Radio Broadcast (Nov 1926-Apr 1927)

488 RADIO BROADCAST MARCH, 1927 Radio Broadcast Photograph HOW THE APPARATUS IS ARRANGED This rear view clearly shows the neat layout of the parts on the sub-panel could cause this back action of the plate upon the grid not to coincide with the original impulse, there would be less danger of oscillations being caused by this inter-element capacity. A method of doing this is illustrated in the accompanying circuit diagram. In this circuit we see that we have placed a shunt resistance, R, in the plate circuits of the r.f. tubes. Here we see that condensers C, and primary inductances L, are shunted by a plain resistance R, which kind of circuit is an ideal absorption circuit, its effectiveness as such depending upon the size of the resistance. It will be seen that the plate circuit now contains a capacity as well as an inductance, and that, due to the presence of these two, the relationship between voltage and current is changed, so that the plate voltage variations will more or less coincide with the current variations, as the inductive effect is largely offset by the capacity, which has an effect opposite to that of the inductance. The effect of the condenser will not completely prevent oscillations but will make it much more difficult for them to occur. The remaining tendency toward oscillations is overcome by the variable resistance. The resistance is more effective at high frequencies where the back coupling tending to create oscillations is greatest. The Phasatrol then fulfils its function in two distinctly different ways: First, it changes the phase relationship of potentials and currents in the plate circuit, so that the potential variations of the plate are much less liable to be the cause of oscillations, while in the second place, the device acts as an absorption circuit which can be accurately adjusted to the exact requirement of every individual receiver. THE NECESSARY PARTS REFERRING again to the diagram, we see that, for the exception of the Phasatrols, which are placed in dotted lines, the receiver is a conventional one in most respects. A list of parts used in the receiver is given below: Li, L2, L3 — Three Bodine Matched Coils |6.oo Five Eby Sockets 2. 50 Ri — Four Amperites, Type i-A . 4. 40 R2 — Amperite, Type 112 . . . . 1 . 10 Ti, T2 — Two Thordarson R-200 Audio Transformers 16.00 Ci — Flammarlund Midline Condenser 0.00035 Mfd 4.75 C2 — Hammarlund Midline Gang Condenser^— 0.00035 Mfd. Sections) 8.00 Lynch Single Resistor Mounting . .35 R< — Lynch 2 Megohm Grid Leak . . .50 Seven Eby Binding Posts .... 1 .05 Four Carter Pin Jacks .60 S — Cutler-Hammer Filament Switch . .60 R4 — Electrad Royalty 700,000 Ohm Variable Resistance . . . 2.00 Rs — Electrad Royalty 500,000 Ohm Variable Resistance . . . 2.00 MA — Jewell Panel Mounted Milliam meter o — 50 Scale .... 7.50 Two Eby Dials 5.00 Wire .50 Pi, P. — Two Electrad Phasatrols . . 5 .50 Cs — Electrad 0.5 Mfd. Bypass Condenser .60 C4 — Sangamo 0.00025 Mfd. Grid Condenser .40 Cs — Sangmo 0.001 Mfd. Fixed Condenser .50 G; — Electrad 1 Mfd. Bypass Condenser 1.80 Total $7165 Panel 7 x 23 inches Sub-Panel 7 x 23 inches The placement of the parts is evident from the photographs, and these, in conjunction with the circuit diagram, should give sufficient information to enable a home constructor to build the receiver. The "figure-eight" Bodine coils are placed on the market in sets of three matched coils, and this makes it possible to use the Hammarlund gang condenser and thus eliminate one control. The 700,000-ohm Royalty variable re sistance located near the left center of the panel, is the volume control. Since this control is in the r.f. stage, it is possible to prevent overloading of the detector on strong local signals. The milliammeter to the left on the panel is placed in the common negative B-battery lead. The meter used in the model had a 100-mA. range, but a 50-mA. range is large enough. The receiver ordinarily draws about 25 mils, with a 112 type tube in the output. With a 171 type output tube, the total plate current will be about 35 mils. The meter is an excellent accessory because it will serve to check up on the quality of the reception. To understand this use of the meter, one oniy has to remember that, in order to be undistorted, the signal currents should be pure alternating currents, increasing and decreasing from the mean value of the steady current to the same extent. If this condition is fulfilled, the meter needle should remain still even with the loudest signal. Violent fluctuations of the meter will always be indicative of distortion, which can usually be remedied by adjustment of the C bias. In order to make the best use of the meter for this purpose, there is installed on the front panel a Royalty 500,000-ohm potentiometer, which is shunted across the C-battery connections. The moving arm is connected to the F lead on the second audio transformer, and has also a connection to one of the terminals of a i-mfd. condenser, whose other terminal leads to the general minus line. This control enables us to adjust the Cbattery voltage to the best value possible, the meter giving visible evidence when the correct value is reached. There is still another service, for which the meter will prove of invaluable service, i.e., in the stabilization of the receiver. To do this successfully, the condenser dials should be turned to about 10. When, now, one of the two dials is moved, and the receiver is in oscillating condition, the needle of the meter will suddenly jump, as the dial passes a certain point. As soon as the correct position of the Phasatrols is found, the meter will remain perfectly still when tuning; the Phasatrol is then adjusted to greatest sensitivity on a short wavelength station, and the receiver is ready for operation and logging. In adjusting the Phasatrols, reduce each unit a small amount at a time — otherwise it is quite possible to make the receiver broad in tuning and a loss of sensitiveness will be noticed particularly on the long wave stations. In this way each resistance will remain about equal, in value, and give best results. THE CIRCUIT DIAGRAM OF THE RECEIVER Using Phasatrols in which these units are shown in dotted lines. Separate B-plus binding posts are indicated for the r. f. and first a. f. plate voltages but these two may be combined as was done in the model made up. Also no C-plus post is used on the model since this lead was connected to the negative-A post. Only seven binding posts are then necessary. The antenna and ground are connected through pin jacks, as are also the loud speaker leads