The "Presto" Change-over Local Receiver

Radio Record, Volume V, Issue 25, 31 December 1931, Page 16

The "Presto" Change-over Local

Receiver

By

Megohm

(Continued from last week.)

The Radio-Frequeney Stage. HE tuning-coils consist of an aerial ' primary of about 35 turns of 3 enamelled wire, und a secondary Zin. diameter 28in. high, of 75 turns of 24 ds.c. wire. The primary is wound upon a former to slip inside the seconiary, the winding being at the lower or earth end of the secondary. The lower end of each coil is earthed. The nua ber of primary turns may be altered to suit conditions--nore turus increases yolume and decrensesx selectivity, and -this applies to the detector primary also. The screen for the coils may be a Zin tin canister reduced to 3iin in height. ‘his may be secured to the wide of the cabinet by a screw, neal the top ot the can, with a washer be ‘tween Can and aluminiuiun to give ‘elearance for the lid, "The lead-in for {he aerial is a piece of 18 s.w.g. bare wire ‘secured to the inside of che aerial socke) ‘and projecting {urough a hole iu the can, where the tup of che primary is soldered to it. The Pin secondary former is cement ‘ed tu a piece of celluloid, and throuss ‘a hole in the centre of this and the ‘bottom of can. a bolt is passed to hola ‘the coil in position. The 285 valve assists selectivity, «iu ean handle without distortion a signa of about 20 times the veitage permissibh with the 224. The maximum plate voltage is 250. and the bias minus 3 volts. By employing a variable wire wound resistance of 20,000 ohuus in the eathode lead. the bins may be varied from 3 to 50 volts, at which fatter teu practically no signal will be heard, se that a good method of yolume control is thus provided, This idea could not be very convenientiy used in this reveives ‘pecause of the change-over switchinz . ‘svhich requires 2 separate control fur. each of the (wo stations. _ dn this stage are plaeed the sufeucn yoltage resistors und by-pass condeny er, and alongside the valve is the bixs resistor, with its by-pass condenser, The screen between this and the de tector stage may be of aluminiuzn or copper--it butts close agaiust the cos} can, and at the lower front end: is,

Hanged to bolt to the upright flange on ub-base. At the back edge a similar lin flange permits of bolting to the bach plate. This partition should ve cat : shade high, and when in position can be marked by means of a straight-edge. taken out, and cut to meet the fo}: cover, The bottom edge is flanged on the detector side to bolt to fhe sul hase. . The Detector Stage. (THE coils for this stage are housed in a similar can to that used for the detector stage. An aluminium

bracket holds the can at its proper height, and close against the partition. The secondary coil is wound on a 2in, former 22 high, and comprises 75 turns of 24 d.s.c. The primary is wound upon a former 1#in. diameter, the lower end commencing level with the lower end of the secondary. About 58 turns of 36gauge enamelled close wound will suit for the primary, but a few more turns. say 10 or 12, may be added if selectivity is not highly important. Now we have anv idea that is incorporated in some of the midget receivers. It consists in placing. the primary winding at the filament end of the secondary, with the exception of one turn, which is placed at the grid end. This enables a slight gain in volume to be obtained without introducing instability. The tickler is wound on a former 3 Sin, in diameter, its lower end level with the lower or filament end of the secondary. Probably 40 turns of 36-gauge

enamelled will be found sufficient. If oscillation occurs on low wave-lengrhs before the reaction condenser plates are half in, reduce the number of turns. Odd sized formers to fit inside the secondaries may easily be made of "manilla"’ (folder) paper and seccotine, Wrap about four turns round a bottle of suitable size, take off, and shellac. Replace on the bottle when winding, The tickler coil goes inside the pri"mary. The bottom connects to the 224 plate, and the top to the-reaction condenser stator. Associated with this stage are the r.f. choke and .5 mfd, by-pass condenser, both boited to the upright flange. The choke used is a commercial one, W metal bracket ready ‘attached. Near «i end of the flange another .5 mfd. condenser is placed. This is the condenser between filament end of. the secondary eoil and earth. If a pickup jack is required, it is placed just above this end of the flange, a suitable-sized hole, say. ‘in. diameter, being bored in the wooden side. The jack, insulated type, is then secured in a hole through the aluminium lining. A wire runs across from the detector plate to the 245 grid and to the lower end of tickler through bottom of can.

