Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST In Fig. 4 we have the circuit converted into a double-detection affair by the addition of an oscillator, VT4, one stage of r.f., VT5, and a second detector, VT6 In the unscrambled diagram of Fig. 2 Ti, Si, and Pi are the coils on the plug-in form feeding the first detector. The coil Pi was originally intended for use in the antenna circuit, as in Fig. 1, but an alternative method of antenna coupling has been provided through terminal Ai and the 15-mmfd. condenser. This alternative method is used in the set when adapted, the coil Pi acting as an oscillator pick-up in conjunction with the extra coil P2 on the oscillator plug-in coil form. Such an arrangement has the advantage that as the coils are changed the pick-up coils change with them and the pick-up from the oscillator remains more uniform than is the case with a set-and-leave arrangement. This is a considerable advantage when going over so wide a range as these tuners cover. It will be seen that Pi and P2 in series with the filament leads form an untuned "link" transfer circuit. The rotor of the regeneration-control condenser, C2, is disconnected from the filament and through a clipcord is run to the top of the tuned circuit L1C4 which feeds the r.f. tube, VTs, (See. Fig. 4). This tube may be a 201a, 240 or 222 without causing any change in the system up to this point. The clipcord should be reasonably short and kept tolerably clear of things. Since the condenser C2 is now a sort of coupling control between the first detector, VTi, and the circuit L1C4 it cannot be used to control detector regeneration. This regeneration is too valuable to lose, and it is accordingly suggested that C2 be set rather high and oscillation in VTi controlled by the addition of the 500.000-ohm Frost rheostat, R2, with a 0.1mfd. shunt condenser, C3. This is not essential and one can get along very nicelv without these devices by simply reducing C2, which may in fact be a " postagestamp" mica condenser. The first audio transformer primary has been disconnected and the wires connected together, also the first a.f. tube, VT2, has been removed from the socket. The oscillator is tuned by the condenser C7, which is a duplicate of Ci. The condenser C6 is merely a bypass and may have a capacity of 0.001 mfd. If it happens that the oscillator tends to squeal (audio-frequency blocking) it may be necessary to introduce at X a combination of rheostat and condenser like that shown at R2 and C3. The description above is general rather than applying to the particular unit shown in the illustrations. Having put the converted short-wave signal into the tuned circuit L1C4 we naturally Top view of ''Wasp'' receiver and amplifier would like to know something of that circuit. Li is a common broadcast r.f., coil shunted by a fixed condenser which tunes the circuit to a point above the broadcast band; 0.0005 mfd. is a convenient capacity obtainable in the small "postage-stamp" mica condensers. The output of the r.f. tube is fed into transformer coil L;,L3. The latter is another ordinary broadcast tuner coil shunted by a .0005mfd. mica condenser. Since the condensers -Method of connecting amplifier and oscillator to receiver C5 and C4 will not have exactly the same capacity one of them must be shunted by a midget condenser of some sort that can be adjusted while the set is in operation. Any one of the various "screwdriver" condensers on the market will do, provided the range is adequate. It is, of course, possible to use regular tuning condensers at C4 and C5 or to use condensers of the mica-compression type with capacities high enough to make sure of resonance at some point. This whole thing can be done much more easily than it can be described. The winding L2 depends on the tube used ahead of it. If a 20lA-type tube is used the Front view of "Wasp" receiver and amplifier • february, 1929 . usual primary found on the coil will serve. A 240-type tube will call for about double the number of turns used by the 201a, while the 222 tube should have a winding with a number of turns equal to about 5 the number of L3. The 222-type tube will require a tube-shield and on the whole it is less painful to use the 240 or the 201a. If this tube desires to oscillate use an ordinary "grid suppressor" at R4. Two stages may be used as they are not much more troublesome when worked in this manner. Obviously these two stages, the second detector and the audio system may all be found in the broadcast receiver with which the unit it used. Diagrams differ so greatly in these that the reader will find it safer to devise his own diagram of connection rather than to rely on Fig. 3 which may overlook some of the possible causes of trouble when used on a strange receiver. The points to remember are simply that the output of the first detector is to be applied either to the antenna post or to the grid of the tube in the first tuned circuit in the r.f. system of the receiver, at the same time making sure that the detector plate voltage does not get into the input to the receiver. A stopping condenser, C5, of adjustable nature and a choke, Ls, will do the trick. The choke must, of course, be good over the broadcast range as well as the range in which the short-wave tuner is to work. The chokes furnished with short-wave tuners usually fulfil this requirement. A list of the apparatus used by the writer in the construction of his short-wave superheterodyne receiver is given below. However, the experimenter may substitute electrically equivalent parts, if desired. The complete fist follows: One Pilot "Wasp" tuner; Two Frost resistors, 500,000-ohm; One tuning condenser, 140-mmfd. ; One vernier dial; One tube socket, UY-type (for oscillator coil) ; Two tube sockets UY-type (for coils Li and L2-3) ; Three tube sockets, ux-type: One extra set of Pilot "Wasp" coils (Two coils of a kind are used in the set and oscillator, respectively, while the 200-500 meter coil of each set is used for Li and L2-3. The tickler serves as la) ; One grid leak, 1.5-megohm; One grid condenser, 0.00025-mfd.; One grid suppressor, 300-ohm (R4) ; Assorted by-pass condensers, 0.001-mfd. and larger (C<0; Combination of fixed and adjustable condensers with maximum capacity of 0.0005 mfd. in each case. In the writer's set C4 is a Sangamo unit and Cs is a Sangamo unit plus an XL adjustable condenser); One panel, 7" x 10" or 7" x 14"; One baseboard; Two brackets; Binding posts, wire, solder, usual small hardware, etc. page 263 •