Radio Broadcast (May 1928-Apr 1929)

AUGUST, 1928 RADIO BROADCAST 221 Radio Broadcast's Service Data Sheets on Manufactured Receivers August, 1928. The Fada 480-B RADIO receivers designed for a.c. operation fall into three general classes, i.e., those using a.c. tubes, those using standard battery type tubes with the filaments connected in series, and those using standard type tubes with the filaments connected in parallel and energized from an A-power unit. This latter type of receiver may be operated from batteries when one desires or may be made completely light-socket operated by the use of A and B-power units. Such a receiver is the Fada 480-B, a circuit diagram and photograph of which is given in this data sheet. The Fada 480-B receiver is a dual control set, the right-hand drum dial (in the diagram) tuning the antenna stage and the left hand drum dial controlling all the other tuning condensers in the set. Small trimming condensers are placed across each main tuning condenser so that the several tuned circuits may be accurately aligned. The tuning condensers are substantially made with heavy plates and large bearings so that they will operate smoothly and hold their calibration over a very long period of time. The receiver may be operated from either a loop or an antenna, either indoor or outdoor. This eight-tube receiver consists of four stages of tuned radiofrequency amplification, a detector and either two or three stages of transformer-coupled low-frequency amplification, depending upon the position of the toggle switch indicated at the top of the diagram. All of the r.f. stages are shielded, the plate leads are all bypassed with 0.5 mfd. condensers and each stage is neutralized by means of centertapped primaries on the r.f. transformers^— a type of neutralization familiar to many as the so-called "Roberts" method. An r.f. choke coil, Li, is placed in the positive filament lead of each of the r.f. tubes to keep any a.c. currents out of the plate supply, where it might produce common coupling and cause oscillations. A separate shielded compartment contains the A VIEW OF THE CABINET THE SHIELDING ARRANGEMENT audio-frequency amplifier. This compartment is at the right in the circuit diagram and contains the sockets, three audio transformers, the output choke, output condenser, and bypass condensers. By means of the toggle switch on the panel either two or three audio stages may be used; with the switch thrown to the No. 8 position all three stages are used, while in the No. 7 position only two stages are used. Tubes of the 201A type are used in the audio amplifier as well as in the detector and r.f. tube sockets. A power tube of the 171A type is recommended for use in the output of the amplifier. The power circuit contains in its output a chokecondenser combination to protect the loud speaker from the d.c. plate current. The choke is marked L2 in the diagram and the output condenser Ci. The condenser has a capacity of 1.0 mfd. An A-B-C power unit is made by the same company to be used with their receiver; The B and C unit uses a type 280 full-wave rectifier to supply the necessary plate and grid bias voltages to the set. The A-power unit consists of a rectifier-filter system designed to supply approximately six volts and currents up to 2\ amperes. This supply is used for the operation of the filaments of the tubes in the receivers and the output is sufficiently well filtered to give quiet operation. The alternating line current is converted to pulsating direct current by means of a dry type rectifier manufactured by the Elkon Works, which is supplied with the correct amount of voltage from the secondary winding of the transformer. The pulsating current is then filtered by means of high-capacity dry A type condensers and properly designed choke coils. There are two adjustments provided; one is for various line voltages between 90 and 130 volts, and the other is used to adjust the output of the device for the operation of six, seven, or eight tube receivers. These 6-volt A-power units are furnished in two types: one for the operation of six-and seven-tube receivers and another for the operation of six-, seven-, or eight-tube receivers. Both of these types are supplied in two models: one for operation on either 50 or 60-cycle alternating current and one for operation on from 25 to 50 cycles.