Radio Broadcast (Nov 1926-Apr 1927)

NOVEMBER, i9a6 CONSTRUCTING THE R. B. "LAB" RECEIVER 37 the metal panel shield to the panel proper by means of four screws which pass through the brass brackets holding the panel in place. The copper should be laid flat on the panel and holes located at the various points where the instruments must project through to the front of the panel. The holes must be made sufficiently large so that no part of the instruments will come in contact with the shield itself. Assemble the Precise regeneration condenser, Electrad Royalty 500,000-ohm variable resistance, Frost pin jacks, and Electrad filament switch on the panel in their respective mounting holes. Then set the panel aside and assemble the parts on the base board. A layout of these parts is shown in Fig. 5. Without fastening the panel to the base board, much of the wiring can be done, as shown in Fig. 8. It is better to do the simpler and shorter connections first. For instance, the audio transformers and sockets are close enough together so that with the aid of a lug on the terminal of the socket, direct connection, without the use of wire, can be made from the grid and plate posts of the sockets to their respective grid and plate transformer terminals. After this is done, the filament wiring may receive attention. Here is where cabling of the leads is beneficial. First, with a piece of gray (natural) hook-up wire, connect the minus F terminals of all the sockets together, leaving about f-inch of slack wire between the terminals. Next, beginning at the minus A binding post, run a piece of the same colored wire along the Direction of Winding . 21" — } A" Hole ft -t <H Hole fx — 3" >\< 6" ;%"Hole -%"Hole % Hole .No. 27Hole&Csk. H Hole 1 ' fcfc::f-f<27H0,e& Csk. I No. 27Hole&Csk.r_ k"H^No:27Hole&C5ki ~y Hole It •it FIG. 4 The panel for the "Lab" receiver shojld be drilled in accordance with the directions on the diagram above. A steel template furnished with the Marco illuminated dials enables accurate drilling of holes for the windows on the panel to be accomplished FIG. 3 The diagram above, together with that on page 36 and the text in the article will enable the veriest of home constructors to satisfactorily produce the "Lab" circuit coils. The connections of these coils in the circuit are explained by the diagram given here amount can be purchased); a piece of f-inch bakelite or formica; a strip of ^ x j^-inch brass strip; four No. 6 brass round head wood screws; a mailing tube if-inches in diameter, and a sheet of celluloid, such as is used in photography. A strip of bakelite or formica is also required as a coil support. This may be obtained by cutting a strip of the desired width from a sheet of iV-inch panel material. With this is required one i x A inch round head brass machine screw. back of the base board and down the left-hand side, and cut it when it is long enough to reach the position the filament switch on the main panel will take. This piece of wire is run parallel to the back edge of the base board and, as it advances, it is wound over the previous socket wiring threading it under and over, etc. From the second approximately estimated position of the contact on the filament switch a similar piece of wire is run back to the minus F post on the last audio socket. It also is twisted with the other lead emanating from the filament switch. Continuing the filament wiring, a piece of green wire is started from the plus A binding post, wound or twisted around the gray wires, and attaches to the right-hand end of each filament ballast. In the radio frequency, detector, and first audio stages, the plus F posts are connected together with more green wire, which is twisted over the rest of the wires. At this time, it is not well to connect the remaining contacts on the Brachstats to their respective terminals on the sockets because the presence of these wires will impede the completion of the wiring of the B battery and C battery leads. From both audio transformers, connection must be made to the B and C binding posts on the terminal strip. As these connections are made, the wire is twisted around the mass already forming a formidable cable. The B battery leads are done in red wire, the C battery leads in blue. COIL DATA HP HE tuning coils may *■ be directly wired into place without difficulty. Winding and assembling the coils for the "Lab" circuit is not as complicated as a first glance at the sketch in Figs. 2 and 3 would make one believe. First procure the material, namely: A J-lb. spool of No. 22 d.s.c. wire (a |-lb. spool is too much, but it is doubtful whether a smaller Radio Broadcast Photograph A CLOSE UP OF THE CABLING Shows how all the wires are formed into one big twist. This is accomplished by threading each wire as it progresses from terminal to terminal under and over those already there