The Output Valve. A FOUR-PRONG sovket is. provided for the 245 valve, and. across: its filament: a 20-ohm balancer is placed. Irom the centre-tap of this the high tension current is drawn to point A, the beginning of the wire-resistance chain. Alongside the valve socket is an 21.f. choke to Which comes the current from the primary of the output transformer in the base, continuing to the 245 plate. A Jead from the grid connection runs to the 6 meg. resistor. Gear Under Sub-base. UNDER the sub-base is a 15,000 ohm " voltage divider, of which part is used as a main resistor, the carrying capacity of the winding, 40 m.a., being very suitable. The two 25 meg and one .5 meg resistors, a 2-meg. two wirewound resistors, and four by-pass condensers, complete the list. A strip of tin 1fin. wide is bolted on to fill in the space at the front of the sub-base. he screws holding the .& mfd condensers and partition will also hold this. Tin is ‘used because if wan be soldered to. . A diagram of connections is shown with the components in miniature, so that the leads are clearly shown, but actually there is little room to spare

where all the components are in place. Where possible, secure. them to bolts used for the valve sockets, partition, ete. Condensers. ‘next to the tin strip may be soldered to it. In order to save space, it is a good plan to turn round.-the legs of the yoltage divider. When this is done, the central screw rod is inclined to be short, but if din. of the Pirtoid tube is sawn off, reassembly is easy. A few counter-sunk headed screws will be required for positions where the head is required to be flush, as, for in_Stance, under valve-sockets. Exactly half of the voltage-divider is to be used, so place one of the movable clips exactly in the centre-this will connect to the hum-bucking resistor, and one end is point A shown on the theoretical diagram, It would be possible to divide up a further portion of the voltage divider for the hum-bucking portion, but a trial showthat a separate resistor would anAver better. The idea is to fix the hum-bucking point permanently, and stow the fixed resistor away under the sub-base, thus saving the space of a potentiometer. A high-voltage dynamic speaker field may be supplied by including it as resistance, deducting its resistance in ohms from the 7500 ohms on the voltagedivider, on which 7 millimetres represents 1000 ohms, and 3.5 em. exactly 5000 ohms. If necessary, the resistance as far as the hum-buck connection may. also be taken out, and, in fact, with the extra smoothing introduced: by the field, there is little hum left to work on. The Hum-bucking Resistor. TPIS resistor should be of the fiat type, 650 ohms, or up to, say, 700 ohms. If such cannot be procured. the constructor can easily made one by winding on a stiff strip of mica about 4in. wide and length 3 inches or so to suit, sufficient turns of enamelled resistance wire to make about 650 ohms. ‘The gauge should not be larger than 38 s.w.g. Clean. off one edge and fasten on a. board as shown in the diagram. Cut three strips of 20 hard brass 2in. long, tapered and screw to board about in the position shown eee OT,

not too. tightly, so that they may be moved over the edge of the resistance. Wires from the receiver are connected as shown to solder-tags under the screws. Move the arms to yarious positions. It will be found that moving the. hum-buck connection well. to the left produces a loud hum, and it will also be found that the hum-buck and point ( must be fairly close together for good results, with not too great a reisstance between point C and earth. In the laboratory model best results were obtained with 200, 10 and 4i+4

ohms, as shown on the diagram, though these must only be taken as a guide because they may not work out for another receiver. If moving the humbuck arm does not get rid of the exexperimenting. "Nettsinsc eutsetspeemsnncneminceane cessive hum, move the central clip on the yoltage-divider about 1-8inch to include a little more resistance. Mark correct, positions on the resistor with white ink or other medium, take off board, and twist two turns of cleaned 32’s round each position, soldering a thin rubber flex lead to the twisted end of each. Lay a piece of mica on: each side of the resistor, and cover with tape from end to end, when it may be stowed away under the x.f.. valve. The leads to the resistor will have been reduced in length after ~

