Radio Broadcast (May 1928-Apr 1929)

336 RADIO BROADCAST OCTOBER, 1928 FIG. 4. THE RECEIVER SEEN FROM ABOVE when adjusted to maximum sensitivity. This condition was thought to indicate a very satisfactory degree of sensitivity indeed — a value so high that it could not be utilized in the warm, summer weather due to the high noise level. During preliminary tests small 50 and 100-watt stations 1000 or more miles away were received with loud speaker volume, and the more powerful stations on the Atlantic and Pacific coasts, and in fact in all parts of the country, were received with excellent volume whenever the noise level was low enough to let these stations be separated from the noise. CONSTRUCTION AND OPERATION THE construction of the receiver itself is quite simple. The parts employed in the model described herewith are listed at the end of this article, the listings being accompanied by the designating letters seen in the various illustrations and in the circuit diagram. Despite the apparent complexity of the circuit, the receiver is very simple to put together for it is built up upon a very ingenious pierced steel chassis which is available in the open market. On this chassis, which incidentally is suited to the mounting of a number of different types of receivers, all necessary wiring and mounting holes are pierced so that to assemble the set it becomes necessary merely to put the various parts in the different stage shields as seen in the photographs and then to bolt these stage pans to the chassis itself, along with a few other parts. The wiring is quite easily handled, as all leads are short and direct and the use of the metal chassis as the A minus, B minus and C plus common circuit return eliminates a large number of connecting wires. The only parts mounted beneath the chassis are the three i-mfd. bypass condensers, Qo, Cn, Q2, the antenna resistance, R12, and the rheostat, Rio, (the adjusting screw of which is visible in Figure 4). The connecting cable used simplifies the battery connection to the receiver and eliminates the multiplicity of binding posts ordinarily used for this purpose. The testing and operation of the receiver is most simple, it being necessary after it has been assembled, wired, and checked, merely to connect the necessary A and C batteries (or light-socket power units) to the battery cable and then to connect the proper leads of the cable to the Bvoltage binding posts of a single-stage power amplifier and B supply. (The receiver may not be operated satisfactorily from dry B batteries, since the current drain on the 45-volt circuit is approximately 30 milliamperes — though the total current drain of the whole receiver is only 40 mA). A good standard B-power supply should be used for the set if a power amplifier is not employed, though the latter is distinctly desirable since it is never wise in the interests of good tone quality to operate a loud speaker directly out of a single audio stage following a detector tube. Nevertheless, this may be done for test purposes and until the builder can afford an amplifier, for the receiver will give very good loud speaker volume without the use of a power output stage (which should preferably be a 210 or 250 pushpull stage). In operation, stations are tuned in upon the Selector 1 and Selector II dials, and volume and stability is adjusted by means of the two small knobs on potentiometers Ri and R2, the set is turned off by turning R2 to the off position. The stations will be received at only one point upon the left-hand dial and at two points upon the right-hand dial. This condition, however, is no disadvantage, for the selectivity of the r.f. amplifier is so great that there is never any possibility of two stations, for which a single oscillator setting will serve, coming through the receiver at one time — the short-wave t.r.f. amplifier absolutely prevents this usual source of super difficulty. This receiver cannot be operated at maximum amplification adjustments except in locations having a most unusually low noise level and under the best of weather conditions, for the sensitivity of the receiver is such that in ordinary residential districts noises not heard on most receivers can be brought in with ear-splitting volume; and of course many weak stations ordinarily not heard come in with them. This, however, is no disadvantage, for by turning down the gain controls of the receiver local programs can be received with quietness, freedom from interference, and satisfying tone quality obtainable from few other radio sets. LIST OF PARTS THE substitution of electrically and mechanically equivalent parts may be made in the list below, at the builder's choice. The apparatus in kit form, with all the necessary hardware, is obtainable from several mail order houses, at about $96.00 list price. B — 1 S-M 701 universal pierced chassis BPl BP2 — 2 moulded binding posts, consisting of -g4, screw, nut, and moulded top Ci to Cj — 7 j-mfd. midget bypass condensers C& — 1 0.0001 5-mfd. grid condenser with clips C9 — 1 0.002-mfd. bypass condenser Cm. Qi, C12 — 3 i-mfd. bypass condensers D! — 1 S-M 806L (left) vernier drum dial D2 — 1 S-M 806R (right) vernier drum dial E — 1 S-M 809 dual control escutcheon Ji, J2 — 2 Yaxley 420 insulated tip jacks Li, Li, L3 — 3 S.M 132B plug-in r.f. transformers L4 — 1 S.M 132C plug-in oscillator Lis — 1 S-M 275 choke Ri, R2 — 2 Yaxley 3000-ohm midget potentiometers, type 53000 R3 to R8 — 6 Carter RU10 io-ohm resistors Ro — 1 2-megohm grid leak Ri« — 1 Carter A3 3-ohm sub-base rheostat Rn — 1 Carter H| §-ohm resistor Ri2 — 1 Durham 1 50, ooo-ohm resistor Sj to Ss — 8 S-M 5 1 1 tube sockets S9 — 1 Naald 481 XS cushioned tube socket SHi to SHj — 7 S-M 638 copper stage shields SW — 1 Yaxley 500 switch attachment Ti, T2, T3 — 3 S-M 210 long-wave transformers T4 — 1 S-M 255 first-stage a.f. transformer TCi, TC2, TC3 — 1 S-M 323 3-gang condenser, 0.0003 5-mfd. in each section TC4 — 1 S-M 320R 0.00035-mfd. Universal condenser 4 S-M 512 5-prong tube sockets 1 S-M 708 10-lead, 5-foot connection cable 2 S-M 818 hook-up wire (25 ft. to carton) 1 Set Hardware (furnished by Set Builders Supply Co.) To make the set operative, the following accessories are necessary: 6 cx-322 tubes 2 cx-i I2A tubes 1 CX-301A tube Power amplifier (210 or 250 push-pull preferable) B-power unit (135 volts maximum) Source of A and C voltage