Radio broadcast .. (1922-30)

370 RADIO BROADCAST FEBRUARY, 1927 - No. 28 Hole No.28Hole R.F STAGE DETECTOR FIG. 4 For those who desire to make their own inter- stage shield the specifications given here will prove helpful furnishes, for a small extra charge, a set of in- sulated bushings designed to fit snugly over the various mounting parts of their dial so that this objectionable feature of contact with metal panels is successfully overcome. Reference to Fig. 6 will show how the bushings are employed to insulate the dial frames from the panel. In order to simplify the assembly of the output terminals, a single-circuit jack was substituted for the pin jacks formerly used. It is not a difficult matter to make the change since one side of the output circuit returns to the minus A circuit—very easily accomplished by mounting the frame of the jack in the metal panel, which in turn is connected to the minus A-battery circuit. In Fig. 7 are shown the specifications for the new panel layout, and with few exceptions there is very little difference from the original layout. In the neutralizer circuit, a o.oi-mfd. fixed 1 St. A. F STAGE 2nd. A. F STAGE 201A 171 C 7 ~JU 1 234 567 -4(4 -40!* A- A* +45 +90 +180 ^T' c+ Gnd. =" •* — Indicates groundingto metal D<*"et Ground A Terminal of 6-Volts storage battery FIG. 5 This is the circuit diagram of the four-tube R. B. "Lab" receiver in its altered form. The i-mfd. bypass condensers, shown in dotted lines are to be recommended especially where the power supply is obtained from an a. c. operated B power unit. In any event, the inclusion of these condensers tends toward greater stabilization in the operation of the receiver condenser is connected in series with the regular neutralizing condenser, which has a value of 10 mmfd. In this newer construction the Precise type 940 condenser has taken the place of the mica insulated type of condenser which was formerly employed. To conserve space and prevent the crowd- ing of parts and wiring, the terminal strip sup- porting the binding posts has been replaced by a Yaxley receptacle and 7-wire battery cable. There are just enough wires in this cable to make connec- tion to all the batter- ies. A separate binding post mounted on a strip of bakelite is mounted in a convenient spot as shown in the illustrations, to accom- modate the antenna lead-in connection. Since the ground connection can be made direct to the minus post of the A battery, no separate ground terminal in the receiver is necessary. Fig. 8 gives the specifications for the antenna post mount, brackets, and other insulating strips necessary for the assembly of the receiver. The circuit diagrams and photographs ac- companying this article are illustrative of the changes and alterations made, and serve well to guide the constructor in the duplication of these modifications. The following lists of parts show the appa- ratus that was used in the original circuit and that specified for the present receiver: RADIO BROADCAST Photograph THE RADIO-FREQUENCY AMPLIFIER STAGE Which comprises the first tuning circuit, amplifier tube, neutralizing condenser, r. f. choke coil, etc., are situated at the extreme right of the base-board as may be seen from the illustration above. The loading coil which is connected in series with the antenna circuit is situated forward of the interstage shield. Practically all of the apparatus employed in the construction of the receiver retains the same position it originally occu- pied as described in the November, 1926, RADIO BROADCAST Knob FIG. It is really a simple job to mount the illuminated controls on the metal panel with the aid of the insulated washers. These latter prevent any connection between the dial and the metal panel. The sketch above will clearly indicate the method employed. Explained, it is briefly this: An insulated washer is slipped over the two mounting bushings of the dial and another smaller washer is fitted in the hole in the panel. Then the dial is held to the panel while, on the front, another washer is passed over the mounting bushing. The dial is then securely fastened by means of the metal washers and nuts provided