The "Presto" Change-over Local Receiver

Radio Record, Volume V, Issue 24, 24 December 1931, Page 16

The "Presto" Change-over Local

Receiver

incorporating the famous LoftinWhite circuit and multi-mu valve

By »

MEGOHM

f---- 10 be able to sit at ease and ics) change from one local station to the other by the mere movement of a switch is the object of the two-station receiver described below. Moreover, the two stations must come in at good and equal loudspeaker volume, however they may differ in actual power output. This receiver has been designed to accomplish this desirable end. Quality reproduction, needless to say. is also imperative in the modern local receiver, and this condition has been attained by the adoption of the LoftinWhite type of amplifier, which is preceded by the new yariable-emu UX235 type screen-grid valve. In order to strengthen the reception of weak local stations, the amplifier has been slightly modified to inelude reaction, which, however, is not intended to be used too freely. Incidentally, it may be possible to bring in certain distant stations under favour- | able conditions by making use of reaction, but the receiver has not been designed with this object in view, the | hs Se

aim being to keep the rudio tuning gear its compact as possible. The Cabinet. HE neat appearance of the cabinet will be appreciated by a glance at the general view of the receiver. Woodwork gives a more distinctive air than can be produced in metal by the amateur craftsman, though, of course, some constructors will prefer to use aluminium alone. Actually, the necessary woodwork has been designed to be effective und at the same time present as easy i. task as possible to the amateur woadworker. The wooden case is lined with sheet uluminium of 16 gange, which forms

wu complete shield against radio interference and at the same time isolates the power-pack which is housed in the lower portion of the cabinet, with the radio gear above, and smoothing condensers and output transformer in the base. It may be stated here that if built as specified, no background whatever is heard from the unwanted station When tuned to either 2YA and 2ZW at a distance of two miles from the former station. Although great yolume may be obtained from either station, quality remains remarkably good at the full output, Although selec. tivity is good, it is not so sharp as to preclude slight de-tuning of the aerial circuit in order to decrease volume without affecting quality, and by adopting this method, extra controls are dispensed with, so that each station is received with equal volume, only the operation of the switch being re. quired to change over. The switching is arranged so that the stators of either of two pairs of tuning condensers are connected to

their respective valve grids, and in accomplishing this the chief: care is to keep the respective grid leads well separated. Experiments showed that a specially-designed switch would be necessary to carry this out, and an easily-constructed pattern was worked out, having the advantage of being cerfain in operation and not too bulky. Pick-up Provision. PROVISION is included for the oper- x ation of a pick-up directly into # the two-stage amplifier, in which case a pick-up with good voltage output is an advantage. Excellent results and adequate volume are obtained from a heavy duty magnetic speaker of the 66R or Farrand types, but if desired a dynamie uit with low-wattage field may have the latter excited by inclusion in the re sistance. chain, as will be shown. Constructional Details of the Cabinet. ‘HH front, sides and base of the cabinet are made from #in. rimu, all dimensions being given in the diagrams. ‘The top may be plain metal, or wood lined with metal. . Make the base first, then build the upper portion upon it, allowing the base to project din. at front and sides, and gin. at back-this allows of screws being placed in holes drilled in tha base to hold the upper part in The ornamental bars on the front are strips Sin. wide and 3-16in thick, glued on,

The panel should be enamelled black or other contrasting colour before placing in position and before the knobs are put on, Stain that will show up the grain of the wood is the best finish-walnut for preference,

{ The depth of the base may be regulated to suit the output transformer to be used. The wooden case is lined with 16 or 18-gauge aluminium sheet, the most convenient method being to cut the pieces separately for the power-pack compartment, then rest the shelf on the top edges of these and place the panel and side pieces on top of the shelf, all pieces being held in place by screws into the wood. Aluminium to cover the top and back completes the shielding. Above the shelf is the sub-base. raised 1tin. by means of four feet made of din. strips of aluminium cut fin. long, bent up 4in. at one end, and a tin. hole drilled in the centre of this portion for a bolt. One of these is \ bolted to each corner of the sub-base. The front of the sub-base is bent upward to form a flange 2$in high. To this flange is bolted the bracket to hold the detector coil shield can. The Power Pack. S this portion of the receiver should be installed first, it will now be described.

