Radio Broadcast (May 1928-Apr 1929)

GENERAL VIEW OF HOME-MADE AUDIO OSCILLATOR An Inexpensive Audio Oscillator ONE of the greatest problems that confronts the average experimenter is the lack of adequate apparatus for conducting the tests that are so vitally important to the final outcome of the experiment. In nearly all cases the cost of the apparatus involved in making an experiment which leads to an authoritative answer places it beyond the reach of the individual. It was with the idea of solving this problem that the writer spent considerable time in designing the inexpensive audio-frequency oscillator presented in this article. The various experiments and measurements to which an oscillator of this type may be put to use are not only quite varied, but at the same time quite useful. In connection with measuring the frequency characteristics of audio-frequency amplifiers, this oscillator has proved itself indispensable. Amateur or commercial radio stations may use it in place of the usual buzzer to modulate the carrier. In the broadcasting field, the oscillator may be used for lining up telephone circuits which connect the broadcasting station with remote apparatus. Many other uses may suggest themselves to the experimenter, such as determining the frequency characteristic of an audio-frequency transformer, filter coil, loud speaker, etc. THE CONSTRUCTION OF THE UNIT THE oscillator which is described here is made up of two component parts, the oscillator proper and the one-stage amplifier. For practical purposes it is desirable to have this stage of amplification built in the same housing, and combined with the oscillator to make one complete unit. In constructing this oscillator, it is first necessary to obtain a Ford ignition coil (or some similar spark coil), such as used on the model T car. A photograph of this unit is shown at the left in Fig. i. The wooden housing, which the ignition unit is molded in, is carefully removed with a screwdriver. While the process of dismantling the unit is in progress, an electric soldering iron CT"HE oscillator which Mr. Stanko de■*■ scribes uses apparatus that nearly every experimenter has, or can obtain. It will generate audio frequencies from about 60 to <yOOO cycles, which is sufficient range to conduct any and all. tests on present-day audio apparatus. In the Laboratory, a similar oscillator has been in use for several years; we used a push-pull output transformer instead of a Ford coil, but the results are the same. The values of capacity indicated as C\ in the circuit diagram must be determined experimentally. The larger the capacity the lower the generated frequency. The Laboratory Staff will be glad to learn of other apparatus of similar nature that experimenters have developed. — The Editor. should be kept handy for unsoldering all of the connections on the inside of the housing. When the entire housing has been removed, place the ignition unit in a moderately warm oven. After it has been given a thorough warming, remove the unit from the oven and cut off with a knife all of the surplus insulating compound in which 186 the coils and condenser are imbedded. Remove the paper condenser and lay aside. It will be used later. Dig around in the insulating compound until the primary winding of the coil is located. The primary winding can be easily distinguished from the secondary by noting the size of the wire. The primary is larger in diameter than the secondary. When this winding is found, the wire is pulled out endwise until all of the wire is removed from the iron core. Care should be taken not to damage the secondary winding while the primary is being removed. When the ignition unit is completely dismantled, as shown in the center in Fig. I, it will be necessary to locate the secondary leads. The two outside leads will not be hard to find as they are at the extreme ends of the two secondary coils. The difficult problem is to locate the connection that connects the two secondary coils in series. This connection is usually found between the secondary coils imbedded in the insulating compound. Progress at this stage must be very slow, as considerable pains should be taken not to damage any of the windings or leads. When this connection is located, cut the connection at the center. Flexible wires are now soldered to all of the leads that extend from the two coils. The unit is now ready for reassembly. The simplest and easiest way of assembling these parts is to get a tin can that will accommodate the two secondary coils and the iron core. Place the coils in the can, one on top of another, slip in the iron core and center it with respect to the secondaries with small wooden wedges. Pour the can full of hot paraffin and let it cool. When the paraffin has hardened, warm the can over a gas flame or immerse it in a bucket of hot water. After