If soldering of resistance wire is necessary, use "killed" acid as flux, because resin is often unsuccessful. Gear in Cabinet Base. ‘PHD smoothing condensers, imfd., . 2Qmfd. and 4mfd., and the output transformer are the only components. The "in primary" of the output transformer is connected to the high voltage side of the 4mfd. condenser, while the "out primary" is connected by a shielded wire through the power-pack compartment to the rf. choke and 245 plate. _ he secondary of the output transformer connects to two terminals on an ebonite strip. outside the base. A third terminal is for earthing, and connects internally with. the B- wiring and shielding. An18-gauge wire should run from this terminal up through the power-pack and connect to a solder-tag under the right-hand leg bolt of the sub-base. See that all parts of the shielding are properly earthed. Run the secondary centre-tap connection of the power transformer direct to

the earthing wire. arth cores by ‘tucking between one bolt and the core, 1 bundle of tinfoil containing the end of a thin wire, the other end of which is to be earthed. All shellac should be scraped off the core laminations of choke and transformer where the tinfoll is to contact. The earth contact must be made only at one point on any core. The general plan to be followed in assembling the receiver proper is to first enamel the front panel, then fit. components on it, with the reaction condenser in centre, ‘ The sub-base components and partition are then bolted in place, and wired up as much as possible without the coils. Next, the sub-base is supported in. an inyerted position and the components are fitted underneath in the positions shown. It is often advantageous to solder a tag to a condenser case so that it may be secured to a neighbouring bolt. The Ferranti and other resistors are assembled without holders in order to save space. The writer first soldered a lead to each end of the Yerranti's, half-way up the slope of the cone at each end, then wrapped each in adhesive tape, covered each with #2 cylinder of copper foil, and soldered the two cylinders together, and the ends of these to the tin strip. Needless to say, condensers must be of high tet and small bulk, so a new type, imfd., 1000 test, and only 5-Sin. thick suit admirably. The two .25 meg. and one 4 meg, resistors should be of the substantial composition type. These are all connected together at one end by a short piece of hare wire, leads of suitah'e length are connected to the opposite end of each, and then each one is covered with adhesive tape. All three can then be laid together, and taped into one bundle, effecting a great saving of space, There is room to use an ordinary holder for the 2 meg. See that the shelf is drilled in suitable positions for the twisted filament. leads, and full B positive, and B posttive from output transformer primary. All the above-mentioned leads should (Concluded on page 27.)

\ The Presto Set (Continued from page 17.)

be connected to their components under the sub-base and left of sufficient length to reach their other connections. The change-over switch is secured in place on the shelf, and the panel held temporarily in its relative position. Hach stator can now be connected to its corresponding switch spring, the bottom condensers to the back springs, using short rubber-covered leads. Do not trust the condenser rotors to connect to the panel, but make to each of the tags provided a proper wire connection direct to the earthing 18-gauge wire. The shelf and panel may now together be lifted into the cabinet, after whi¢h the sub-base will follow, finally plage the aluminium sides in position, securing with a screw or two through holes already drilled near the top. The sub-base may still be lifted at the back edge sufliciently to pass leads through the holes in the shelf. Long wires must be already provided from the jack or bottom of detector secondary coil to point C, from the cathode 1 mfd. condenser, and from the lower end of the voltage divider resistance, for the purpose of fixing the humbucking resister. ’' The jack may now be connected up, if one is provided, otherwise the bottom of the coil, .5 mfd. by-pass condenser, and lead to point © are all connected together. To connect the jack, the spring concting to tip of the plug connects to point C; the spring connecting to sleeve of plug connects to bottom of coil, and the contacts made by each of the two springs when the plug is withdrawn, are both connected t* the .56 mfd. bypass condenser. Be sure that the red marked cord of the pick-up is connected to the tip of the plug. The Diagrams. N order to clear up any points that may have been missed, we will now take a run through the diagrams. In the. theoretical there is little to comment upon, If desired, .5 mfd. bypass condensers may be used in place of the two mica .008 shown. The 280 filament winding is shown twice, chiefly to show that there are three filament windings in all, The aluminium sizes are given so that an idea of quantity may be formed although the size may not be adhered to exactly. Shielding is essential for the power-pack, but not for {le receiver, if more than two miles from a main station, If the back of the upper portion is left open, the heat from the valves is more readily dissipated. As well as the pieces shown. there is the sub-base. Flexible wire from the transformer to mains must now always be the solid rubber variety, in which the two conductors are embedded in the rubber. The 280 rectifier is in a horizontal the socket being screwed inWde the front of the cabinet. Iligh-tension and filament leads from transformer to 280 should be sufficiently slack to allow of the choke being easily extracted from the cabinet, all leads to the choke being of sufficient length to allow this. The plan of the receiver shows the general lay-out, the coil cans being omitted and their positions shown by

