Radio Broadcast (May 1928-Apr 1929)

40 RADIO BROADCAST MAY, 1928 detector is impressed on the first intermediatefrequency amplifier tube grid. The reader will no doubt recognize the intermediate amplifier as the well-known Silver-Marshall "Jeweller's Time-Signal Amplifier" unit. The high amplification, the sharp cut off of its accurately tuned air-core transformers, and the consistently excellent results obtained with a considerable number of these units, are the reasons for its selection for this receiver. After the second detector, the audio component of the signal is amplified by one stage of audio amplification. A choke coil together with the bypass condenser included in the amplifier, combine in bypassing the radio-frequency component of the signal to ground, thereby keeping it out of the audio amplifier. The output of this stage is fed to the second audio stage, which has been omitted in the circuit diagram. To combine the various ideas described above in a receiver employing battery-operated tubes is an easy matter. Fig. 2 shows such an arrangement. True electric operation, however, is convenient and obtained in simplest, cheapest, and least troublesome form with the tubes lighted from an alternating-current source of supply. Tube life when a.c. tubes are used is an important consideration. The writer's experience indicates that excessive filament voltage is the cause of complaints of short life of the a. c. tubes. An almost total lack of measuring instruments is responsible for this condition which time and an increase in knowledge will undoubtedly correct. As all tubes in this receiver are worked at a point well under the rated voltage, uniform and highly satisfactory results are to be expected, the voltage adjustments being extremely easy to make. GENERAL CONSIDERATIONS THE omission of a number of details from the * review of the receiver, while enabling the reader to obtain a clearer idea of the main features, has no doubt set up a number of questions. The numbers for the switch terminals are determined by counting from right to left from a rear view, as explained previously. The first detector and oscillator circuits may be easily traced with this information at hand. These tubes in these two circuits are of the cathode type in order that the beat note, tremendously amplified in the intermediate stages, will be absolutely free from hum. The cx-326 (Ux-226) type tube is used in the three intermediate stages, the 20-ohm potentiometer, R3, across the filament circuit, providing a mid-tap >o.7 for the grid returns. The 1000-ohm potentiometer, R2, biases the grids of these tubes to prevent oscillation and hum, the usual method of running the grids positive being impossible where alternating current is employed. The method of securing the bias voltage will be recognized as that used in biasing the last audio stage in the modern power amplifier. A 50,000ohm variable resistor, R4, is shunted across the primary of the audio transformer in order to provide an additional means of reducing the volume when the super-heterodyne is employed. The remaining resistor, the 3000-ohm potentiometer, Ri, provides a common bias voltage for the second detector and first audio stages. The adapter which is necessary in order that the cathode type tube may be employed in the standard socket in the "Time-Signal Amplifier" is omitted from the diagram for the sake of simplicity. It is referred to in the list of parts. A detailed account of this device will be found later, in the wiring instructions. The various colored leads, ten in number, noted in the diagram, are provided in a single Jones ten-wire cable. This enables the user to disconnect the power from the receiver in a second or so. A Silver-Marshall filament transformer is used to supply the two filament voltages for the a.c. tubes and the voltage for the dial lights. Three of the cable wires provide B voltage to the receiver while a fourth connects the plate of the first audio tube to the primary of the second transformer. ASSEMBLY \A/ '"^ a SrasP °f tne main facts and an idea * of the principles employed, the construction of this receiver becomes an extremely simple matter. The first step is that of assembly. All of the apparatus, with the exception of the "Time Amplifier," is put in the positions noted in Fig. 3. The pointers listed below have been gathered from the experiences of a number of builders, and if followed carefully, will insure perfect results. After mounting the panels and dials the first point to be noted in the assembly is the method of mounting the condensers. The slotted bars provided with the dials are removed and a oneinch machine screw is slipped into the slot in the dial frame. The three collars or bushings furnished with each dial are slipped over the screws. The condensers are then held in the position shown in the photograph and the screws are threaded into the holes provided in the condenser frames. An extremely solid mounting is the result. The tube sockets, audio transformer, <(90 C5 Time Amplifier 0002 mfd OOO R5 ' Dial Lights 90 V. 6V.AX.orD.C. B O 45 V -4HC FIG. 2 Circuit arrangement for battery operation switches, bypass condenser, choke-coil, the 50,000-ohm resistor on the panel, and the antenna coil require no special description. The remaining resistors are mounted on the resistor strip which is raised two inches above the baseboard by brackets, in the following order: In No. 1 position place the 3000-ohm potentiometer; No. 2, the 1000-ohm potentiometer; No. 3, the 20-ohm potentiometer; No. 4, the 50,000-ohm resistor. The double rotor coupler (consisting of L2, L3, L4) used in the first detector circuit to secure variable antenna coupling and regeneration requires altering before mounting. All but eight turns are removed from the antenna coupling rotor, L2, which is controlled by the lower of the two knobs. The oscillator coil also requires alteration before mounting. Eight turns are removed from the outside end of the large or grid winding of the coil, L5. This is done so that the detector and oscillator control settings will match although tuned 112 kc. apart. The wire removed from this coil should be added to the plate coil, L.6, at the bottom, insuring sufficient feedback to cause oscillation. This coil is then mounted three eighths of an inch from the coupler, as shown in the photograph, to insure proper coupling. The wiring of the first detector, audio stage, and oscillator circuits is the next step. A twisted pair from the pink and blue terminals of the cable to each pair of dial light terminals eliminates these from further calculation. From the yellow and black terminals another twisted pair is connected to the contacts of the mid-tap resistor mounted on the filament posts of the audio tube socket. From this point the pair is continued to the detector socket and from there one wire goes to the oscillator tube socket while the other connects to Contact No. 5 on the triple-pole switch. A wire from No. 4 contact on the switch to the remaining filament post completes the wiring of the heater circuits. The remaining wiring to these three tubes may be easily traced from the diagram. All filament, cathode, and B battery wiring should be formed along the main cable, as shown in the photograph, wherever possible. The leads from the plate of the detector to the switch, from the antenna coil to the coupling coil, and other leads at a high potential from a radio-frequency standpoint, should be formed in a secondary cable close to the panel. Avoid right-angle bends on grid and plate connections; the shorter they are, the better. In wiring the oscillator circuit be sure that the grid and plate connect to the outside ends of their respective coils, or the tube will not oscillate. Pin-jacks may be mounted at the ends of the resistor strip so that a phonograph pick-up or one of the "homebroadcasting" microphones may be employed. One pin-jack should be connected to B minus and the other to the plate terminal of the audio transformer. The drilling instructions for the resistor strip are shown in Fig. 3. Only two of the three connections on the 1000 and 3000-ohm potentiometers are employed as their function is that of a variable resistor rather than a bridge resistance in this circuit. The "Time-Signal Amplifier" should be put in position next. The filament wiring should be twisted together and the remaining leads running the length of the receiver should be formed into the main cable. The B minus, plus 45 volt, and plus 90 volt connections should be picked up at the nearest point in the wiring of the other tubes and connected to the proper posts on the amplifier. The adapter for the second detector tube, referred to previously, is provided with a pair