The "Rotorua" Portable Receiver

Radio Record, Volume II, Issue 23, 21 December 1928, Page 26

The "Rotorua" Portable Receiver

By

Pentode

(Concluded from last week)

Sean Ty OLLOWING on with the i) Eigay|. description of the portbaa able set, the cabinet of which was described in the special issue of the = "Radio Record"’ last week, we have the receiver itself to construct. . Some readers appear to be having.a little difficulty in obtaining the parts as mentioned in the list of components. This list is given as representing the parts used by the writer, but there is not the slightest necessity to keep exactly to the list. Any good makes of components are quite suitable, and the reader is advised to consult his dealer. The point to watch is, that each part must be small and light in weight.

Carefully lay out the components on the baseboard before screwing down, and make quite sure that the moving plates of the variable condensers clear all wiring, ete., when moved. Before anything can be done the coil and coil screen must be. made. This coil has to be totally enclosed in a metal case fer one reason. The grid coil of the high-frequency valve is wound to constitute the frame aerial, which is hinged with the lid. The position of this coil therefore varies with relation to the grid coil of the detector valve, and a strong coupling would exist between the two coils, unless one is shielded. The result would be unstability and difficulty to neutralise. As reaction is employed, the size of this coil and screen can be considerably reduced without sacrificing any efficiency, and any damping introduced by close proximity to an earthed screen is compensated by the application of reaction. Details are given for the construction of coil and screen, but if the reader prefers to incorporate any of the well-known makes of English sereened coils, the numbers given here apply to the numbers found on the Sixpin coil base. This English component is known as the "split primary high-fre-

quency transformer, with reaction," and the numbers will either be found on the base or the coil itself. | Out of a piece of sheet copper or aluminium mark and cut the shape as shown in the diagram. The length over all is 12 8-Sin. and 8 3-8in. wide. The dotted lines should be marked, and show where to bend. By clamping the sheet between two pieces of wood with sharp. square edges. the lines shown dotted can be bent down at a right angle by tapping with a flat piece of wood. Before bending, drill 1-8in.

"holes where shown. This sheet is bent to form a box measuring 4fin. high and 8in. square, with a flange at the bottom,.so that when inverted over the coil, the screen can be serewed down permanently. If alumin-: ium’ is used the seams will have to be either riveted or bolted with small nuts and bolts. If made of copper, then all the seams can »%e soldered and no holes will be necessary except in the flange. This screen is screwed to a metal base 83in. square, which can be provided with three drilled projectors to screw to the wooden baseboard.

The Coil. Now the coil itself. This consists of a centre tapped primary, coupled to a secondary with a coil for the Reinartz method of reaction. Details are given for winding a 2tin. and a 2in. diameter coil. A few more turns are necessary on the smaller former. Two small brass or aluminium brackets are best and drilled. to hold the coil upright. Starting about 3-8in. from the end on which the brackets are screwed, wind on twenty-five turns. of the 24 DCC wire. Make a loop and carry on winding in the same direction, another 65 turns. Fasten the end by pass- . ing in and out of two holes drilled in

the ebonite former. This constitutes a coil of 90 turns, tapped on the twentyfifth. ON the smaller piece of former wind on twenty-eight turns of the 30 DCG wire, tapped in the centre. Allow about 6 inches of wire on the two ends and centre tap for connecting. This coil can then be slid inside the larger coil and fastened by wedges so that it is under the middle of the 65 turn secondary coil. Reference to the diagram will help the constructor to understand the arrangement. These three leads from

the primary coil can be brought out to three soldering lugs arranged cn the end of the ebonite former. The number of turns given here are for a coil of 24in. diameter. [* a two-inch former is used, then wind on thirty turns, make a twist for a tapping and follow by seventytwo turns. This makes a coil of 102 turns tapped at the thirtieth primary winding 82 turns centre tapped. After wedgin in the primary, this can be fixed into place by sticking with seccotine or celluloid cement. This finishes the coil, and a note can be made of the leads for connecting up. These are numbered, so chances of mistakes are very remote. R.F. Choke. The radio frequency choke can now be given attention. If the constructor wishes to buy one already assembled he should choose one with a_ small enough diameter to go in the space provided and should cover the broadcast range. . A good R.F, choke can be made on « celluloid fr.me constructed from strong motor-hood celluloid joined with cement made by dissolving chips of the -celluioid :.in-- liquid. acetone,

which may be obtained from a chemist. The cement should be of a creamy consistency, and is best made in a small bottle, the bulk of small chips bgng about half that of the acetone The cement is applied to joints With a small sharpe-’ 7 stick and dries solid in two or three hours if not too thickly applied. | Two strips of celluloid are cut fin. wide by 4in. long, and one strip lin. wide by 4in. long is cut the shape shown, 1 in. wide and itin. from point to point. The diagram of parts is given full size so that the celluloid may be placed over it and the lines scratcheed in with a sharp point. The corners of the notches should be rounded as shown so that

the wire will not catch on the edge wk n win‘tine Two hc = are shown‘in the base through which fin. serews can be placed to hold the choke in position. The position of the upright strips is shown by ‘otted lines. The two #in. strips are to be cemented to the itre of the inch strip, one on ‘ach side, cemenu' being placed along .ach side of the joint. To do this the fin, strip can be supported on edge by a strip of wood each side, the top edge cemented and the inch strip put in place. supported level by strips of cardboard packed underneath. The other 4in. piece is then cemented on and similarly propned up. When the cement ‘is partly set, the supports may b taken away and the upright cemented to the base and left to dry.

