Radio Broadcast (May 1928-Apr 1929)

OCTOBER, 1928 THE SARGENT-RAYMENT SEVEN RECEIVER 355 FIG. I. THE SCHEMATIC WIRING DIAGRAM If a 112A or 210 tube is used in the power stage, the plate lead of the power stage should he connected to terminal 3 of the output device, T%; if a 171 a or 250 tuhe is used, to terminal 2. Also note that if 180 volts or less is used with a 1 12A or 171 a tube in the last stage, the speaker should he connected to terminal 4 of T„ as indicated in solid lines. If more than 180 volts is used — as with a 210 or 250 tuhe — the speaker must be connected through a 600-volt 2.0-mfd. condenser to terminal 2, as indicated by the dotted lines ordinarily be quiet for other receivers. Models of the receiver brought to Chicago and tested under the trying conditions produced by twenty or more local stations in simultaneous operation, have brought in as many as one hundred broadcast stations in a single evening's tuning. One receiver was tuned over the broadcast band, beginning at 550 kc. and going up the frequency scale. As rapidly as the dial could be turned and the verniers trimmed for maximum signal strength, new stations could be logged. When the evening was over, it was found that a station had been logged for every transmission channel, beginning at 550 kilocycles and going on up to over 1200 kilocycles before any gaps were found (channels upon which no station could be heard). This in itself is a remarkable record, and one which indicates the high degree of amplification that may be had in a carefully designed tuned radio-frequency receiver taking full advantage of the possibilities of screen-grid tubes and adequate shielding. CIRCUIT DESIGN A LL of the four r.f. stages consist of essentially similar tuning coils and tuning condensers associated with screen-grid amplifier tubes and the necessary bypass condensers and choke coils to insure absolute isolation of the various amplifier circuits. Each stage embodies an r.f. transformer with the secondary wound of 72 turns of No. 25 plain enameled wire on a threaded bakelite tube 2§" in diameter, the winding occupying a space 2j" inches long. The turns are spaced 32 turns per inch. The r.f. resistance characteristics of this coil are most excellent. To each of the interstage r.f. transformer secondaries is coupled a primary consisting of 25 turns of No. 28 d.c.c. wire, wound upon a 2j" diameter bakelite tube, fitting inside the secondary at the filament end. Upon close observation, the antenna coupling coil, Li in Fig. 2, will be found to differ slightly from the interstage coupling transformers in the succeeding r.f. stages. This coil is of the tuned rejector type, having a primary winding of 20 turns of No. 28 d.c.c. wire with the turns spaced |" apart on a tube 2j" in diameter. This winding is common to the antenna circuit and the grid circuit of the first r.f tube. Surrounding this coil, and coupled closely to it, is a second coil which is similar to the secondary windings in the succeeding stages. This coil is tuned by the first or extreme left-hand tuning condenser and serves to reject effectively undesired signals, without having its tuning greatly affected by various sizes of antenna. Examining a typical r.f. stage, it is seen to consist of the r.f. transformer; the 0.0003 5-mfd tuning condenser with its associated 0.000025mfd. midget vernier condenser; a tube socket for the screen-grid amplifier tube; two f-mfd. bypass condensers, and two radio-frequency choke coils. Each amplifier circuit is complete in its own shielded compartment, and the only leads carrying r.f. current running from stage to stage are the plate leads. One of the j-mfd FIG. 2. WITH THE TOP REMOVED From left to right are the four r. f. stages, the detector stage, and the two-stage audio amplifier housed in one compartment. In this compartment the second-stage audio transformer, Ti , is mounted on top of the output transformer, TV The five trimming condensers, V\, Vi, V3, Vi, and V%, are mounted just under their respective tuning condensers. The mounting of the remainder of the parts above the sub-panel is clearly shown in this picture