Radio Broadcast (May 1928-Apr 1929)

JUNE, 1928 BUILDING AND OPERATING THE A. G. "R. B. LAB" RECEIVER 9-7 <D © © © ® FIG. 2 When the adjustment is properly made there will be a slight "swishing" sound as the r.f. circuit is tuned to the station and the signal will clear up. If the detector is placed on the point of oscillation, there will be no tendency for the r.f. stage to make it oscillate as the r.f. dial is turned. It may be necessary to readjust the regeneration control, (C3), slightly to make the balance adjustment more exact although the setting is not very critical. After the set is balanced, the effect of handcapacity should be tried on a weak station. If there is any appreciable effect, a bakelite or hardwood shaft should be substituted for the brass one used with each condenser. The effect may be further reduced after this change has been made by grounding the metal frame of the dials. If the antenna stage tunes broadly it is an indication that the antenna is too large and the effective capacity should be reduced by connecting a fixed condenser about 0.0001-mfd. capacity in series with the antenna lead. The selectivity of the first stage should be measured with the volume control in the full "on" position since this resistance reduces the selectivity. When the set is tuned to a station where the volume control is near the "off" position, the question of selectivity is never important. Since the volume control is in the radiofrequency circuit, special provision must be made for controlling the volume of the phonograph pick-up arrangement. Nearly all of the standard pick-ups with which we are familiar are sold with a special volume control. To minimize hum, the B-power circuit should be grounded. Usually the ground works most satisfactorily on the minus B lead. In some cases the hum is reduced by grounding the plus 45-voIt tap which in this set connects to the heater winding mid-tap. Only one of these leads should be grounded, however. If an outdoor antenna is not available, the experimenter may try connecting a 0.002-mfd. fixed high quality mica condenser from one side of the house-lighting circuit to the antenna binding post. Both sides of the line should be tried since one side may be grounded. In this case the line acts as the collector and the signal is brought to the receiver much as it is in carriercurrent telephony. The list of parts below are those used in the model described here. Other parts, electrically and mechanically similar, may of course, be used. The coils L2 and L3 are special and are supplied by Aero Products. If the builder desires to revamp the standard Aero coil set, No. U95, he should follow instructions on page 94. The dimensions for all the coils, in the event the constructor prefers to make his own coils, are indicated on the diagram, Fig. 2. PARTS LIST L2, L3 Aero Products Co., (1) Pr. U-95, or Special Lab. Coils $ 8.00 Ci, C2 Hammarlund, (2) ML-23 Condensers . . 1 1 .00 C3 Hammarlund, (i)-MC-i 5 Midget 2.00 L4 Hammarlund (.1) RFC-85 Choke 2 .00 C4 Hammarlund, (1) Equalizer . . .50 Micarta (Westinghouse Co.), (1) 7" x 21" x \" Black Panel . ... 2.20 Ri Electrad (1) Type P Volume Control 1.50 R2> R3 Electrad (2) 500-Ohm Grid Type Resistors .50 Rs Electrad, (1) V-30 Resistance . . .75 R4 Electrad, (1) V-10 Resistance . . .75 Yaxley, (1) 669 Cable 3 .25 Yaxley, (1) S.P.D.T. Switch No. 30 . 90 Ti, T2 Silver-Marshall, (2) 240 Audio Transformers 12.00 T3 Silver-Marshall, (1) 247 Filament Transformer 5.00 Eby, (3) 5-Prong Sockets .... 1 . 50 Eby, (2) 4-Prong Sockets .... .80 Eby, (2) Binding Posts .30 C7, Cg Tobe, (2) 1.0-mfd. Bypass condensers 1.80 C6 Aerovox, (1) 0.00025 Grid Condenser .40 Kf, International Resistance Co. (1) 2.0 Meg. Grid Leak .50 International Resistance Co., (1) Leak Mount .50 National Co., (2) Single Drum Dials . 9.00 $65.15 ADDITIONAL PARTS (Needed to complete the set as described). (1) Phonograph pick-up, with volume control. (1) B Power-Unit, furnishing a maximum voltage of 220, at 40 mils., with taps as follows: 45, 90, 180, -40. (3) a. c. 227 type tubes. (1) Power tube, Type 112-A or 171-A (See Text). (1) C Battery (Optional, see text; rating depends on power tube used). (1) Corbett cabinet to accommodate panel (7x21 x |"). 11 n 11 n ' Blue ♦ 45 FIG. 3