dotted circles. : Connections to the detector coils are shown as far as the ean. The aerial wire should be fitted with a plug to fit the socket provided. A piece of 18 gauge bare wire connects to the back of this socket and projects inside the can to permit of soldering the primary to it. The top of secondary coil comes through the can and joins the wire running from switch to grid, as in the detector..stage. The primary and secondary coils are earthed by connecting the lower end of each to a solder-tag, which is fastened by a screw passing through the centre of the bottom of the can. The rf. screen resistors are shown unmounted -space is economised by soldering leads to the ends, then wrapping the whole in adhesive tape. The .008 mfd. condenser across the 235 eathode resistor is conveniently of the upright type, the resistance being connected across the terminals. With the Igranic jack, shown, the » mfd. condenser is connected to both contacts on one side, and when the plug is withdrawn, both springs contact the condenser. Any jack that is used must be connected so that there is no connection to the .5 mfd. when the pickup is in use. A _ oneinch hole should be made in the side of the cabinet for insertion of the plug into the jack, By winding all coils in the . one direction connections will work out correctly as shown. The space of 1 1-8in. under the siibbase is the minimum, but may be increased about jin, to suit components, though care should be taken to save as much space as possible. Clips were first used to find the hum-control resistances, but did not prove very satisfactory, so the resistor was fixed to the edge of a piece of lin. wood by a nail through each end of a lath laid over the resistor. There is plenty of room in the cabi-

net base for the smoothing condensers., Harth one side of each condenser. and all cases, if of metal, as shown by the dotted line. The wire at A rungs up through the power-pack to the r.f, choke and plate of 245. The wire at B carries full voltage to the .5 meg. of resistor chain. Wires passing through the power-pack should be of the metal-shielded type. The wire at D connects to the input side of the smoothing-choke, ‘and B to the choke centre-tap, ~The secondary of the output transformer connects to the pair of terminals outside the bases. Connections under the sub-base are shown clearly by drawing the components in their relative positions, but much reduced in size. This wiring is all done with rubber-covered flexible wire, and looks. somewhat confusing when completed. the shortest route from point to point being taken, A little ingenuity is required to use available bolts to hold the components. The inner end of the voltage-divider is held by a countersunk screw under the 22+ sotket. Any condenser may haye one or tivo solder-tags attached, so that it may be held by adjacent screws, Nonmetal condensers may be held by a strap of wire over them from one screw to another, or may be glued in Dosition. The B positive wire from the .25 meg. : is shown through a hole, but this hole is actually in the shelf, ‘and not in the sub-base. The hole at x is for the wire from below to the s.f. choke and 245 plate. Final Adjustments. MALL .variations may. be tried by S moving the clip 1-8in. or so either

way along the voltage diyider,. he 2meg, resistor may be higher than its rated yalue, and better results may be obtained with a high-rated one-meg..The 20,000 resistor may be reduced to as low as 15,000 or a little less. There should be 250 volts on’ the 285 ‘plate and 90 volts on the screen. Point A should show about 180 volts.. A good wire-wound 15,000 ohm resistor should be used in the 224 cathode lead, as this value is critical,