The "Record" Short-Wave Receiver Set

Radio Record, Volume I, Issue 22, 16 December 1927, Page 16

The "Record" Short-Wave Receiver Set

Full Shielding Recommended and Described

(By

Megohm

Below is given the second instalment of "Megohm’s" comprehensive ariicle upon the manufacture of a compact and efficient, ully-shielded, three-valve short wave receiving set. This set was on exhibition in a Willis Street shop for several days last week and was closely examined by a number of radio enthusiasts, who expressed admiration for the lay-out and workmanship of the set. For the benefit of distant enthusiasts we give below in addition to further diagrams photographs of the interior and panel . board of the set.

fHilS is the second instal- } ment of the short-wave receiver description, ‘Lhe full shielding of the set with copper sheet is desme} cribed, so that those who nake the outiit complete may do so, and it is left to constructors to determine whether they will discard any or all of the shielding. But we recomend those who construct the full set to shield the receiver itself fully, as a considerable amount of static aud interference, including A.C, hum, is thereby cut out. ‘The shielding also makes oscillation easier, and smoothes out or reduces the tendeney to blind spots at certain positions, Where there is liability to special kinds ‘of interference, it will certainly pay to shield the batteries and leads as well. In the casé of the converter only, the front panel should be of metal to cut ont any chance of hand-capacity, and full shielding. of the detector unit would in this case alsv be an advantage, carried ont ,

exactly ‘as for ‘the full set, with the reduced width. VARIABLE CONDENSER. The Hammarlund condenser specified for tuning the secondary coil is of .0OUL capacity, the plates being double-spaced specially for use in short-wave recepfion. The object of using a condenser of such small capacity is to provide eisy tuning, and to those especially who are new to short-wave work this is a great advantage. ‘This small capacity makes the use of interchangeable coils necessary, but as two of these cover the wave-lengths most used, and there is only one piece to be plugged in, the operation of changing is very €asy, especially as the shield ‘is designed to tilt up conveniently for coilchanging. Some constructors will perhaps be unable to obtain the .0091 con-

denser, or even a slightiy larger one, but a substitute can be made by removing all but three maying plates of a .0U025 as a last resort: The reaction condenser is a .00025 of low-loss, ceitre-line type, with not less than 10-1 vernier. Both tuning condensers are set back from the panel to keep clear of shield capacity, and in order to allow of this, the spitidles have to be lengthened by ‘means of a. brass gollar, or conuector, with set-screws, which hold a_ short piece 4 inch brass rod extension. ‘his also necessitates a separate support for the condensers, so each is provided with a small upright panel of ebonite 4 inches high, and two imches or more in width, to suit the holding screws of condenser. This ebonite is screwec to a small piece of wood, which is in turn screwed to the baseboard as shown in a diagram. The positiott for these is found after the extension spind). and yernier dia] are in place.

THE TUNING COILS. There are four secondary coils, each accompanied by its own suitable tickler permanently attached. ‘The set ot coils covers a wave-length range from 15 to 110 metres, with a reasonable aimount of overlap between each, as follows :- Secandare WWoelelas Ranga in

AdOCe T1gures must He taken as proximate, because differences in receivers or tuning arrangements will ‘affect the exact wave-length covered by

coils. If a larger capacity secondary condenser is used, each coil will covet a wider range. By providing coils with a suitable number of turns, ordinary broadcast wave-lengths can be efficiently received. The secondary coils are made from 18’s tinned copper wire held together by celluloid strips both inside and ontside in three places. One of these celluloid supports is long enough to run back and fix under the first pin of the group of three in the mount. ‘The method of making low-loss coils has several times been described, but for the benefit of new readers a brief description will be given. A collapsible former 8in. in diameter and about 8in long must be provided. This may be of cardboard, ent-along its (full length, with some provision for supporting from inside whilst winding. Six strips of motor hood-light celluloid are cut fin. wide and the length of the former. ‘Three strips are spaced out and tied to the former at each end with twine, ‘The end of the tinned wire is then passed through a hole in end of forniet, three turns wound over celluloid, with space between cach turn equal to thickness of wire, then the next turn runs diagonally to leave a space of fully 2 inches, and the six-turn coil is then wound. ‘hen leave a space of about 2 inches and wind eleven turns. Then celluloid cement is liberally applied to the wires where they cross the celluloid strips, and for a distance each side. Another strip of celluloid is pressed on top of each, and put aside for at least two hours to set, after which the former: may be carefully taken out and the celluloid strips cut to the desired lengths, leaving one long end on ¢€ach coil for bolting down, The 20-turn coil is subsequently wound in the samme way. Tickler coils are made of 24's (not 22’s) enamelled wire wound close _ together and secured with celluloid The former should be of @}in. diaimeter. These coils are secured by sticking two of their celluloid supports inside the two upper supports of the secondary, with the coil rather less than fin. from the first turn of secondary. Allow three inches »f wire on each end of ticklers for connecting to soldering tags on pins of mount. THE R. F. CHOKE COIL. Many forms of radio choke have been tried, atid the one found to be best is 1 properly-constructed low-loss coi] of {00 turns made on the same principle -as the Browning-Drake tuning coils. The choke is only din diameter, and the 100 turns of 380’s enamelled wire occupy about 33in, in length, which means about 26 turns to the inch. ‘The former is made from a six-inch length of round stick about $in. diameter, sawn down the centre lengthways, the thickness of saw-cut being made up with one or two narrow strins of cardboard longer than the stick, so that they can be puljed out to loosen the former. A single layer of thin paper is to be put round the former before winding, sticking as little as possible anywhere. CelIuloid strips %3-16in. wide are applied at opposite sides hotl inside and outside the wires. The strips are left an inch long at the lower end of the coil, and these ends may be glued to a. cork which is m turn glued to the board. Any constractor who wishes to make a choke more easily may wind it on a fin. test-tube, but of course the presence of the glass dielectic detracts from its efficiency. WOODWORK. eee The baseboard is jin. rimu, 20 by 11i$ inches, battened underneath in three places with inch strips of the same thickness. ‘Ihe frame to hold the panel is also constructed of one-inch

