The "Picnic Portable"

Radio Record, Volume V, Issue 23, 18 December 1931, Page 18

The "Picnic Portable"

~ A Cheap, Compact, and Powerful Portable weighing | only 24 pounds By the Assistant Technical Editor.

ITH the approach of summer we have received quite a number of inquiries concerning the design of portable setsso many in fact that we set to work to design and build one for description in the Christmas issue. Hence the "Picnic Portable." It has been designed in accordance with the limitations imposed on us by our readers’ suggestions, whith were in general as follow:1. Light A" and "B" battery drain, enabling the set to be adequately supplied with current from a small portable two-volt accumulator, and the lightest "B" battery possible, consistent with good service and the necessary voltage. "2, The set to operate from a built-in frame antenna, but provision to be made for the attachment of an aerial and ear th to enable distant stations to be-secured either at home or wherever a temporary aerial and earth carried in the set might be fitted up. ‘8. Complete cost to be under £10. 4 To be as compact as possible, but using only parts the constructor might happen to have on hand or which may ‘be easily obtained. 5. Good loudspeaker performance within reasonable range of a high-pow-ered station. The above is rather a "tall" order, but we are confident that we have fulfilled it. The third condition, and that of’ eost, limited the number of valves

to three, which, because of the restrictions imposed:on "A" and "B" supply, must of necessity be of the light "A" and "B" consumption type. This turned our thoughts immediately to the new 230 type valves, and these we have incorporated in the set, A 232 has been used for the screen-grid stage and two , 230’s for the detector and audio stages. Relatively high plate drain of the 231 type power valve and the necessity for high bias prohibited its use in the last stage. As an alternative to the 230 used in this position, a small power valve of the type P215 may be substituted to improve the tone. The total "A" drain of the set, using the P215 type valve in the output stage, is .27 of an amp, and the "B" drain is about 9 m.amps.-hboth valves being well below those permissible with the batteries used. If preferred, English valves may ve used throughout with equal success. The "A" drain will be slightly higher, while still well within the maximum permissible, but the "B" drain will, if anything, be slightly less. As the wiring of the r.f. stage as depicted in the photograph and layout sketch is that for a 282 valve, it will have to be slightly modified if an English screengrid valve is used, for the connections to the valve base and cap are different. In the American valve the ordinary grid is at the top, the screen-gri.l connections are taken to "G" on the valve hase, and that to the plate. to "P." In the English type, however, the plate

is at the top, the ordinary grid connection is taken to "G,’’ and the screengrid to "P." The extra components necessary to fill the second condition are six in number, namely, a three-point switch with one common terminal, two banana plugs and sockets, a .0001 mfd. fixed condenser, and a suitable secondary coil, with a valve base for mounting. These have heen arranged in such a wiy that by operating a switch on the front panel the set may be worked

-_- from either the frame antenna or an aerial, which is plugged into a banana socket on the front panel. Another socket is provided for the attachment of an earth. A length of insulated cable, and a length’ of earth wire to which is attached a small pointed copper rod, are carried to the set itself, for use in suitable locations. To eliminate all danger of shorting, hoth are rolled up and wrapped in oiled silk, as shown in ‘the photograph. As regards the third condition. the

Set can be built with everything incinded at a price under that stipulated, We found the fourth and last condition the hardest to comply with. By using specially selected components, which, however, could not possibly be easily obtained by the home-construc-tor, and by using a home-made loudspeaker cone, the size of the set could have been reduced. to about two-thirds its present size. Even though standard parts have been adhered to, however, the set is still quite compact, and, what is perhaps more important, is quite light-the complete outfit weighs just over 24lb. Building the Set. HE set is built on an aluminium baseboard, 14 x 4 inches, with 4inch turnover all round. A list of parts is given, and these should all be ebtained before the work is commenced, | The baseboard parts are then laid out ‘In accordance with the photograph, and the layout diagram,.great care being taken in the positioning of them, The two de-coupling condensers in the r.f. stage are mounted as shown in the photograph, by means of a suitably bent strip of aluminium, bolted to the baseboard. Coils and valves should be placed in ‘their sockets before the latter are mounted, to ensure that sufficient space has been left for the inserting of these components. Coils should be about one inch distant from the screen, not less. After the baseboard components are mounted, the panel, which forms one side of a four-sided aluminium box, enclosing the set, should be drilled to take the panel components. Space conditions have rendered it imperative that bakelite dielectric variable condensers be used. These are quite readily obtainable and are cheaper than the more bulky air dielectric type. In the

