Radio Broadcast (May 1928-Apr 1929)

THE EVER-POPULAR ROBERTS SET WITH A. C. OPERATION The illustration is lettered to agree with the list of parts on page 101 and with Fig. 3. The circuit diagram. This model of the a.c. Roberts reflex circuit was built to fit in a phonograph cabinet. It is unnecessary for the constructor to follow this layout for wide latitude is allowed by this circuit in this respect A Three-Tube A* C. Operated Roberts Receiver By ELMER G. HERY OF THE thousands of set-builders who have constructed the original three-tube Roberts, few have been won over to any other circuit unless they have gone to a much more elaborate and expensive layout. For those who know the merits of the Roberts circuit, a.c. operation as described in this article is almost inevitable. Beginners will find in this receiver a set which is easy to construct and uniformly excellent in results as to the quality, selectivity, sensitivity, volume, and distance reception. An important feature is that this circuit does not require the most expensive pieces of equipment. It may be constructed from whatever materials are at hand. The skeptical may make a rough assembly of old parts, and then convince himself that the substitution of any good lowloss parts will give better results. [The list of parts and the photographs show exactly what was used in the set described by the writer. The parts are all standard and readily available. Wide substitution can be made, according to the desire of the constructor. — Editor.] Another good feature is the fact that the arrangement of parts and the panel shape do not affect the results. This circuit has been built by the writer on square panels and on long narrow panels, and to fit in all kinds of cabinets with excellent reception in all cases. The first thing to consider is the coils. Manufactured coils may be used if desired. The writer has used Sickles and Hammarlund-Roberts with perfect satisfaction. If the set-builder desires to make up his own coils they can easily be made. Thirteen-point spider-web fiber forms are required. They may be obtained from the 10-cent stores. The antenna coil consists of thirty-five turns wound over-two-and-under-two spokes, with a twisted loop or tap every five turns. There are two secondary coils Lj and L4, and each consisting of forty-four turns wound over-twoand-under-two spokes. The tickler coil, L5 consists of twelve turns wound over-two-and-undertwo spokes. These four coils are all made of No. The A. C. Roberts Receiver THE Roberts receiver, first introduced by Radio Broadcast in 1924 won many friends for radio and many for this magazine. It has been constructed by more than 100,000 radio fans and, in one form or another, is still giving satisfactory service all over the world. The circuit is so efficient— considering the number of tubes employed — and so easy to build and operate that its popularity, like a certain famous cigarette, is deserved. Many readers are still interested in building the circuit and Mr. Hery's article here gives them full instructions and a wide latitude in construction. For those who are interested in making over their present Roberts set for a.c. operation, the last part of this article provides sufficient information. The circuit shown here employs the original reflex arrangement. Those who desire to employ a straight audio stage and eliminate the reflex may secure information by writing to our Technical Information Service. — The Editor. 22 double cotton covered wire. The NP coil L3, is made of No. 26 double cotton covered wire. This coil consists of a double winding; that is, two parallel wires wound over-one-and-underone spoke, with eighteen turns. In other words, there are two concentric coils of eighteen turns each. The wire may be twisted or kept flat, preferably the latter. The antenna coil and its secondary coil are mounted on the same shaft about f" apart. A long brass machine screw (3§" or more) makes an ideal mounting arrangement, using a nut on either side of each coil to hold it rigid. The machine screw is bent to give any desired mounting angle to the coils, and fastened through the baseboard with a nut. (See Fig. 1) Fig. i-B also provides a very simple mounting through one of the spokes of one coil. A standard mounting for the tickler coil is an arm. with knob for panel mounting. The NP and secondary coils may then be mounted on a screw through the baseboard as shown in Fig. 2. The dotted lines show the tickler coil in the raised position. It is important that the antenna coil and its secondary be mounted so that the direction of rotation of the windings is the same for both coils. Likewise the NP, Secondary, and Tickler coils should be wound in the same direction. One way to avoid trouble from this source is to proceed as follows: when the coils are wound, mark on the forms an arrow which points around the form in a clockwise direction. Then, starting from the inside and proceeding to the outside of the form, the wire should be wound 99