Radio broadcast .. (1922-30)

420 RADIO BROADCAST ADVERTISER Control Volume with this Modulator Plug "V\ T"ITH your radio set VV operating under full power, you can now regulate tone and volume to suit your mood, by simply turning the knob on this Centralab Mod' ulator Plug! Replaces ordinary loudspeaker plug. Provides perfect control of volume from a whisper to maximum, without touching the tuning dials or rheostat. Cuts down powerful local stations, and brings through programs sweet and clear — improves re' ception wonderfully! $1.50 at your radio dealer's — or sent direct if he can not supply you. Write for literature describing this and other Centralab controls. 22 Keefe Ave., Milwaukee, Wis Central Radio Laboratories :Now, I HAVE FOUND A Department for the Exchange of Ideas and Suggestions of Value to the Radio Constructor and Operator CONTRIBUTIONS to this department are welcome and those used will be paid for at the usual rates, that is, from two to ten dollars each. A pri^e of twenty-five dollars is given for the best idea used daring each three-month period. The pri^e winner for the last period is announced in the August RADIO BROADCAST. Manuscripts intended for this department should not exceed about three hundred words in length, and should be typewritten. Little consideration can be given to manuscripts not typewritten. Envelopes should be addressed to this department, RADIO BROADCAST, Garden City, New York. A NEW PRIMARY FOR THE BROWNINGDRAKE R. F. TRANSFORMER N ACCOUNT of its extraordinarily high efficiency, the Browning-Drake circuit has won much favor with radio fans; especially since the construction of this type of set is simple and straightforward. The only difficulty I came across when building the coils for my first receiver of this kind was in the construction of the former for the primary of the r. f. transformer. This primary is wound in a nar , Primary Winding Cut fro hard rubber shee Cut away as shown v 3 Tube for Secondary FIG. I row groove, and a turned form made of wood or hard rubber is generally specified. Having no lathe, I made some experimental forms consisting of three cardboard discs. Two of these were cut just large enough to go into the coil former with slight pressure, the third was about iV less in diameter. The three discs were cemented together with their centers coinciding. Quite a useful form was the result, but for the finished set, something better was desired. Having some small pieces of /«" hard rubber in my junk box I set to work and finally evolved the new low loss form shown in the accompanying sketches Figs, i, 2, and j. It is merely a starshaped pattern with eight arms. The top ends of the arms are slotted to take the primary winding (as shown in Fig. i.) The tube used as a form for the secondary winding had an outer diameter of three inches; the form for the primary was filed and sandpapered down until the arms made a good fit within the outer, larger, tube. To mark out this form, paste a piece of drawing paper over the piece of hard rubber selected. Draw the center lines, and divide each ninety-degree section formed bv these center lines into four equal parts, as indicated in Fig. 2. Next draw the pitch circle for the \" diameter holes, and Toted and approved bv RADIO BROADCAST mark the holes with a center punch. Drill these holes before proceeding with the marking. Draw the outer circle X in Fig. 2, with a diameter equivalent to the inside diameter of the cylindrical tube and mark the width of the arms on this circle. This width may be about |". From the points A and B, draw tangents to the small holes drilled previously. Using a sharp new hacksaw blade, cut out the form along the lines marked. Some care must be exercised during this operation or the hard rubber sheet may split. The cutting is made much easier if the sheet is clamped in a vise. The sheet of paper which had been pasted to the hard rubber sheet is now washed off with warm water and the coil form is throughly dried. Finish it by rubbing it all over with a little machine oil and polishing it with the finest sandpaper you can lay your hands on. It only remains to slot the arms of the form for the primary winding. Again it is clamped in the vise, with the top of the arm just projecting. To prevent it being marked by the jaws of the vise, place a thin smooth board on each side of the hard rubber form before clamping it. The slot should be about iV' wide. A FIG. 2