strips jin. thick, with a short diagonal ‘brace at each’ end. Attached te the underside. of the bottom shield by screws, are two s‘rips 20in, long, one at the back, the other at the front; raising the set to allow the battery cable and "phone leads to come away to the right through holes in the bottom shield. The exact method of at.aching ‘phones or speaker is left to the individual to suit requireinents. Some may prefer to fit a jack, others to lead the ’phone cords in and attach inside, or to attach to the output wires outside.

Shellac varnish makes a suitable fiuish for the woodwork, GENERAL DETAILS, The biake for the -aerial coil is made from two sttips of .18’s hard brass, 23 by 8-8 inches, bent as shown, drilled one end to screw to baseboard, and drilled near grip to take tightening bolt, A small piece of felt is placed between grip and spindle, and, with proper adjustment, very smooth action is obtained. The aerial coil spindle is kept from having end-play by a metal pin put through it on each side of the back bearing, but as the scraping of metal

against metal must be avoided in a short-wave set, a thin celluloid washer must be slipped over the spindle on each side of the bearing, and against these the pins will work. All connections on the R.F, side are carried out with 14’s bare copper wire, and in sight on the audio side, with bare 18’s tinned, whilst all under-base Wiring is done with 20’s bell wire of good quality. . | WIRING-UP DIAGRAM. , All wiring above the board is easily seen from the plan, but for the underneath wiring a special diagram is given. A full set of varying B and C voltages is provided for, and any not required may be cut ont. It should | be noted that all under-base wires are to be run parallel as much as possible, and, where convenient, may be bound together with thin: twine. Dot-. ted lines indicate how the wires are to. be bundled together. All wires: that run from any point in the set to a battery are to be of 20’s bell wire, and must be cut a suitable length to run without join from the point to the battery, ‘The circles on this diagram represent a hole in the board, through most of which two wires pass to the’ upper side, The panel and bottom shield are to be connected to each other, and at a convenient point to A positive. A fixed resistance is shown, instead of a theostat, to regulate amplifier filament voltage. THE SHIELDING CABINET. The copper shielding really forms an easy method of making a neat cabinet for the set, as it is easier to make a metal case of smart appearance than one of wood. With the black cycle enamel finish on panel and shield the appearance leaves little to be desired. The copper sheet used is in pieces four feet by two feet, tinned on one side, in which case the tinning is used as the inside, on which no enamel is placed. The cutting out of the various parts from the one sheet must be done exactly as shown in the diagram, which leaves about sufficient copper. left over to mint a farthing. :

Every piece is clearly shown in the, diagram, but the bottom piece needs a little explanation. In order to get this ont, pieces 4 and 5 will be found to nearly fill the missing cerier. Pa.t of piece 6 is then soldered over the remaining gaps, and other narrow s.rips remaining are used to cover any remaining join. Of course, if the ccupper is bought in a different size of, sheet, this joining may not be necessary. Strips 1 and 2 of the cover are turned in to add ‘stiffness, but must not be allowed to add much thickness, as these edges have to go insile the -turndback sides of the front pinel. ‘The tcp

front edge of cover is turned back inside, but the edge of this should foria a one-eighth inch tube in order to add stiffness. A thick wire can be put iu the fold. The front panel is turned up one inch all round.- This should be Cone after the woodwork, with the 38-ply panel in place. The copper pan:l then fits oyer this close at top, aud under battens at bottom. At the sides tl inch turned back stands one-eighth in away from the upright to allow the sides of cover to drop inside. The turned-up back edge of bottom slopes back at 45 degrees to allow the cover to tilt back for coil-changing,

The bend of the three turned-up edges of the bottom must be one-eight inch away from the edge of the baseboard in order to allow room for edges of cover to drop gnside. ‘The front edge of bottom flanges goes outside the side flange of panel. The top and sides of the cover are in one piece, The back has a half inch turned at right-angles on three sides, and these ‘are soldered in to form the back of cover. The Lottom edge of the back is turned in flat to add stiffness. All flanges and turned edges are half’ inch except on panel (1 inch), and the two sides of bottom, which are fiveeighths inch. ‘ The hole in bottom for cable must be clear of the inch strip : of wééd screwed underneath at back edge.. Next week the question of valves will be dealt with, and the wiring diagram and instructions for the converter will be given. On receipt of stamped addressed envelope with query, "Megolum’"’ will supply the address of a firm making the condenser connectors. Constructors are reminded that B eliminators cannot he used for shortwave, and that a B battery must be provided. . :

turns. 11