Radio broadcast .. (1922-30)

386 Radio Broadcast lead, 82 to the center and 83 to the inside. When switch Si is to the left, or short position, only half the loop is used. When it is to the right, the whole loop is used. This means all low wave stations up to 380 meters will come in on dial C2 from o to 100 degrees. Stations from 350 meters up will come in on C2 with switch Si thrown to the right, or long position. This means that in the long position, C2 will read about 20 for 345 meter stations, and about 70 for 536 meter stations. On the short position, C2 will read about 85 for 345 meter stations, and about 45 for 270 meter stations. These figures are approximate, but show that to cover the entire wavelength range, C2 must be varied from o to 100 degrees to go up to 370 meters with Si to the left, then Si turned to the right and the remaining wavelength range secured by varying €2 again from o to 100, allowing, of course, for over-lapping. Ci, the oscillator, starts around 18 for 270 meters, and brings in the lower heterodyne point for 536 meters at about 70. Two points can.be found for each station on this dial, which will help in tuning, as when interference is experienced on one point, the other may be used. TUNING AND TESTING THE COMPLETED RECEIVER THE first test should be to check tube Vi for oscillation. Insert only tube Vi, set R4 just on and connect the phones in series with the 845 lead. Then touch lugs 3, or 6 of the coupler. If a plucking sound is heard, this tube is oscillating. If not, adjusting the rotor coil will cause it to oscillate. When this rotor winding is in the same plane as the stator windings, turning it 180 degrees around will either start or stop oscillation. When once set to produce oscillation, this coil should never be touched. If the tube squeals at low settings of Ci, reduce Ri to .25-megohms, or try another .5 megohm leak. Use the highest value of leak possible — (up to i meg). The receiver will probably squeal below 10 degress on Ci in any case. R2 is not critical and may vary from i to 3 megohms. With tube Vi oscillating constantly, insert the remaining tubes in the set, turn the rheostat seveneights on for 20 1 A tubes, or on one-third for 190/5, and rotate the potentiometer from positive to negative. If both Ci and C2 are set at 100 degrees, a plunk will be heard at some point as R3 is adjusted, indicating amplifier oscillation. If Ci is adjusted, squeals will be heard. R3 should be set with its arm just positive of the point where squeals can be heard, and either left set at this point, or used to control volume. Now, with Si to the right, and Ci set at 50, rotate C2 over its entire range. A click will be heard near the center of its scale. The condenser CX connected between terminals 3 and 4 of the coupling unit should now be slowly turned out in small steps until rotating C2 fails to produce a click. The receiver is now balanced and CX, CX should never be touched unless tube Vi is changed. In tuning, C2 will appear rather broad, which is correct, while Ci will be extremely sharp. This permits of extremely easy logging, since C2 need only be set approximately correct, and Ci rotated in order to find a station. The chart printed on page 385 will help in preliminary tuning. The set will log definitely, and a station once heard may be brought in again at the same settings of Si, Ci and C2, providing CX, CX have not been tampered with. Due to the sensitivity of the circuit, a small amount of hand capacity may be noticeable on Ci. This may be overcome by grounding the negative filament line to a piping system, or it may be compensated for by tuning-in a station, increasing Ci slightly until the volume begins to decrease, and removing the hands from the set. The signal will then return to full intensity. It will be evident in those few cases where it may be encountered, only on weak, low wave stations, and is seldom bother An Explanation THE similarity in name of Dr. Walter Van Braam Roberts and J. E. Roberts has led to some confusion among the readers of RADIO BROADCAST. Doctor Roberts, of Princeton University, is responsible for the inception and the design of the Roberts Knockout receiver. Doctor Roberts wrote two articles describing this receiver which appeared in the April and May, 1924, RADIO BROADCAST. Immediately after the publication of these two articles, a great deal of interest was shown by radio con' structors all over the country in the design and operation of the set. Many investigators started building the receiver, making additions and alterations according to their own ideas. One of these enthusiasts was J. E. Roberts, of Cleveland, Ohio, who prepared an article describing how to build the original two'tube circuit in a cabinet with an additional stage of audio amplification, making three tubes. We have received many letters from residents of Cleveland and vicinity who have taken issue with what they regarded as an unfortunate use of the similarity of last names of these two men. The situation which drew the protest was in Cleveland and other cities. And in especial, a number of our correspondents did not like printed matter which was distributed by a Cleveland radio shop. Mr. J. E. Roberts has no connection with RADIO BROADCAST other than that of a former contributor to its pages. The only approved models of the Roberts Knockout circuit have been described in the magazine. We can take no responsibility for any printed matter or representations of individuals relative to this circuit released by other sources than this magazine except that of approved manufacturers of parts which may be used in the Roberts circuit.