Radio Broadcast (May 1928-Apr 1929)

356 RADIO BROADCAST OCTOBER, 1928 condensers is connected from the screen-grid to the grounded shield, and one from the B plus side of the r.f. transformer primary to the grounded shield. Electrical isolation is further insured by the use of two r.f. choke coils, one connected in the screen-grid lead and one in the plate lead of each r.f. amplifier stage. These chokes are placed on the under side of the chassis. An additional r.f. choke is used in the detector plate circuit to prevent any r.f. currents from straying into the audio amplifier. With all of these precautions, the receiver is remarkably stable; however, it is capable of being made to oscillate when desirable. A detailed analysis of the functioning of the screen-grid tubes as r.f. amplifiers indicates that even though the plate-to-grid capacity of the screengrid tube has been reduced to an almost negligible value, this value is still high enough to allow oscillation if sufficiently good circuits are used with the tube. As very good circuits have here been employed to provide as high amplification and selectivity as is possible, the volume control has been combined with a stability control so that the r.f. amplifier stages may be operated at peak efficiency at every wavelength, regardless of oscillation tendency. Measured amplification curves of the different stages show repeater voltage gains varying from 17 at 550 meters to 30 at 200 meters, these comparatively low values having been selected in order that the full merit of the tuned circuit might be taken advantage of to obtain the selectivity required by modern broadcasting conditions. The rising characteristic of the r.f. amplifier at short wavelengths is compensated by the tuned antenna input circuit, which has an opposite characteristic in that it shows greatest voltage step-up at 550 meters with a decreasing step-up at shorter wavelengths. The five tuning condensers, Ci, Co, C3, C4, and C5, are all connected together, and are operated by a single drum control dial, this connection being effected by means of the floating removable shafts, and flexible couplings arranged to link the condensers. The receiver can be tuned over the entire broadcast band with the single tuning drum, no difficulty being experienced in ganging, due to the high accuracy of the double spaced condensers employed. It was felt desirable, however, to equip each stage with individual tuning verniers, Vi, V2, V3, V4, and V5, so that there would be absolutely no question in the mind of the operator that his receiver could always be tuned to absolute peak efficiency on any and all wavelengths in the broadcast band. NOTES ON CONSTRUCTION THE construction of the receiver is quite * simple, for there is available for it the complete shielding assembly, fully pierced, and requiring only the insertion of some 88 -fz screws with their nuts and lockwashers, to put it together. The use of this large number of screws to hold the shielding together is the result of an interesting fact discovered during the development of the set. At first an endeavor was made to use the simple and attractive cornerpost type of assembly, attaching these posts to the chassis and slipping the partitions, ends, front and back panels into the slots of these corner posts. The result was a very attractive mechanical job, but of very poor electrical characteristics, for the electrical joints provided between the partitions and the chassis (and for that matter between the partitions and the corner posts) were of such a variable nature as to change the entire performance of the receiver. It was necessary merely to strike the shielding with the palm of one hand to change the electrical contact between the different portions of the shielding, thereby altering their shielding effects on the circuits. From these results it was found that it would be necessary to use lap-joints and thick aluminum and to insure positive contact at many points, which accounts for the use of nine fastening screws to each partition. The parts, and accessories, used in the Sargent-Rayment Seven are listed at the end of this article, and being of standard manufacture, may all be procured upon the open market, including the especially prepared aluminum cabinet assembly. The coils may be wound at home from the data given in the text. The assembly of the receiver is quite simple, involving only the mounting of the parts upon the pierced chassis with machine screws, wiring them up, and finally, the attachment of partitions and front and back panels with the 88 machine screws previously mentioned. The wiring of the set is surprisingly simple for a receiver of this type, as may be seen from a study of the two pictures in Fig. 2 and Fig 3. The schematic wiring diagram in Fig. 1 also shows the simplicity of the wiring. LIST OF PARTS In the list below the substitution of equivalent parts may be made at the builder's choice. Q to C5 — 5 S-M variable condensers, 0.00035 mfd., type 320-R C6toCi3 — 8 Polymet Bypass condensers, 0.25 mfd' C14 — 1 Polymet Grid condenser, 0.0001 5 mfd. C16 — 1 Polymet Bypass condenser, 0.002 mfd. C16, C17 — 2 Potter Bypass condensers, 1.0 mfd. D — 1 National Velvet vernier dial, type F, with illuminator Li — 1 S-M antenna coil, type 141 L2, L3, L4, L5 — S-M r.f. transformers, type 142 Le to Lu — 9 S-M r.f. chokes, type 275 Ri — 1 Carter resistor, 3 ohms, type H-3 R2 — 1 Carter resistor, 1.0 ohm, type H-i R3 — 1 Grid leak, 2 megohms R4 — 1 Durham resistor, 150,000 ohms, with leads Rs — 1 Yaxley Junior potentiometer, 3000 ohms, type 53000-P Si — 1 Yaxley Junior switch, double circuit (d.p.s.t.), type 740 SH — 1 S-M aluminum shielding cabinet with control legends, type 705 Ti — 1 S-M first-stage audio transformer, type 255 T2 — 1 S-M second-stage audio transformer, type 256 T3 — 1 S-M output transformer, type 251 Vi to V5 — 5 S-M midget condensers, 0.000025 mfd., type 340 1 S-M walnut finish base moulding, type 706 1 S-M 10-lead battery cable, type 708 2 cartons S-M hook-up wire, type 818 7 S-M tube sockets, type 5 1 1 2 Yaxley insulated tip jacks, type 420 1 set hardware (obtainable from manufacturer) The accessories necessary to make the set operative are as follows: 4 cx-322 r.f. tubes 1 CX-301A or, preferably, CX-II2A detector tube 1 cx-i 12A first a.f. tube 1 CX-371A power tube 1 6volt storage A battery or A-power unit 4 45-volt heavy-duty B batteries or a B-power unit (180 volts), such as the S-M 670-B Reservoir Power unit ! 4o|-volt C battery 1 45-volt C battery If it is desired to use a cx-350 type power tube in place of the 371 in the last stage, a highpower A-B-C supply, such as the S-M 675 ABC Hivolt Power supply, should be used. In this case only the 45-volt C battery is necessary, and a 2-mfd., 600-volt condenser must be placed between the speaker and terminal 2 of the output transformer, T3, (see Fig. 1.) FIG. 3. THE UNDER SIDE This view gives an idea of the simplicity of the wiring of the receiver, resulting front the use of the metal chassis as the A minus circuit return. The mounting of the condensers and chokes on the under side of the chassis is also clearly shown