Audioclinic (Q and A) (Oct. 1970)

by JOSEPH GIOVANELLI

AFC, and Receiver Drift

Q. I have an FM receiver which works beautifully, except that every so often it must be retuned slightly. I can see the tuning shift by observing the center-of channel meter. I thought all modern receivers employed AFC in order to prevent just such drift. However, I am unable to find any reference to AFC being built into this rather expensive receiver. How come?

-Dick Prokopowich, Ashborough, North Carolina

A. Modern solid-state FM receivers do not employ AFC because transistor oscillator circuits are so stable that AFC is not required. Heat was the major cause of drift in tube-equipped gear. Heat is, of course, produced in much smaller amounts with solid-state equipment eliminating the need for AFC circuits.

This is to your advantage, for, as you know, AFC does not correct completely for mistuning. There is always a certain amount of error present. Further, some AFC circuits can lead to a loss of low frequencies and to a small amount of distortion.

You indicated that you retune via your center-of-channel indicator. I wonder whether the mistuning ever reaches a point of producing audible distortion, especially on stereo broadcasts. It might be that your tuning meter is extremely sensitive to small changes of frequency, so that even extremely small amounts of drift show up as large meter deflections. In other words, maybe your "problem" isn't a problem at all.

If your drift problem is indeed serious, then you will have to look into the oscillator circuit in order to bring about a cure of the condition. Oscillator circuits are voltage stabilized. If the regulator supplying voltage to this oscillator is defective, or if some other element in the regulating chain has failed, the oscillator voltage will wander, especially as the transistors heat slightly. This will produce a change in the internal capacitance of the oscillator transistor, which, in turn, will result in oscillator drift. You may need a service manual to determine the correct oscillator voltage. If the voltage is stable, it may be that there are defective temperature-compensating capacitors.

You should also check the bias resistors for proper value because it is altogether possible that the transistor is improperly biased. Further, as the values of the bias resistor change during operation, the resulting bias-voltage change will produce oscillator drift. Also, the oscillator transistor itself might be defective.

Playing 78's

Q. I recently started to acquire a large number of 78 rpm records. Modern amplifiers have no provisions for the equalization curves used with these old disks.

How do I get around the problem? Another question I have relates to reproducing these records from the stylus end of the system. Is a modern diamond stylus of appropriate tip radius suitable or is the old steel needle a more "realistic" answer? Will the diamond stylus of today be too severe on the grooves of yesteryear, and destroy them?

-Samuel Jacobs, Endwell, New York

A. The curves used when recording 78 rpm records were not uniform from one company to the next nor were they seemingly always uniform from one record to the next within the same company. When playing these old 78's, you will find that your best friends are your bass and treble controls.

It would also be helpful if you have high- and low-pass filters in order to eliminate rumble and record scratch. I set my filters so as to remove as much noise as possible, but without reducing whatever high-frequency response was on the records. Better signal-to-noise ratios could be produced at the expense of the highs. I personally would rather have my high frequencies, and put up with the noise. If a disk has highs up to approximately 6 kHz, then the filter should be set to cut off at approximately this frequency. If the frequency response is, say, 3,000 Hz, then the filter settings can be adjusted accordingly, again without degrading the response. This same approach is followed when dealing with bass response.

The biggest problem I have found in reproducing the low end of some 78's is record warpage, and this can be reduced by proper application of small amounts of heat and a small amount of weight placed on top of the warped disks. The disks should, of course, be placed on a flat surface. As to the exact temperature used, I would say that it should not be greater than about 100 degrees F. A few hours is all it will take to remove most warps.

Because of the wide difference in frequency response from one record to another, even when produced by the same company, I simply to do not care what the curve might have been during the recording process. I simply adjust the playback system to suit my taste.

I definitely suggest that you use a modern diamond stylus for playing these old 78's. The disks will both last longer and sound better. Of course, you must use a stylus having the correct tip radius, which is generally considered to be 3 mils.

You might have to use slightly more tracking force than you do with your LP's.

In any case, you will not be tracking at nearly so great a force as was common when the 78 was king. Because of these light tracking forces involved, these disks will last infinitely longer than they would with the original phonograph equipment common in those days, including steel needles. The diamond stylus holds its shape even despite the friction produced by playing the rough surfaces of 78's. Because it does maintain its geometry, there are no sharp edges produced, and this improves record wear. Steel needles do not hold their proper tip for more than a side or two. This, together with heavy tracking forces, ruined many a 78-rpm record prematurely.

Using FET's

Q. Can I directly substitute an FET for a vacuum tube? Arthur Darrow, Albany, New York A. You cannot directly substitute an FET for a vacuum tube, and there are a number of reasons for this.

1. I have not seen FET's which can deliver large amounts of power. Hence, an FET cannot be substituted for an output tube.

2. The voltage applied between plate and cathode of most vacuum tubes is much higher than the FET can withstand.

The voltage must be reduced to a safe level before it can be applied to the FET. The excess voltage must be dissipated in the form of heat, and this might be a problem in some circuits.

3. The FET must be wired into a circuit, whereas most tubes are plugged into sockets. You would either have to wire the FET into the circuit point-to-point or you would have to mount it in an appropriate plug so that it can be fitted into the existing tube socket.

4. In the case of r.f. circuitry, the substitution of the FET will require a complete realignment of the circuit.

5. Circuits of the type normally found in phonograph and tape-head preamplifiers often have feedback to provide the necessary equalization. Some changes in component values for such equalization circuits must be made so that proper frequency response is maintained.

6. Bias values will almost certainly have to be changed.

7. I suggest that you consider only N-channel-depletion type FET's. They are more like tubes in their operation than other types.

(Audio magazine, Oct. 1970)

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