Radio Broadcast (May 1927-Apr 1928)

234 RADIO BROADCAST JANUARY, 1928 a 1000-cycle source and the output of the bridge is connected to a single-stage audio amplifier, thus increasing the sensitivity of the system. The operator uses the headphone method of adjusting for capacitance by finding the minimum sound intensity. When the sound is minimum the capacity of the unknown is exactly that of the standard. A condenser with high losses does not permit of complete silence in the phones, and is rejected. The audio-frequency transformers are also tested in an interesting manner. It is highly important to know the response of the transformers to be placed in the receiver, but it would be a tedious procedure to plot a response curve for each individual unit, hence each transformer is compared with a standard on certain frequencies. A tube oscillator, generating a 500-cycle note and a 6000-cycIe note, is connected to a standard amplifier. The output of this amplifier is connected to the transformer to be tested, and the output of the transformer under test is connected to a vacuum-tube voltmeter, the output circuit meter of which gives visual deflections indicative of the response of the transformer under test. The circuit is so arranged that, by means of an anti-capacity switch, the transformer under test can be replaced by the standard and the deflections compared. By means of a switch, the oscillator can be adjusted to generate either the 500or the 6000-cycIe signal. This signal is free of harmonics and is constant at all times. The deflections with the standard transformer are therefore constant. The complete receiver undergoes several tests, and the method of testing is also original and novel. A buzzer-modulated master oscillator, tuned to 200, 400, and 530 meters (1 500, 750, and 566 kc.) feeds a master antenna. The buzzer modulation is accomplished by breaking the plate voltage supply with the interruptions of the buzzer. The operator testing the receiver (several operators are testing at one time) has his own antenna of standard inductance and capacitance value. He first adjusts the receiver at 200 meters by tuning it to resonance with the 200-meter oscillator signal. The receiver output is then noted by means of a tube voltmeter connected to the output circuit of the receiver. After the receiver is adjusted on 200 meters, adjustments are made on 400 and 530 meters, and the second harmonic of 530, which is 265 meters (1 130 kc). In this way each receiver is tested on four wave THE GREBE CONE LOUD SPEAKER lengths. This is indeed a comprehensive test, for it will bring to light any defects in design upon any of the wavelengths within the range employed for broadcasting purposes. If the tube voltmeter does not show standard output on all four waves, the receiver is rejected for a reexamination. The problem of conductive coupling in the receiver to adjacent leads was overcome by the use of the chassis as the negative filament lead, thus eliminating numerous long leads. The filament wiring in the receiver consists of only the positive polarity wires. The negative lead is formed by the chassis. The condensers are all grounded upon the chassis. The mechanical alignment is facilitated by punching the complete chassis in one piece. It is made out of aluminum and stamped out on a 60-ton press. The chassis, after the stamping, carries all the mounting holes and brackets, thus assuring correct alignment. The receiver, from start to finish, is carried from one operation to another by means of a conveyer system approximately 1000 feet in length. This conveyer consists of a belt or a roller as the occasion demands, and the partially assembled receiver moves from one operation to another until it finally reaches the final department. THE GREBE CONE THE electrical development of the Grebe loud speaker is also of interest, particularly because this field of endeavor requires highly trained engineering. The development of a loudspeaker does not consist of the mere selection of steel, iron, wire, and a paper cone. Let us consider for a moment an important consideration which the fan generally passes over very lightly. This is the angle of the apex of the cone, and the size of the cone. The information relative to the size of the cone will doubtless be of interest to constructively inclined radio fans. According to the engineer in charge of cone construction, their experiments showed that a 20" cone was the best compromise and that increases above this diameter did not justify the additional space required. Experiments showed that very little is gained by using a cone of larger diameter. Reduction in size, however, showed a material loss. As to the angle of the apex, 20 degrees is also the best compromise for efficiency and quality. The greater the angle, the greater the efficiency but the poorer the quality of reproduction. The 20-degree angle was considered the best for quality and efficiency. Many articles published in this and other periodicals have stressed the importance of a large value of inductance for loud speaker windings in order to produce satisfactory response on the low frequencies. With this in mind, a value of 1.7 henries was selected as the loud speaker coil inductance. The shape of the armature is also an important consideration, and by using an armature that is wide and short, the lowest moment of inertia is obtained. The selection of the material for the armature also requires care and silicon steel was chosen instead of Swedish iron, because the losses of silicon steel are less on the higher audio register. The difference between Swedish steel, iron, and silicon steel is not appreciable on the lower audio register but it approaches an appreciable value on the higher audio frequencies. The elimination of harmonics in the average loud speaker is a paramount item because their presence will not permit true reproduction. To attain this result it is necessary to minimize magnetic saturation. The testing of the loud speaker is carried out by first subjecting it to a series of audio frequencies obtained from a beat note oscillator. This beat note oscillator consists of two radiofrequency oscillators adjusted in a manner which permits the generation of a beat note; this note is passed through several stages of audiofrequency amplification and then into the loud speaker. One of the radio-frequency oscillators, is variable in tuning and the frequency of the beat note is variable between 50 and, approximately, 20,000 cycles. This test will bring to light any defects in the loud speaker mechanism which would result in a rasping sound or a rattle when it is placed into operation. Another test consists of the reproduction of an organ record played upon a talking machine and fed into the loud speaker by means of an electric pick-up and amplifier combination. The organ selection has a wealth of low notes, and these are particularly desired for testing purposes, since the amplitude of these low frequencies is high. This test will bring to light any defects in the placement of the armature. Another test consists of the application of the plate current of a 171 tube through the windings of the loud speaker, first in one direction and then in the other, to test for magnetic saturation when it is in operation. AN INTERIOR VIEW OF THE "SYNCHROPHASE" SEVEN