Radio broadcast .. (1922-30)

52 RADIO BROADCAST NOVEMBER, 1925 FIG. 2 The apparatus behind the panel. This view clearly shows the location and placement of the sockets, ballast resistances, and pin jacks on the shelf. Note that the coil units are at right angles to each other the set. Instead of working from a set of blue prints on to a panel, the constructor has the opportunity of buying the panel ready-drilled, together with the other essentials, and merely assembles the rest of the material which goes to form the completed receiver. Brackets, cut and bent to shape, support a shelf upon which a majority of the apparatus, such as tube sockets, audiofrequency transformers, ballast resistances, and the neutralizing condenser are mounted in the completed job. THE APPARATUS USED IN ITS mechanical and electrical design, 1 consideration has been given seriously to the employment of none but the best apparatus obtainable (all of which has been tested and approved by the Laboratory of RADIO BROADCAST). In its present form, the construction and operation of the receiver has been modified to simplicity with the inclusion of ballast resistances for all but one tube, one output jack for the loud speaker, and pin jacks for battery terminals mounted on the rear of the tube shelf. Following the trend of modern design, the receiver has been constructed on a slanting panel and is entirely self-contained thereon, the apparatus being mounted either directly on the panel or on the shelf supported by the brass brackets which are fastened to the panel. High grade audio-frequency transformers employed in the audio amplifier, together with the peculiar parallel arrangement of the last two tubes, insure distortionless quality output. In this receiver the tubes are not situated in the conventional manner, but in order from left to right looking over the top of the receiver are: fust audio, radio frequency, detector, second audio (this last named consists of two tubes connected in parallel). Volume is controlled by means of the filament rheostat connected in series with the filament of the radio-frequency tube. The tuning coils used permit of the reception of signals from those stations which operate on the higher frequencies (low wavelengths) and will amply cover those stations situated at the other end of the scale. The tuning is reasonably sharp on the antenna coil control due to the absence of reflexing. Tuning in the detector circuit is the same as before and is comparable to tuning a regenerative receiver by the squeal method. Briefly, to tune with this method, the tickler is well advanced to produce regeneration and by rotating the detector tuning condenser, squeals will be heard every time the circuit beats with the carrier wave of a station transmitting at that time. Once a desired station is located in this manner, the squeal can be eliminated by loosening the coupling between the secondary and tickler. While, in the standard three-circuit regenerative receivers this system would play havoc with other receivers in the neighborhood, in this receiver, due to the use of the Roberts system of neutralization which is a positive preventive, no squeal is passed along to the antenna to cause disturbance. Three views of the receiver shown here indicate its commercial appearance and mechanical design, and by means of the prepared parts, duplication in design by all those attempting its construction is assured. The models shown differ in some points of mechanical refinement from the receivers it will be possible to construct from the commercially available units. Considering the individual variations in the construction of receivers described in radio periodicals, and realizing the troubles encountered by constructors in modifying original designs to suit their own fancies, it is not difficult to appreciate the special attractions and favor of a plan which will minimize the detailed dimensioning, layout, and assembly of receivers. Analyzing the circuit in Fig. 4 the salient features herewith described are apparent. ELECTRICAL DETAILS OF THE CIRCUIT TN THE receiver illustrated, the variable *• condensers Ci and C2 are shunted across their respective secondary coils, the first secondary functioning as a combined primary-secondary, in auto-transformer fashion, but in the finished mode!, a separate antenna coil has been provided. These condensers are of the standard .0005 mfd. value. The condenser Cj is that with which neutralization is obtained and is of the value of .000032 mfd. Two by-pass condensers, 05 and C6, are employed, one across the primary of the first audio transformer, and B battery in its circuit, its value being .001 mfd. and the other a .006 mfd. one, connected from the minus A to the