The power transformer has a lin, stalloy core built to bare 1#in. thickness. The "window" measures 3#in. x fin., and the length of the respective pieces of stalloy is 48in. and lfin., 144 pieces each size, requiring 25 3ft. strips. Shellac all the pieces after cutting. Olamps (lin. x fin. iron) drilled 3iin. centre for jin. bolts, The transformer stands upon one end of the clamps in order to reduce the height, and to facilitate fastening down, four angle-pieces of copper or brass, say, 24, should be drilled with two holes, one being secured under the clamping bolt, as shown in diagram. The two front angle-pieces may extend nupward and serve to hold the fuse-panel, which is of ebonite or fibre. The primary winding consists of 1875 turns of 28 enamelled wire, occupying 12 layers, with paper between each, the supply being 280 volts, 50 cycles. The secondary high-tension winding is in two separate halves, 3164 turns of 36 enamelled on each side, occupying 19 layers, averaging about 166 turns per half-layer. This winding. gives 400 volts with the filter specified The

paper strips should be cut 1 9-16 wide. with the full width strips 3}in. bare. An idea now adopted by the writer for obtaining a perfect balance in divided secondaries is to wind the lefthand coil in the usual way, finishing the winding at the centre, by spread-

ing out the last layer turns if. necessary. Then turn the spool round the opposite way on the spindle, so that the wound secondary is on the right. Now wind the second coil in exactly the same way as the first, with beginning lead-out through left-hand cheek of spool, and finishing at centre, These two centre ends may be brought out separately and joined outside, which will be the centre-tap connecting to earth. This method is better than connecting the two inside, and bringing out only one lead. ‘The high tension ends leading to the plates of the 280 rectifier come through their’ respective spool ends, When the above method of winding is adopted, any required taps should be put in near the beginning of each Secondary winding, that is, for 800 volts at 789 turns and 250 volts at 1164 turns from the beginning. Filament windings are for the 280, 45 turns 18 dec. tapped at twentythird turn with a thinner wire, 24 or 26; 245 filament, 23 turns 18 or 20 enamel or d.ecc. wire; 224 and 235 joint winding, 23 turns 18 4.c.c. Place the transformer as shown, so that fuses may be readily examined and replaced. Detailed information on the construction of power trans formers will be found in the 1921 "Guide." The exact height of the transformer over all should be 4 5-8in., which leaves about #in, clearance below the shelf. The overall length is 5 1-Sin.

The Smoothing Choke. QNLY one smoothing choke is re quired, as the Loftin-White humbucking system allows of considerable reduction in the smoothing equipment, Nevertheless, constructors must not eut the size of the choke down unduly or the result will be a persistent hum that cannot be eliminated except by the substitution of a better choke. The following specifications are for choke "IH" as given on page 97 of the "Guide," and have already proved satisfactory on many amplifiers of this class. The core is of lin. stalloy, cut 34, 2 7-8, 24 and 1 T-8in., a one-inch pile of each. Spool ends 2 5-8 square, and spool length 2}in. outside, leaving tin. of the core to be gripped by the (Continued on page 28.)

