Radio Broadcast (Nov. 1925-Apr 1926)

336 RADIO BROADCAST JANUARY, 1926 Hard Rubber < 3 -> FIG. l6 To keep radio frequency currents where they are useful, a choke coil is shown in use in Fig. 8B. Such a coil is illustrated above and may consist of No. 30 wire wound 100 turns to the slot. With such a coil the feed-back condenser may be a small " midget" condenser recommended. Furthermore, the dimensions on the sub-base are large enough to allow the use of any sockets or transformers now on the market, without making necessary any changes in design. Perhaps there are those who would like to improve on the general design of this receiver in one way or another and the point which might well be expected to be attacked is the control of regeneration by the resistance across the tickler. Some of the attempts at this which were made in our laboratory, during the development of this receiver are indicated in Fig. 8, but for a number of reasons we have found the system finally employed here to be most practicable with the type of coils employed. Since the inductive relation of the tickler to the secondary of the radio-frequency transformer, that is coil L4 to coil L3, is always the same there is no change in wavelength or detuning in the radio frequency circuit, which is sometimes noticeable to a marked degree in receivers where a tickler of the rotary type is employed. Then, too, the number of moving wires and the breaking of connections they sometimes cause has been eliminated and with the proper detector plate voltage and the proper variable resistance, the control of regeneration is remarkably smooth, which is a distinct advantage. CONDENSER FEED BACK A VARIATION of the resistance control is the condenser feedback, probably due to Weagant and used commonly in the Reinartz circuit. A fixed coil is placed near the detector secondary and coupling to the plate is effected by means of a series condenser. The condenser and coil is then a shunt path for the radio frequency currents, and a choke coil may be necessary to keep these currents from escaping through the phones or amplifier primary. The circuit is shown in Fig. 8B and a drawing of a choke in Fig. 16. There should be no condenser across the output in this arrangement. This method of adding regeneration is particularly smooth in operation, and it avoids the movable tickler with its varying field. And now there is little to do but the soldering and wiring. Wherever possible, the home constructor should fit himself out with a good soldering outfit, and a set of those small wrenches which comes in so handy in getting the nuts on and off transformers, tube sockets and such places. He should have a good supply of bus bar and spaghetti or flexible rubber-covered wire and a goodly supply of small sized lugs which may be directly fastened to the various units which go to make up the circuit and to which the soldering is actually done, rather than to the units themselves. By using this method of construction, it is possible at any later time to remove the holding nuts and off comes the wire with no fuss whatever. Then it should also be remembered that a good small screw driver is valuable in placing the soldering lugs under the heads of the screws in those units provided with screws instead of binding posts, and there are a great many of them on the market. That's about all there is to the building, and now we come to the point of putting our prize on the air. OPERATING THE UNIVERSAL THE antenna used with Radio Broadcast's Universal Receiver should be about 1 50 feet long, from the receiver itself to the outside insulator, including the length of the lead-in wire. With such an antenna, if you are located within 25 miles of a powerful broadcasting station you may find that the receiver is not selective enough to permit you to cut out the local station and bring in distant stations on frequencies near that of the local. This objection may be overcome by inserting a .0001-mfd. fixed condenser in series with the antenna or by reducing the length of the antenna a little. The former method is easier and usually more effective. It will be found that the two dials will run just about even over the entire scale, if they are properly set when they are attached to the shafts. If the wiring is correct the receiver should respond as soon as it is put on the air, if there is any broadcasting going on. The only adjustment other than that which usually characterizes tuning is the setting of the neutralizing condenser and that is a simple matter, which once taken care of need cause no further worry. In order to set the neutralizing condenser properly, some broadcaster whose frequency is about 1000 kc. (300 meters) and whose volume is not very great should be tuned-in with the detector oscillating. The detector condenser should be tuned until the whistle from the station is quite loud. Then the first, or antenna, condenser, should be tuned. It will be noted that the whistle will change in pitch as this condenser is varied. When the set is exactly neutralized, this whistle will not change, and the problem is to adjust the neutralizing condenser until such a state of affairs exists. The neutralizing condenser should be varied a little at a time, each time noting the change in pitch of the whistle. On one side of the neutralization point, the pitch will rise in frequency; when the neutralizing point has been passed, the pitch will lower in frequency. By listening for these changes in pitch, the listener can tell on which side of the actual balance point he is. The usual method of turning out the first tube and adjusting the neutralizing condenser until no sound is heard is not satisfactory. The grid-plate capacity of tubes differs by a large factor in the two conditions of tube unlighted and tube lighted. In other words, the tube will not be neutralized when it is lit if it is balanced with the filament turned out. It should be neutralized under actual operating conditions. A more practical all-round receiver than Radio Broadcast's Universal will be hard to find. FIG. 17 A typical example of the Universal circuit worked into the small dimensions of Radio Broadcast's Phonograph Receiver. A Hanscom single-control unit with model 2 RK Clarotuner coils provides the tuning system, and the sub panel with special sockets was supplied by Osborne & Company of Boston to our dimensions. Note the freedom from visible wiring