original model three small slow-mo-tion tuning dials were used. Only two of these, however, are strictly necessary, as the third is for reaction,

which may -be controlled by a small knob. A word about the three-point switch used to change over from frame antenna to ordinary aerial. The switch

has one common terminal, which goes to the grid of the r.f. valve. Two connections are taken from one of the remaining two terminals, one from the top of the secondary coil, and one through a .0001 fixed condenser to the nuerial socket mounted on the panel. One end of the frame antenna is taken to the remaining switch terminal. Notice that the tuning dials should be mounted upside down to render tuning more convenient. When all the panel components are in position, all the baseboard wiring it is possible to do without mounting the boxing should be completed. If, by the way, it is found that a little more baseboard space is needed, the bias battery may be mounted under the lid, which otherwise is really not required. Instead of employing a battery cable or terminals, ordinary rubber covered

flex leads for the "A" and "B" supplies are taken straight through the holes drilled in the chassis, and, where necessary, fitted with wander plugs for battery connections. The "A’’ — terminals

yf the valve sockets are taken straight to the chassis via the bolts holding the sockets in position. The wiring is thus greatly simplified.’ Note that a midget fixed condenser has been used

to couple the r.f. stage to the detector. This is mounted under the baseboard, so that the aluminium partition separating the r.f. stage from the rest of the set has no connections made through it. Thus it may be fitted in (Concluded on page 35.)

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position and screwed down after the Whole job has been completed. The aluminium boxing should now be placed in position over the chassis and bolted in position. Before doing this, however, it is a good plan to solder suitable lengths of wire to the various panel components to enable the final connections to be easily made. Notice that there are only three of the panel components which need to be insulated from: the aluminium panel. These are the on/off filament switch, the three-point aerial switch, and the socket to take the aerial plug. The others, comprising the variable condlensers, the reaction condenser, and the earth sockets, are mounted directly onthe aluminium panel. The moving vanes of the variable condensers are thus connected to the chassis and are virtually earthed. Hence only one connection, that to the fixed vanes, need be made, Winding the Coils. HE coils, which are of very simple construction, are wound on iin. diameter valve base extension former. A winding length of about 2tin. will be receded for the r.f. coil, and about 3in. cor the detector coil. The connections to the various pins should be carefully followed, because they have been arranged so that if by chance a valve is plugged into a coil Socket, the filament will not be damaged. After mounting the two pieces of former of the required lengths on, the valve bases, the longer is laid aside and the winding of the r.f. coil commenced. This consists of but one winding, a secondary, and so only two connections of the valve base are utilised, namely, plate and grid. About 3-8in. from the bottom of the former bore a small hole and pass the end of a coil of No. 28 (d.s.c, wire through the plate pin and se- » c

cure it there with a dab of solder. 105 close-wound turns are now put on, and a hole drilled above the grid pin, through which the end of the wire is taken and secured as before. The Detector Coil. HE secondary winding of this coil is put on in a manner exactly similar to that followed previously, only the number of turns in this case is reduced to 88. The reaction winding is now commenced. Exactly 1-8in. above the secondary, and directly over one of the filament pins, drill a small hole and pass the end of a length of No. 82 d.s.c. through the pin and secure it there. Then wind on 42 turns in the same direction as the secondary. The end of the winding is taken down through a hole to the remaining filament pin. The pins of both coil valve-bases should now be smoothed off with a file so that they Will slip readily into the sockets

Final Hints. HIE set is now complete, and should be tested out befor proceeding further. Before inserting the valves conduct a final test of the wiring by the torch bulb method, which has often been outlined on these pages. If the set will not oscillate reverse the connections to the fixed vanes of the differential reaction condenser, try 4 higher "B" voltage on the detector, a larger value grid-leak, and, as a last resource, increase the number of turns on the tickler. It is extremely unlikely that the latter will be necessary, and so, before making any alterations to the coil, carefully check and re-check all wiring. [Owing to unusual pressure on space, we regret that we have been forced to hold the remainder of this article until next week’s issue.-Hd.]