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The difficulties of accurately tuning modern superhet receivers are eliminated by this ingenious "tuning corrector."

This tuning corrector is fundamentally a plate current meter with a small condenser mounted in place of the indicating needle. The action is a form of "bridge" which seeks a balanced condition.

The New “Self Tuning” Radio Receiver!
Radio-Craft, January, 1936

The importance of having a receiver of the super-heterodyne type, whether it is operating on the broadcast band or one of the many short-wave channels, tuned exactly to resonance with the station, has been recognized for some time. It is for this reason that manufacturers include visual tuning indicators of the meter, neon and cathode-ray types.

However, a new circuit has been developed in Europe to eliminate the need for such exact tuning–and still prevent the introduction of distortion which was encountered in sets, up to this time. The new device–which automatically tunes exactly to a station carrier–was one of the outstanding attractions at the recent Paris Radio Show.

Fig. 1. Two circuits illustrating the electrical action.

A glance at Fig. 1A will explain the basis upon which the unit operates. A sensitive galvanometer movement is connected in the plate circuit of an R.F. tube and to this galvanometer is connected one plate of a small variable condenser, which is shunted across the regular tuning condenser of the amplifier. Now, as a station is tuned in, the plate current of the tube increases. This makes the meter movement operate, tending to increase the capacity of the small condenser. If the main tuning condenser is off-resonance with the station in such a way that an increase in capacity will bring it closer to the resonance point, the movement of the meter (and small condenser) will tend to increase the signal strength which will keep the meter turning until the signal strength no longer increases, at which time, the meter movement will cease to turn. If, on the other hand the condenser is off the exact tuning point on the other side of resonance, the increase in the movement of the meter will reduce the signal strength and cause the meter to return toward normal position (set by the spring shown in the heading illustration, which will cause an increase in signal strength and thus a state of balance is reached.

In actual practice, the "automatic tuning circuit is made in the form shown in the headings illustration and is connected across the plates of the oscillator tuning condenser, which is the most sharply tuned in the usual superhet receiver. One coil on the movement is connected in the "B" return of the entire radio receiver, to activate the core, while a second coil (the moving one) is connected in the "B plus" I.F. lead. By carefully adjusting the spring tension and the position of the moving plates (which are normally at 1/2-maximum capacity) the effects of mis-tuning can be entirely done away with.

Additional Resources:

Self Seeking Circuits
The 1936 receiver requires careful tuning if the best tone quality is to be obtained. Manufacturers have tried all kinds of systems, tuning meters, squelch systems, neon lights and the magic eye, to make the customer tune the station right "on the nose" but the majority of listeners are not using these devices to their full advantage.


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