Radio broadcast .. (1922-30)

How to Build a One-Dial Reflex 729 enough to light the filament or the tube will be burned out. A negative C battery is used to cut down the B battery consumption and to improve the quality of signals through preventing distortion. To operate at its best efficiency, the set should oscillate or be very near the oscillating point over the entire range of broadcasting wavelengths, when the balancing condenser is at its minimum. At this point the dial controlling this condenser should read 100 degrees. When this dial reads near zero, oscillations should not occur at any wavelength. TUNING THE method of tuning this set is very simple and can be mastered in a few minutes. It will be observed that the large dial at the left is labelled "Tuning." The next dial to the right is labelled "Volume." To the right of the dial controlling volume are three rheostat knobs, which control the current in the filaments of the first, second, third, and fourth tubes. Immediately below the volume control dial is the filament current control switch. Continuing to the right of this switch and below the first rheostat knob is a jack for use of phones with only one stage of amplification. Then there is a second jack for the speaker, using both stages of amplification and the last jack employs all four tubes. At first it will probably be best to try to tune the set with powerful near-by stations. To do this you set the volume control dial near zero and slowly rotate the tuning dial. Then having picked up a local station, by increasing the reading of the volume control dial, the signal should come in loud and clear. The setting of the tuning dial should be recorded and thereafter that particular station should always be found at that setting. To obtain maximum volume on distant station reception the procedure is slightly different. In this case it is best to set the tuning dial approximately at the desired wavelength and for each of these settings increase the volume control dial until you get a hissing noise. If you go beyond this point you will get an audio note and the volume dial setting should be reduced. Generally THE BALANCING CONDENSER When fully opened as shown has a minimum capacity reading. The distance between the edges of the two plates is | inch. The semicircular movable plate and the narrow \ inch wide plates make up the full circular form if inch in diameter. The fixed plates are spaced \ inch apart and one semi-circular plate rotates between them. The plates are mounted on a bakelite base by means of bolts, spacers, and a shaft of the required thickness. All these parts may be secured from a radio shop with average stock speaking, for every setting of the tuning dial get the maximum setting of the volume dial. The rheostats should be adjusted on a moderate signal until maximum volume is obtained. At the same time, for the sake of economy the tubes should be run no brighter than is necessary to give satisfactory performance. The following is the list of parts for the four-tube circuit. It is entirely possible to use any standard type of condenser, tube socket, jack, rheostat, etc., when building this receiver, and for that reason, no especial make has been listed. However, for the tuner and radio-frequency transformer units, it is well to use the same parts as specified in the article and used in the construction of the receiver described. If the reader wants to construct his own components, he can readily do so if he follows the constructional data given. From the data given about the radiofrequency coil, it will be observed that this is no ordinary radio-frequency transformer, such as is used for the common radio-frequency amplifier, but must be especially constructed and connected in the circuit as shown in Figs, i and 3. — THE EDITOR.