Radio Broadcast (May 1928-Apr 1929)

176 RADIO BROADCAST JANUARY, 1929 main foundation base plate. All of the wiring in the receiver (with the exception of the external power leads), is placed within this \" of space. Originally the H. F. L. Isotone was designed as a battery-operated receiver. The engineers realized that, while electric operation was highly desirable, a satisfactory receiver would have to be built around direct current tubes. Tubes of the a.c. type, and particularly those of the a.c. screen-grid type, are not considered by the writer to be conductive to the best possible results. Therefore, in designing the Isotone particular attention was paid to stability, ease of operation, and economy in operaion. When a receiver was realized on a direct-current basis which furnished these desirable factors, it was decided that the practical way of electrifying such a set would be by a dry power-supply unit furnishing all A, B and C voltages. POWER SUPPLY UNIT THE Model 5 ABC power supply was designed as a special current-supply device for the H. F. L. Isotone. The A voltage is furnished by an Flkon dry rectifier unit operating in conjunction with large filter chokes. The power supply also furnished plate potentials of 50, 135 and 180 volts for the plate circuits of the various tubes. In addition to these voltages there is also a connection marked "90 volts" which has an individual variable resistor as its controlling device. From this terminal any voltage from o to 180 volts may be obtained. Thus, the unit will deliver a set of voltages which will operate practically any receiver in existence to-day. Regarding the performance of the receiver. In an actual test in the City of Chicago on October 22nd, 1928, the Isotone brought in THE POWER UNIT This power unit, which was designed especially for use with the Isotone receiver, supplies all necessary grid, filament, and plate potentials. The A current is obtained from a dry rectifier unit, and a 280-type tube is employed to provide the B voltages One assembled and wired tuning unit; One assembled and wired screen-grid amplifier; One assembled and wired audio amplifier; Eight shield cans with tops; One base assembly plate; One drilled and engraved front panel; One seven-wire cable and socket; Two gold escutcheons with knobs (attached); Two dial lights (inside of drums); Two large walnut switch knobs; One small walnut switch knob; Two steel panel supporting brackets; Twelve plated connecting strips; Fifty-five 6-32 hexagon brass nuts; Fourteen f-inch hexagon spacer studs; Fourteen f by 6-32 inch r.h. machine screws; Six j by 6-32 inch f.h. black machine screws; Four f by 6-32 inch r.h. machine screws; Eleven tinned copper lugs; Six feet push-back wire. Assuming A.C. operation, the constructor will require the following accessories; station pwx, Havana, Cuba, with full loudspeaker volume for a period of one hour starting at nine p. m. Central Standard Daylight-Saving Time. The temperature at this time was around fifty degrees, and it is estimated that at this same hour over twenty seven local stations were operating. LIST OF PARTS THE Isotone receiver is sold only in semicompleted form as described in this article; that is the various units are supplied completely assembled and wired. However, it may be well to list the actual parts of the H. F. L. kit just as they come to the set-builder: Three 222-type tubes; Three ii2A-type tubes; Two i7iA-type tubes; Two 2oiA-type tubes; One Model 5 ABC power supply; One 280-type tube (for the power supply); One Pacent phonograph pick-up unit (optional); One loop antenna. For D.C. operation the following batteries will be required: One 6-volt storage battery (120-ampere hour); Two 225-volt C batteries; Four 45-volt heavy-duty B batteries. COMPLETE SCHEMATIC DIAGRAM OF RECEIVER Although the five basic units of the Isotone receiver are supplied will be found a great aid to the set-builder when constructing the completely wired by the manufacturer, this diagram shows the set, as it follows closely the mechanical arrangement of the parts, circuit of the entire set. The peculiar arrangement of the diagram The dotted lines of the diagram enclose the shielded circuits