The "Rotorua" Portable Receiver

Radio Record, Volume II, Issue 22, 14 December 1928, Page 2

The "Rotorua" Portable Receiver

An Ideal Equipment for the Approaching. Holidays

By

Pentode

‘TH the coming cf summer the general opinion is that the radio enthusiast loses interest in the hobby in preference for some outdoor sport. He cannot be blamed for leaving his radio set at home unused and packing a hamper for a trip into the country. The average receiver, when once installed is not adaptable to portability and _ it would need many bulky packagés of batteries, speaker, etc., to be looked after if one wis ed to have the set during a holiday for company. The possessor of a true portable receiver is to be envied if he can merely open the lid and tune in a station to get the latest cricket results, ete. Take the man touring the country in a car. He camps in some out-of-the-way spot and takes out his portable receiver and can enjoy the afternoon session and later have a summary of all the latest news. Out on the river, what could be more pleasant than a little music. When listening in on the heach there is far more fascination than: listening at home with the receiver connected to an unsightly aerial.

Points in Design. [TN the design of a portable receiver there are numerous points that have to be considered. In the first place the set has to be truly portable, and self-contained. It has not to be too heavy. Hspecially as it has to be used outside, the speaker will be called upon to deliver ; reat volume and good quality reproduction. So many portable sets give horribly distorted music as soon as the volume is turned up. The question of running costs also enters, although not quite to such a great extent as with a home receiver. And last, but not least, the price has to be moderate. In designing the following portable set, consideration was given to all these points, and if the reader desires to

make one of his own and follows the details set out below, he will "be in possession of a portable set that will be the. envy of all of his friends. Provision is made for the use of an outdoor aerial, so this receiver can be constructed as a home receiver and will receive distant stations with just the same ease and clarity as the popular Browning Drak-. Also, if the constructor desires to develop the audio frequency side a little more fully, such

as incorporate pushpull in the last stage or use a higher plate voltage and larger power valve, Le can use it with any of the high grade gramophone pick-ups and obtain music at good volume and wonderful quality. For thos. who can work from a theoretical diagram, by referring to the one below it can be seen that the first grid coil is actually the frame in «ihe lid tune? by a .0005 mfd. condenser. This frame is centre tapped, so that an outside aerial can be attached to bring in more distant stations. It may be stated here that this is not a freak set, that is uncertain in operation and ticklish to tune in. There are various circuits for portable receivers employing reaction in various ways, but these sets as «a rule are not all that

can be desired in the quality reproduction of music, and furthermore they are not adaptable for use on an outside aerial. This receiver is neutralised and although reaction is used, the set cannot radiate if properly used. In a portable set the constructor is confined more or less to a limited choice of components. Appended is a list,of the components used by the writer, but if the builder wishes to incorporate parts he already has, or if he is

unable to obtain the parts specified, he will have to adapt the box container to suit the size, ete. of such components. The speaker, for example, is one that is easily obtainable, but there is no reason why a Sferovox or similar cone should not be used. The ease, will, however, have to be made accordingly. The small Hellesens B batteries are used, but, if unobtainable, the ordinary upright 45 v 't can be used with a little extra weigit to carry around. Those wishing to build exactly to specifications have the list of parts actually used and space is allowed for these components. The writer would stress the point that there is not too much room for larger parts and it would he time well spent to spend an hour in

measuring everything out before startinz to make the cabinet, unless, sf course, those actual components are used. The valves also were chosen to suit the circuit, but whatever valves are used, it is always better to employ one having a high amplification factor as a detector. As the full description of this receiver makes rather a long article it will be divided into two issues. This week the construction of the cabinet will be dealt with, confining the receiver construction to next week’s issue. The Cabinet. V ITH all portable sets the cabinet is of importance, inasmuch as it has to be light and yet strong. It has to be of such a size and shape that no complicated manoeuvres are necessary té examine batteries or valves. Omitting the lid, the inside measurements of the case are 18 inches long, 13 inches high, and 6 inches from back to front. As can be seen from the diagram, the lid makes an additional 1 inch inside when closed. The writer’s box is made of 3-8 inch oak, and although slightly heavier than one made of, say, Oregon, it is very strong. The top, bottom, and two ends can be of oak, while the sides will do well if constructed of three-ply. The best way to commence on the box is to treat the case and lid as one, | and after fixing on each side a line can/ be drawn around where the lid cut has to be made, and the lid can be cut off afterwards. From the local timber yard obtain timber to these measurements :-- 2 pieces 74 inch x 13% x 3-8 inch dressed oak or rimu. 2 nieces 7# inches x 18$ x 3.8 inch . dressed oak or rimu. 2 pieces 183 inch x 184 inck threeply wood. 1 piece 18 inch x 52 x 3-16 inch three-ply. 1 piece 18 inch x x 3-16 inch threeply.

Tf the reader is any hand at woodwork, he can proceed to dovetail the four pieces of oak or rimu together to form an oblong, with inside measurements 18 inches x 13 inches x 7} inches, and a rabbet on each side the thickness of the three-ply and + inch deep, to allow the three-ply sides to be glued and nailed in position. Quite a presentable container can be made by just glueing and nailing the four pieces fegether, but the two side pieces will have to be cut down to 18 inches long, so that the inside measurements are the same as specified. The outside of the box can now be cleaned up with sandpaper, and the position for the saw cut to divide the lid from the -box can be marked. This is a line right round the top, two ends, and the bottom. It is 6 3-16 inches from one side and 1 3-16 inches from the other side. This leaves two parallel lines 1-8 inch apart, and gives plenty of room for a saw cut and the necessary cleaning up. When the edges are planed up the lid can be fitted with hinges at one end and _ temporarily hung. An attache case lock gives the whole a distinctive appearance, and is fastened to the opposite end of the lid to the hinges. Three small pins projecting about } inch should be fitted to the edge of the lid with corresponding holes in the edge of the body to carry the weight of the lid and frame aerial when the box is closed. Now unscrew the hinges and lock and fix the shelf inside on which the receiver rests. As the ebonite panel is five inches deep, this ledge will have to be five inches from the inside of the top. The top being when the box is standing with the hinges on the left. This is only a three-ply shelf and need ‘not be very strong, as the set is ar\Tanged on a baseboard, and the whole

