Radio broadcast .. (1922-30)

No. 42 Radio Broadcast's Set Data Sheets March, 1930 ERLA MODEL 224 A.C. SCREEN-GRID RECEIVER GANG CONDENSER cmomm. \ LOW \f CURRENT ..L CHOKE • ON-OFF SWITCH 'T'BE CIRCUIT of this receiver chassis employs -1 three stages of tuned-radio-frequency am- plification with four tuned circuits and uses three 224-type screen-grid tubes. The detector is a 227-type tube operated in a grid-bias or plate-rectification circuit. In the first audio- frequency stage a 227-type tube is employed with resistance-coupled amplification. In the output stage two 245-type tubes are used in a push pull. Reference to the diagram will show that two antenna connections are provided. For most purposes the "S. Ant." connection should be used as it provides the greatest sensi- tivity, but if a very long antenna is used, con- nection should be made to the " L. Ant." post. It will be noted that the " L. Ant." connection consists of a resist- er (attenuator) in series with the S. Ant." connection. This serves to cut down the signal pickup and consequently the noise level when a very long an- tenna is used. The local-distance switch func- tions to tap the primary coil of the first r.f. transformer. When in the tapped or "local" position the amplification is decreased. This setting is best for most local stations as that proper control of volume is obtainea. By moving Position of Tube Rectifier 2nd A.F. 1st A.F. Del. R.F. this switch to the distance position extreme sensitivity is obtained, but if powerful local stations are tuned-in improper volume control action is obtained for these strong signals tend to overload the screen-grid tubes. The volume control consists of a 6000-ohm wire-wound potentiometer connected so as to vary the screen voltage on the screen-grid tubes. In combination with this volume control a single-pole, double-throw toggle switch is em- ployed to throw the receiver from "phono- graph" to "radio." When the control is turned all the way to the left the input to the resistance- coupled a.f. stage is switched from the output of the detector circuit to the phonograph pick-up Type of Tube 280 245 227 227 224 AVERAGE VOLTAGE READINGS Screen-grid Plate to Fit. Volts to cathode cathode (A.C.) 4.8 to 5 2.4 to2.5 2.35 to 2.4 2.35 to2.4 2.35 to2.4 volts 75 to 80 volts 340 to 360 240 to 250 90 to 100 60 to 75 160 to 170 Ground to cathode volts 4.5 6 to 7.5 1.5 to2 jacks on the rear of the chassis. It will be noted that a small clip-type switch is connected across one of the jacks and the chassis frame. This clip should be connected in this manner when the pick-up unit is not inserted in the jacks, as other- wise a hum will be evident when the volume control is set so that the receiver is in the phono- graph position. This hum results from having the input circuit of this first a.f. tube open. A small toggle switch located alongside of the 280-type rectifier tube is used to tap the pri- mary of the power transformer. The approximate line voltage ranges for the two positions of this switch are shown. It is well to keep this switch in the 115— 125-volt position wherever possible. The loud speaker used with this receiver is of the electrodynamic type and contains in its assembly the output transformer for coup- ling the output of the two 245- Gridto filament mils 45 to 50 (The above are based on a line potential of 110 volts and the in the 95-115 position, no signal and volume control at maxim switch um.) type tubes to the moving coil of the loud speaker. The field of this loud speaker has a d.c. resistance of 1000 ohms and is designed to carry 100 milliaraperes. All con- nections from the loud speaker are made by means of a five- conductor cable and the special five-prong plug which avoids any danger of improperly connecting the loud speaker to the receiver. 290 • • RADIO BROADCAST FOR MARCH