Radio Broadcast (May 1928-Apr 1929)

194 RADIO BROADCAST AUGUST, 1928 1 ~r 1 1500 ohms I • Shields^-. 0.00025 mfd. ~\ 1 I ci2 . ./Tonv T, A+ B+ 90 B+45 4ohms Input FIG. 4. THE CIRCUIT OF THE RECEIVER The receiver described in this article consists, as shown by this circuit diagram, of four stages of tuned r.f. amplification, a grid-leak condenser type detector and a single stage of audio amplification, the second stage of amplification being incorporated in the power amplifier and B supply. For those that desire to use a phonograph pick-up it is suggested that an additional socket be included in the set. The placement and wiring of this additional socket is indicated in the picture diagram, Fig. 1. 1-1 1 -vVyVO^v -OB + 45 -OB r^-OC -OGnd. FIG. 5. THE CIRCUIT OF THE POWER UNIT This power amplifier and B supply, designed for use especially with the receiver described in this article, supplies A, B, and C voltages to the type 210 power amplifier tube and B and C voltages for the receiver itself. This power unit, in conjunction with an A-power unit,such as the Abox, makes possible the operation of this receiver directly from the light socket extra socket connects to the P terminal of the first stage audio transformer. The two filament terminals are wired together and are connected to the B plus terminal of the same transformer. Therefore, when the adapter from a phonograph unit is plugged into this socket, the pick-up is connected directly across the transformer primary. When the pick-up is being used, the detector and radio-frequency amplifier tubes may be turned off. This is accomplished by tuning the filament rheostat on the panel to zero, in which position all the filaments except that of the first a.f. tube are turned off. HOW TO ASSEMBLE THE RECEIVER '"p H E assembling of the receiver should not be 1 very difficult if the circuit and picture diagrams are carefully followed. The circuit diagram of the receiver is given in Fig. 4 and the picture layout in Fig. 1. Specifications for home-made r.f. transformers are given in Fig. 2, the lettering on the coils corresponding to the same numbers in the circuit diagram. Fixed primaries are used in these home-constructed coils because they are more easily constructed than coils with adjustable primaries. A top view of the receiver is shown in Fig. 3. If manufactured coils of the make specified in the list of parts are used, it is essential that the first and third coils (Li and L2, and L5 and L6) be mounted by means of only one mounting screw at the primary end. If mounting screws are placed through the hole at the grid end, the coil will be short circuited by the shield. The bypass condensers, Q, C2 and C3, should also be mounted with one screw. The accurate alignment of all four tuning condensers is accomplished by loosening the clamping screws on the tuning condensers and then adjusting, by hand, the position of each rotor so that maximum volume is obtained. The power unit illustrated in this article has been constructed especially for use with this receiver. It is a combination power amplifier and B supply. The circuit diagram is given in Fig. 5 and the picture layout in Fig. 6. In adjusting the complete installation, the power Clarostat in the power unit is adjusted so that the voltmeter, V2, on the receiver panel reads 90 volts. The 150-ohm Centralab variable resistance supplies C bias to the first audio tube. The parts used in the receiver and power unit illustrated in the article are given on page 192. Other equivalent parts may, of course, be used. The set illustrated in this article was made for Radio Broadcast by the Rossiter, Tyler & McDonnell Service. FIG. 6. C Ground B B+ 45 THE PICTURE WIRING nput Speaker DIAGRAM OF THE POWER UNIT