Radio Broadcast (May 1929-Apr 1930)

-z. RADIO BROADCAST that no sacrifice of this characteristic be made due to low field excitation, a separate power supply unit is provided to furnish field current. This unit includes a power transformer, a 280-type rectifier, and a filter capacitor. The use of the extra rectifier tube allows full voltages to be applied to both the output tubes in the chassis and to the loud speaker field without overloading either rectifier. The operation of this receiver is very simple and extremely convenient. As stated above, there is a "visual tuning meter" provided which indicates when the received station is tuned to resonance. This arrangement is essential in a receiver employing automatic volume control as when a powerful station is tuned in, the control functions to keep the volume from the loud speaker at the same level while the dial is rotated over the audible range of that station. However, it can be noticed readily that the fidelity of reproduction is correct only at the resonance point. For this reason the meter is provided in order that proper tuning can be obtained easily and accurately. It should be remembered that the automatic volume control circuit varies the sensitivity of the receiver in proportion to the strength of signal. That is, when a strong signal is received the receiver is relatively insensitive and when a weak station is received the sensitivity is increased. Thus, when no signals are being received the sensitivity is at maximum, and when tuning from station to station the sensitivity is extremely high. In order to achieve "silent tuning" whereby the listeners would not be an noyed by "static noises" picked up on account of this high sensitivity and by undesired stations, a "Silent Key" is provided. This key shorts the moving coil of the dynamic speaker through a predetermined resistance. This arrangement allows just enough signal to come through the loud speaker to let the operator know FIRST AUDIO AMPLIFIER TUBE Fig. 8 — Top view of chassis with cover over radio and detector tubes removed, showing the location of all the tubes and parts on the chassis. if the carrier wave he has tuned in by the "tuning meter" is being modulated. The necessary resistance in this case is obtained by properly proportioning the lead wires to the key. The manual volume or level control, in a receiver employing the automatic volume control circuit, must be a control to vary the signal input to the audio amplifier as the signal level at the detector is practically constant when the received signal is at or above the value that actuates the automatic control. Of course, the detector can never be overloaded due to the action of this automatic control. Normally, the manual volume control is set for the desired room level and does not need to be touched unless a station is tuned in which has very low percentage modulation or the signal strength goes down below the value required for operation of the automatic control. In regard to the latter case it should be emphasized that such a signal' is a very weak one, and on most receivers would not be "received" at all. The "Sensitivity Control" previously mentioned is a potentiometer connected in such a manner as to vary the input from the antenna to the r.f. amplifier. This is used to reduce any extremely strong local signals which would cause the overloading effect in the r.f. amplifier (indicated in Fig. 5) when signals above 100,000 microvolts are received. In locations where there are a number of powerful stations nearby, it is advisable to keep this control partially turned down all the time, particularly when only the local stations are desired. Of course, when very distant stations are being searched for, this control should be at maximum sensitivity. One can readily check the action of the automatic volume control by tuning in a strong signal and varying the sensitivity control. The signal from the loud speaker will not change over a wide range of movement of the dial, but the visual tuning meter will vary, indicating the different strengths of signal supplied from the antenna to the r.f. amplifier. The capacityresistance series network between the (Concluded on page 366) Fig. 9 — Complete schematic diagram of the Stromberg-Carlson No. 846 receiver, showing the circuits of the chassis, the loud speaker, and the speaker field power supply. • OCTOBER 1929 • • 357