Radio Digest (Oct 1923-July 1924)

R A D I O DIGEST — Illustrated March 29, 192k SUPER-HETERODYNE •:uinued from page ill eter for control of grid potential in the main super-heterodyne circuit, including the detector, and one for the audio frequency amplifiers. Two Oscillators Figures 7 and S show two oscillators Figure 6. Separate B batteries are used for the Radio amplifier side of the circuit. The main point of interest in this circuit is the use of a Hartley oscillator, whereas most of the previously shown circuits make use of the Colpitts circuit. The coil in the plate circuit of the oscillator is a Radio frequency choke, used to avoid instability of the circuit by leakage from the oscillator to the B battery circuit and the Radio frequency amplifiers. It is important that the oscillator variable condenser also has a high electrical efficiency rating with the best possible maximum to minimum capacity ratio. On that have been m; le up following the specifications given in the February 2nd issue of Radio Digest, but using different coil construction. Figure 7 shows one in which the coils are self-supporting, but Figure 7. this fact depends the actual working range of the heterodyning action. The addition of two stages of audio frequency with jacks makes the circuit ideal for loud-speaker use. Only two rheostats are used, one for rigure S. are held in position by means of a narrow piece of tubing, which also takes care of the rotor bearing. Figure 8 shows the" oscillator coupler made up with the two stator windings bank wound on a bakelite moulded form. The rotor winding is spaced. This unit is unusually neat in appearance and workmanship. In the next part details of a filter circuit recommended for use after the detector stage will be given. This filter eliminates any radio frequency leakage that is the cause of noisy reception and howling. (TO BE CONTINUED) THIRTY MINUTE A-B-C (Continued from page 22) rounding air, the aerial so energized, sets up a similar motion in surrounding space. Audio Frequency In Figure 9, a toy telephone line is shown. Two diaphragms are connected by a taut string. When air waves strike the diaphragm at the right, this diaphragm moves back and forth and pushes and pulls at the string so that the wave motion moves the diaphragm at the left, setting up similar air waves which may be heard. Or if a string is tied between two chairs, a hairpin hung at one end can demonstrate energy transmitted when a ruler strikes the string near the other end. The string can be arranged so that this is seen, or if very short and taut, the wave motion set up will be at a high enough rate to be heard. Audio frequencies are such as can be heard. In Figure 11 a diaphragm is shown in circumstances 1 to 5. At 1 the diaphragm is at rest. A voice pushes air against one side of the diaphragm as shown by 2 and air on the other side is pushed out. As shown at 3, the diaphragm will flex back, and in moving in and out set up a series of waves recognizable as sound. Now if this diaphragm were a bell, provided that it could still send out the sound waves from the applied voice, it would also send out its own audio frequency. Diaphragms are accordingly constructed as far as possible to minimize their own natural frequency, so as to transmit the applied audio frequency. How Sound Is Changed into Electrical Pulsations Electrically this is done in much the same way. The diaphragm is arranged to jostle some graphite grains held between two contacts. This device is called a microphone because the path it offers to a current of electricity varies according to the positions taken up by the graphite grains according to the ■ vibrations of the controlling diaphragm. More or less electric current can thus pass the microphone according to the motion of the diaphragm. A battery connects the microphone with a receiver in which an electromagnet gets a variable current supply governed by this microphone. This electromagnet is then used to op / !> > ^PENDULUM '^WEIGHT <tT~ ~-3 Figure 12. erate a reproducing diaphragm, so that the electrical means does what the string did in the example of Figure 9. For Radio, there is a substitute for the wire line, the Radio link between the broadcast sender and receiver. Broadcasting wave lengths are expressed in terms of meters. This means that the wave is the length stated, a meter being about 3.28 feet. Thus a 360 meter wave can be thought of as a wave about 1150 feet long. How Radio is broadcast will next be further discussed. (TO BE CONTINUED.) A steady hum in the headphones is not always caused by alternating current electric wires, although this is the most frequent cause. An open circuit somewhere in the set will produce the same noise. Thordarson POWER AMPLIFYING TRANSFORMERS Price per pair, $13.00 The new Thordarson Power Amplifying Transformers (push pull) are designed for use as third stage audio frequency amplifiers, to provide high power amplification for operating loud speaking devices. With power amplification, not only is it possible to increase volume, but, since two tubes replace the usual one, the distortion and howling which usually accompany the overloading of a single tube on the third stage is done away with entirely. The Thordarson Power Amplifying Transformers are well constructed electrically and are capable of indefinitely carrying the additional load without breaking down. In tonal purity these transformers equal the Thordarson Super Audio Frequency transformer whose rich quality and even amplification has made it the popular transformer of the day. Thordarson ELECT. MFG. CO. 500 W. Huron St. CHICAGO Panel Attached Variocoupler The necessary parts for constructing this attachment can be had for a few cents, and it is simple to make. The rotor should have a hole drilled straight through and a piece of brass rod yV or 14 inch in diameter 2 inches long to fit in the one WOOD ROTOR PIG TAIL LEADS rotor binding POSTS COIL BINDING POSTS ^^s3 o V BRASS BRACKET z>4 ■SHAFT side. The two leads from the rotor are taken through the center and through the other hole out to the two binding posts as shown in the illustration. The bracket can be made of two small pieces of brass drilled and fastened to the panel as shown. A washer should be placed between the bracket and rotor to avoid wearing the windings. By following this method the rotor can be fastened to the panel by itself and the coil placed under it. In making my set I assembled the rotor first and attached it to the panel, then I was able to place the coil where I wanted it. The rotor may be removed by drawing out the shaft. This enabled me to make a neat job of soldering the tops with the rotor out of the way. — R. f. Keller, Br ton, Pa. RADIO DIGEST PUBLICATIONS 123 West Madison St. CHICAGO. ILL.