cal.

A. When a tuner operates properly at some frequencies and not at others, this condition can be traced to a fault in the h.f. oscillator circuit. As the tuning dial is turned to reduce frequency, the amount of feedback in the oscillator tends to fall off somewhat. If the oscillator is just on the edge of operation, any reduction in feedback will cause it to cease oscillating. That is the reason your tuner works at the higher frequencies, but does not work at lower frequencies.

From what I have seen so far, most of the troubles of this kind are caused by bias resistors which have changed in value. The bias network should be replaced. This replacement will likely solve your problem. It is also likely that the calibration of the tuning dial has shifted. The oscillator will need to be realigned.

Presumably, the service shop already tried a new oscillator transistor. Assuming that the bias is correct, that would be the next logical step.

These oscillators are often of the grounded-base variety. The base is not directly grounded, but is bypassed to ground. This bypass capacitor might be defective. If all else fails, replace it.. Of course, if you have an oscillator which is marginal in its operation, coupled with a home in which the line voltage is somewhat low, then the tuner might not operate properly in your home, but might work all right in the service shop. That might be why you did not have any success when sending in the equipment for servicing. Oscillators are usually regulated by Zener diodes in order to hold their voltage. However, if something in the power supply has shifted so that the Zener is not working properly, then your tuner's oscillator voltage will not be stable.

Minimum Speaker Driving Power

Q. I am confused as to what the minimum power input to a speaker means exactly. For instance, if a speaker requires 20 watts rms minimum, does this mean that 20 watts are needed before the speaker will "drive"? If it does not, will the sound be distorted if less than the hypothetical 20 watts are applied? Also, assuming that 20 watts are not needed to drive the speaker but are needed for good response, will volume have to be high?

-Sgt. Spence Blakely, APO San Francisco, California.

A. When we talk about the minimum amount of power for a given speaker, we are discussing the minimum power which will provide reasonably loud listening in an average listening room. If you do not plan to do much loud listening or if your listening room is a small one you will run your speakers at considerably less than this 20-watt minimum you mention. The reason for providing this kind of information in the speaker specifications is simply to give the potential buyer the information needed in selecting his amplifier. You know that you need a minimum of 20 watts per channel in order to drive our hypothetical pair of speakers. Unfortunately, we do not always know if this is 20 watts rms or 20 watts music power. I would say, therefore, that you should assume that the speaker manufacturer means the rms rating if no other indication in this regard is provided. When the speaker is run at less than this 20-watt level, it will sound fine. It should not be distorted.

Two- and Three-Way Speaker Systems

Q. What is the difference between a two way and a three-way speaker system?

-Sgt. Spence Blakely, APO San Francisco, Cal.

A. A two-way speaker system is one which includes a woofer and a tweeter. The sound spectrum is divided between these two speakers.

A three-way system contains a woofer, and midrange and tweeter speakers. The audio spectrum here is divided up into three narrower slices than was true of the two-way system. The names of the peakers imply the range of audio frequencies they cover.

You will never need to be concerned with this aspect of speaker design.

Whether a manufacturer employs a two or a three-way arrangement is a matter of the engineer's preference and sometimes his budgetary requirements. Each type of speaker system can be excellent.

Your only clue about a speaker's performance can come from listening to the unit. If you like it, then this is the speaker for you.

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(Audio magazine, June 1970)

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