Radio Broadcast (May 1928-Apr 1929)

168 RADIO BROADCAST JULY, 1928 ANT. GROUND R C2 VTi T THE UNDER SIDE P Tt -AB+C o+135B FIG. 2 is passed on the tuning dial. As was stated, the screen-grid tube, due to its low grid to plate capacity, eliminates this objectional feature and permits a band of waves to be swept by the tuning condenser, C3, without other adjustments except a minor manipulation of the regeneration condenser, C4. The receiver has also been adapted to the receiving of broadcast programs by the employment of high-grade audio transformers together with a cx-312 power tube in the output circuit. Smooth control in operation has been attained by no small amount of effort. A portion of the success of this feature is due to the splendid characteristic of the choke, L3, which isolates the regeneration circuit, L4, C4, at all frequencies to which the tuner is capable of responding. Stability of control is also obtained by isolating the various circuits as completely as possible. Thus the 0.005-mfd. condenser, C5, assures that the screen-grid will be maintained at ground r.f. potential. In the same way the plate circuit of the screen-grid tube is isolated by means of the 0.005-mfd. condenser, Q. Because several portions of the receiver are operating from the 135-volt tap of the battery, another choke, L5, is inserted. In order to prevent small radio-frequency currents from being carried through the stray wiring capacities of the audio amplifier, which would cause objectionable hand capacity and prevent the detector going into and out of oscillation quietly when wearing the head phones, the cores of the audio transformers are connected to the panel brackets and thence to ground; in addi THE COMPLETED RECEIVER tion the capacity, C6, is employed across the secondary of the second audio transformer, Ti, as well as another capacity, C7, across the output terminals of the receiver. It is a fact that these improvements for eliminating audio noises in the output of the receiver contribute materially to the ease of operation of this set. A further source of troublesome extraneous noises has been negligized by the unique construction of the Amsco 0.0000 1 4-mfd . ( 140 mmfd ) condenser, C3. With a condenser of ordinary construction, unbearable noises are produced when the receiver is tuned at twenty to thirty meters due to the contact voltages set up by the dissimilar metals usually employed in the shaft and bushings of the condenser as well as to the scraping between the metal parts. Strange as it may seem at the first blush, these noises cannot be eliminated by "pigtailing" the shaft of the condenser, because at these extremely short wavelengths the inductance of the "pigtail" is sufficiently appreciable to prevent the variable resistance caused by the scraping contact to be short-circuited by it. This difficulty has been met very well by completely insulating the bearings of the rotor from the frame of the condenser and then running the "pigtail" directly from the shaft to the frame. In this way the variable resistance caused by the shaft-bearing contact is eliminated entirely and the whole circuit is' carried by the "pigtail." The wavelength range of the receiver with the four plug-in coils is from seventeen to one hundred and fifty-five meters, arranged to include all short-wave stations experimentally broadcasting at present or contemplated, as well as the principal amateur phone and telegraphic bands. It should be noted that the construction of the Aero plug-in short-wave coils has been altered from a 3-inch diameter to a 2-inch. This change has improved the structure and rigidity of the coil; it limits the magnetic field of the coil materially by increasing the ratio of length to diameter for a given inductance; it minimizes the pickup from powerful near-by stations, and also provides a more favorable coefficient of coupling between the tickler and secondary for operation at extremely low waves. The constructional work on this receiver has been reduced to a minimum. A foundation unit is available with drilled and engraved front panel, subpanel provided with sockets, and back panel, all of Westinghouse Micarta. NOTES ON ASSEMBLY THE top view of the unit on page 167 shows the arrangement of this subpanel. It will be noted that the socket for the cx-322 tube, at the right, is depressed and held with machine screws and short bakelite studs, in order that this taller tube shall not project above the top of the front panel. The plug-in jacks usually contained in the Aero coil mounting have been dismounted and are supplied mounted directly in the subpanel. To the left and right of the coil mounting are the radio-frequency chokes, L5 and "LJt respectively, and the grid leak mounting directly behind the coil. It is wise in assembling the receiver to do as much of the work as possible on this subpanel before assembling it into the mounting proper. It will be necessary to solder long leads to the plate and grid terminals of the audio and detector sockets in order to facilitate their connection to the audio transformers after assembly.