Presto Local Receiver (Continued f om page 17.)

clamp by the gap, whcih is 1-16in. each side. This. choke will drop only 14 volts when passing 40 m.a. The winding consists of 7000 turns of 32 s.w.g. enamelled wire, tapped at the 8500th turn for connection to a Imfd. smoothing condenser of 400 volts working. The quantity of wire is 14lb. The choke is preceded by a 2mfd condenser and followed by 4 mfds.. "both 400 volts working. Insulate the choke well round the core, and before tapping half-way, eover the whole winding with tape. to prevent any subsequent turns contacting early ones; insulate the tapping, and continue winding, the wire being simply "run in" without paper strips, and not in layers. It should be mentioned here that the "sap" given in choke specifications is for the maximum current to be passed, and to this condition the given inductance also applies. As the amount of current passed is reduced, so the inductance rises, and if the current is much below the maximum, the gap may be slightly reduced, thus further increasing the inductance. In this case the choke will be carrying nearly the maximum current---40 mills. There is ample space for the choke -vehind the transformer, and the only

precaution as to its position is that the central core must be at. rightangles to that of the transformer, which means that the axis of the spool must be vertical. The choke need not be fastened in position, and if all leuds are of rubber flex and sufficiently lone the choke may be withdrawn at any, time fur examination, if the vPS0 is first withdrawn from its socket, A lead comes from the 280 centretap to the choke and 2-mfd. condenser below. At the oulput end a lead runs down to the 4+ mfd. condenser. and up above to the resistor chain 4-meg. Insulate all joints so that there is no chance of a "short" in the shielding. The Change-over Switch. {OUR variable condensers of the bakelite or mica dielectric type, .0005 mfd. capacity, are placed upon the panel. When receiving one local station, che top pair of condensers wil] be used for tuning if the switch is in the top position. and when the switch is down, the two lower condensers are brought into action The main part of the switch is a piece of ebonite tube (rod could be used) 5-8 or #in. diameter. The actual diameter used was 5-8in., and it was found that a 3-8in. brass tube would just fit inside. Two pieces fin. long were sawn off the brass tube and saldered in position on the brass uprights to act as bearings. Failing the brass tube, brass sheet could be coiled round and soldered on. If ebonite rod is

ised, drill each end 1-Sin. and solder a l-Sin, bolt through a 1-8 hole in the brass upright. Two pieces of jin. cbonite are required, 1 by $in., cut as squarely as possible, These are attached to the brass uprights with the lower side 3-8 in. above the base. To each. lin. side is attached a strip of 24 brass aa shown, fastened by two 1-Sin. bolts jin. long. For those who have not so far attempted to tap holes for screws it

rt i will be necessary to explain how this simple process is carried out. A 1-8in. tap is purchased, costing not more than 1/-. The correct drill to use with this is 3-32in. Any hole to be tapped is first drilled with tbe drill mentioned, then the tap is put into the drill chuck and placed in the hole, {urning just as for a drill. The only precautions to be observed are that turning must nor be eentinued after the end of the tap reaches the bottom of the drilled hole, or the thread will be stripped. In hard metals the tap may be worked backwards and forwards to facilitate cutting the thread. Taps are more brittle than drills, so ia little more care is required te pr ‘vent side-strain, Four strips of 24-gauge hard brg#ss 2in. long and 3-1Gin. wide will be ‘required. These are soldered to the strips attached to the lin. by #in. ebonite, as shown in the diagrams. One ‘of these springs is attached by a wire ito the slator of one of the tuning condensers. On the ebonite tubing, four roundheaded brass serews are placed, two to engage each pair of springs. ‘The position of the two screws is at 90 degrees around the tubing, so that one is contacting a spring when the other has lost contact with its corresponding spring. Half-an-inch toward the centre 2, hole is tapped to take another screw. upon which two nuts hold a soldertag to which the grid connection is made. Clean up a piece of 24-gauge wire and with it connect up the lastnamed serew with the two — contact screws by soldering, or by nuts on {he latter. The wires must run out ways from the contacts. : The switch handle is a strip of ebonite 2in. long, tapered from 5-Sin. to }in., and attached to the ebonite tube bv two brass brackets. A hole is also drilled and tapped in the end of the handle to take a screw through the ebonite tube. The front of the ebonire tube is 1}in. from the panel, A diagram suggests an alternative type of switch that some constructors may prefer, although it occupies no less space than the one already showy (To be concluded next week.) *