ean sit on bearers on each end of the box. The diagrams will help considerably to make these small points clear and should be referred to from time to time, The front battery cover comes next. This also acts as a baffle for the speak-

er, and if possible should be rather heavier than three-ply. Five-ply would: be more suitable, although the threeply in the set made by the writer seems to be O.K. A circle should be scribed on this cover with the centre 5iin. from the right-hand side, and 4in. from both top and bottom. It should have a radius of 34in. Plain bars or

QVUEE.EUIOOINUEGSOUIOGTESUAUEEUOCE AEE any fancy design can be drawn on the woodwork, and the space between cut away with a fretwork saw. ‘This is to accommodate the speaker and will be found to be the correct size, allowing for the flange through which

screws can be placed to clamp the speaker unit to this cover. Coloured silk fabric makes a very neat job if a piece of this material is fixed at the back between the diaphragm and the cover. Four or five small wooden brackets are now fixed inside the box

so that the cover can be screwed tightly and the front be flush with the edges of the box. Before going further the whole box container will have to be stained and polished. This is not a difficult job and a better method than applying the coloured varnish direct to the wood is to stain with either water or spirit

Hul dye first and then apply plain shellac varnish. An alternative is to cover the whole with imitation Iorocco leather; hut this is a tedious job and unless the CO1 structor has done that work before it

will not end up as intended. A good water stain is a solution of permanganate of potash in water. A saturated solution is perhaps too dark, so try a little first on a small piece of serap wood. The pink colour when first applied soon changes to a brown, and an idea of the finished colour can be obtained when the wood is still wet. Run over the surface quickly and allow to thoroughly dry before giving a brush over with shellac dissolved in methylated. spirits. If a more polished finish is desired than that given by two or three coats with the brush, the whole should be rubbed with a cloth moistened with the varnish. If inclined to stick or drag the rubber should be lubricated with a small quantity of linseed oil. Next connect the frame aerial. This is arranged in the lid and consists of 20 turns of 22 enamelled s.w.g. wire. It would be preferable to employ the special loop aerial wire sold for that purpose if obtainable, but the enamelled wire was proved quite suitable by the writer. A glance at the small drawing will give the reader an idea how to tackle the problem. Four pieces of ebonite, jin. square and 5in. long, are cut and a countersunk hole jin. from each end; take two small screws to be fastened in each corner of the lid. Twenty cuts are made with a hacksaw at an angle, 1-Sin. deep. These will have to be 3-16in. apart. A blade with a fairly coarse tooth setting is advisable, so that the wire will slip in easily. This coil is tapped in the centre and the three ends brought out to three ter-' minals on a small ebonite block screws ed to the lid close to the top hinge, When using the frame aerial alone the two outside ends are fastened by flex(Continued on page 30.) °

Components for "Rotorua" Portable

Wooden Carrying Case, as specified. Philips P.C.J.J. Speaker Unit. 2 Hellesens 45V Batteries, WIPYT Type. ; 2 No. 6 Dry Cells. 1 C Battery. Ebonite Panel, 18in. x 5in. x 3-16in. 3 Variable Condensers, .0005 m.f.d, (Formo). 1 R.C. Unit (Meter or Polymet). 1 "Audio" Transformer (Philips) 4 Valve Sockets (Antimicrophonic). 1 Neutralising Condenser. 1 Grid Leak (5 meg.), and Condenser (.00025 m.f.d.).

1 .1 m.f.d. Condenser. 1 .0005 Fixed Condensex. 1:12 to 15 ohm Rheostat. 1 Doz. Small Terminals. Aluminium or Brass Coil Shield. 4in. Ebonite or Cardboard Former, 24in dia. 4in Ebonite or Cardboard Former, 22in. dia. t]ib. 22 Enamelled Wire, S.W.G. or stranded, silk-covered loop wire. Zozs. 30 D.C.C. S.W.G Wire. ztlb. 24 D.C.C. S.W.G. Wire. 2 XA209 Philips Valves. 1 XA225 Philips Valve. 1 XB205 Philips Valve. Insulated Sleeving, etc.

The "Rotorua" Portable

(Continued from page 3.) ‘ ible wire to the two terminals provided on the set. Jf the outside aerial and earth are used, then the aerial is fastened to the centre terminal, and ground terminal to the bottom terminal, top and bottom terminals connected as before to the two on the set. The lid and box can now be rehinged and the various fittings screwed into place. A leather carrying handle on the top is fastened by four small nuts and bolts with washers inside to prevent the nuts from tearing through with the weight. Before starting on the receiver itself the batteries can be arranged and clamps’ made for holding them in their places. On the bottom on the left the two Hellesens batteries stand. Two small blocks of wood can be fastened on the bottom and side to hold these in position. The two dry cells also are fastened with aluminium strips as shown so that ‘they will not fall over on the speaker unit. The © battery is similarly fixed. This completes the construction of the carrying case, and on completion the reader is ready to commence on the receiver itself. The next issue of "Radio Record" will contain a concise description of this part of the set. Meanwhile the work laid out above can not be done in a day.