Radio broadcast .. (1922-30)

524 Radio Broadcast FIG. 2 poise is connected to the radio receiver in place of the ordinary ground connection. USING STANDARD PARTS FOR THE ROBERTS CIRCUIT IN THE matter of substitute coils to replace the spiderwebs used in the RADIO BROADCAST Knock-out circuit, it is well to understand from the start that endeavors along these lines so far have been experimental in nature. Many experimenters are working on the problem, and in a short while no doubt the needs of all will be satisfied in this particular connection. RADIO BROADCAST has experimented|with various forms and herewith is presented a few guiding facts which may prove helpful to those who wish to experiment of their own accord. In most instances those couplers now on the market such as Ambassador, Shamrock, Fischer, Eastern picklebottle, and others of a similar constructional nature may be advantageously experimented with by merely adding to, or rewinding the primary coil constituting the N-P winding of the Roberts circuit. The turn ratio between primary and secondary will vary according to the coupler used and no definite value can be given. In general it is well to use the same number of turns as specified for the spiderwebs, then vary for satisfactory operation. Instead of a double wound primary a coil of twice the number of turns as specified may be used taking off a center tap as shown in Fig. 5 and 6. In most instances the placement of the primary N-P coil will have a very decided effect upon the operation of the receiver. The usual practice is to rewind the primary N-P coil directly over the secondary with cambric cloth insulation between the two. Standard neutroformers offer an opportunity for interesting experiment. The present primaries may be removed and double-wound coils substituted. In this case the N-P-coil would consist of as many turns per coil as the removed primary. A variometer in the plate circuit of the detector tube will provide regeneration. Mr. Roberts, in his original article describing the two-tube receiver, mentioned the fact that the two wires constituting the N-P winding should be wound physically as close together as is possible. From Mr. Roger Whitman, Associate Editor of Country Life, comes the suggestion of cutting two pieces of wire long enough to provide 20 turns each for the N-P coil and twist them together. Mr. Whitman has found that with about 3 to 5 twists to the inch TO RECEIVER FLOOR. INSULATOR TU8E /FLOORING RADIAL I T° «KFLAT TOP SUPPORT COUNTER'OISE CEILINS T1M&ERS TYPES OF GROUND LAYOU1S IN CELLAR FOR. COUNTERPOISE FIG. 4 DOUBLE WOUND N-P COIL PLACED ON SAME PLANE WITH SECONDARY FIG. 5 this arrangement provided more stable, sharper operation. Figs. 4 and 5 are illustrations of the various points explained herein. Fig. 6 shows the circuit diagram for the connection of the coupler with the split primary illustrated in Fig. 5 GENERAL POINTERS ON THE ROBERTS CIRCUIT MR. C. J. F'S. question is similar to a number of others received by THE GRID. The following general pointers will serve as an aid in locating and eliminating the troubles sometimes found in the RADIO BROADCAST Knock-out Roberts receivers. 1. Check over all the parts to be used, before assembling, with a view to preventing the use of defective parts. A pair of phones and a C battery, used as a testing circuit, will uncover any open circuits in the various coils, transformers etc., and any possible short circuits in the several condensers to be used. 2. Tubes offer one of the greatest hindrances to proper, efficient operation. This is