lengths is made by winding 200 tfirns in each set of notches, making? 1000 _ turns in all., ‘Thicker wire may be soldered on at the ends for connecting" up. ée A useful gauge of wire is. 34’ or 36's. A useful choke for broadcast The Assembly. | . PROCURE a baseboard 18 x 5% x 4, and after drilling the panel for the condensers, rheostat, and terminals, screw on the baseboard: with fine

serews. Two small brackets bolted to the baseboard and screwed to the panel will strengthen the whole and lessen the strain on the small screws. Holes to take the three condenser spindles can be drilled 4%in. apart, the left one being 2¢in. from the panel end. The rheostat is mounted on the right of these three condensers, while underneath are drilled two holes to take the two speaker terminals. These are brought out to the front, together withthe speaker leads, so that the *phones can be used if necessary by simply disconnecting the speaker. On the extreme left are two more terminals for connecting to the loop with a small piece of flexible wire. ' Reference to the layout diagram will give the disposition of the various parts. A narrow terminal strip is screwed along the back, arranged with six terminals, on which short leads are taken down to the batteries underneath. As will be seen, the grid leak and condenser has to be. suspended, and for this purpose a short piece of thick bus-wire is used to connect to the grid terminal of the detector valve. On the left, close to the. end of the baseboard, screw the first valve holder. Behind this is the neytralising condenser. Next screw the metal plate of the screen and mount the ecoil vertically in the middle of this. Making room for the grid leak and condenser between the coil screen and detector valve holder, this holder is screwed into place. The radio frequency choke and by-pass condenser are fastened as shown. This small fixed condenser will also have to be suspended by stiff buswire and not screwed io the wooden. baseboard. At the side of this is mounted the _ resistance "papacity unit. Before mounting slide out the coupling condenser and _ test with the ’phones and a 45-volt battery. After the first click, no other loud clicks should be noticed. If this condenser leaks, the results will be horribly distorted music and speech, and a faulty condenser here should. always be looked for in the event of poor reproduction. In the extreme right-hand corner at the back is serewed_ the transformer, arranged with the plate

and grid leads convenient for their respective terminals on the valve holders. The remaining two valve sockets can be arranged in the spaces behind the rheostat. In portable receivers the small B batteries which are used are subjected to a fairly heavy load, and may, with

use, develop slight noises. A m.f.d. condenser is therefore used to smooth out the slight irregularities in direct current, and also helps to lower the resistance of the B battery. It is always advisable to incorporate one in any set. Wiring. J{VERYTHING is ready for the wiring, and the easiest way to tackle

this is to wire all the filament wiring first. In the theoretical diagram the filament rheostat was omitted, but this can be arranged in the positive lead from the A battery. All the wiring can be done with rubber-covered flex or 20-guage tinned wire covered with insulated sleeving. Follow each wire carefully, and when fixed cross out from the diagram. This is perhaps the safest way to do the wiring, and it can be seen at a glance if any wiring has been left out. Although the grid and plate wires are shown with right-angle bends in the diagram, make all these wires as Short as possible, so that they run straight from each terminal. When leading wires to the coil, arrange them to be down close to the metal base plate, so that small pieces can be cut out of the shield to allow these wires to pass through. Laying Out. AVING wired and fixed up the components so that a jar will not upset anything, the receiver can be tried

out. This is done before fixing in the cabinet. Stand the set on a support so that two flexible leads can connect up the frame aerial. Connect up the batteries and the speaker or ’phones and insert the valves in the following order :- HF, detector, lst L.F., 2nd L.F. Turn on the rheostat and tune in the local station. The frame aerial will have to be pointing to this station, as will be the case whenever using the frame. To operate, tune-in on the two Iefthand dials, and use the right-hand condenser dial as a reaction or volume control. The rheostat knob is used to control the voltage in the filaments, and should never be used at a higher reading than necessary. As the battery supply is 3 volts and fhe filaments consume two volts only, a certain amount of resistance will always have to be in use, except when the batteries are running down. Neutralising. To neutralise the set disconnect the filament positive lead temporarily from

the first valve socket, and tune to the local station again,» preferably using the outside aeridl for this purpose, The outside aerial can be attached to the cenitre tap otf the frame aerial, and the earth connection to the lower terminal of the three on the frame, Having tuned-in the loéal at its loudest strength, adjust the ueutralising condenser until the signals become inautible, or grow very faint. The set is how balanced, and the neutralising condenser can be screived down firm.

Paneis for the Kxperimenier, {OR purely expesimental work, ebonite or even bakelite panels can be dispensed witi very easily. wvirst of all, proeure a supply of 3-ply wood. This material is not expensive, 2ud when any prolonged amount of experimental! work is carried ovt, it is a good thing to have a stock of this wood handy. Make the necessary panel for the experimental set or apparatus out of this wood. Whilst doing so, procure also one or two old gramophone records and half a pint of naphtha or methylated spirits. «x the records up into very smal] pieces, and allow them to soak for

Disconnect batteries and slide into the cabinet. Tag all the battery leads with their correct connections, so that connecting will be done inside the cabitiet without being able to réfer to the terniinals above, Join the two A batteries in series, remembering that the terminal is the oue in the middle. Join the two 45-volt battéries in series and adjust the detector voltage to the best yalue.

A Few Hints. The receiver is now complete, and one or two hints may be useful for the user of this set. ' Always turn off the rheostat when not being used. This automatically cuts off all the batteries. . If a long outside aerial is used in conjunction with the frame, it may be necessary to use a .00025 fixed condenser in series with the outside aerial, and should be ¢oniected between the

outside aerial and the centre terminal of the frame. Handle the set with reasonable care} always remembering it is a wireless se and not just an attache case contaiuing books.