Picnic Portable

Radio Record, Volume V, Issue 24, 24 December 1931, Page 29

Picnic Portable

(Continued from last week.)

MAKE sure that all holes drilled in { ' | the aluminium baseboard have been © smoothed off with a larger drill. This | is important, for the sharp edges left are quite likely to cut through the inSulation and cause serious shorts. | After making sure that the "B" bat- | tery voltages are only across points that they should be, connect aerial, earth and speaker, not worrying about the frame antenna for the present, and insert the valves and coils. . If the set is operating correctly, and a good aerial and earth is used, it

The carrying ease and rear view of the speaker.

should be possible to tune in on the speaker a number of the larger Australian stations, though it must be remembered that with the approach of summer, dx conditions are in general growing more unfavourable, When the sect is going all right it should be laid aside and the work commenced on the cabinet. speaker baffle and frame antenna. The Cabinet and Speaker. IIE next job to tackle is that of making the cabinet, the speaker baffle, and the fryume for the antenna. The cabinet should be built to the dimensions given in the sketch. Note that these are all internal. Light timber, 3-8in. thick, should be used. If the constructor has had no experience in woodwork, it would perhaps’ be better to have this portion of the set built by a cabinetmaker. The set slides into the cabinet on two half-inch square section pieces of wood serewed to the sides of the cabinet, and when in posi-

tion should be firmly secured in place with two strips of wood, one at each ond of the panel, as shown in the photozraph. It is advisable, before slipping the set into position, to place at the bottom of the cabinet a piece of thin rubber matting or corrugated cardboard, to absorb any jolts to which the set might be subjected. In the four corners of the lid pieces of wood in. square section are mounted of such a length that the speaker baffle, when in position, fits flush with the sides. The baffle is fastened to these means of four screws. The next task is to prepare the speaker baffle, which is built of ply wood. Constructors will have to use their own ingenuity largely in this matter, as with different types of loudspeaker units, different schemes of fretwork decoration will have to be adopted, because the mounting screws of the unit will not be in the same positions in every case. The main point to watch is that the driving rod is in the exact centre of the baffle. The speaker used in our model had an aluminium chassis, but all of this, with the exception of the portion holding the cone itself, was cut away to prevent it having any harmful effect on the inductance of the frame antenna, ‘This rather drastic step, however, is not absolutely necessary, as the difference in efficiency is only slight. The loudspeaker cord is taken out through a hole in the fretwork and is held in position round the side of the cabinet by means of small staples. This is clearly shown in the photograph. The fretwork is covered at the back with brown silk, the unit then screwed in position, and the loudspeaker cone mounted. Do not forget to attach the driving rod securely to the cone. The next task is to build the frame on which the antenna is wound. ‘This cousists of four pegs. three inches Jong, braced by thin slats. A number of saw cuts, placed 1-8in. apart. are made in each of the pegs to hold the windings in place. The wire used is either 23/36 silk covered frame antenna wire, or Litz wire. One end is attached to the bottom right hand peg, close in to the baffle. and the winding commenced. Twenty-four turns are put on, and the wire then anchored to the peg from which the winding was commenced, and both ends are taken out from the set through a hole made by cutting the bottom right hand corner of the baffle off. One of the ends is connected Lo the chassis of the set. and the other is taken to the correct terminal of the three-point switch. The baffle board is now placed into position and screwed down. To make the job neater, the loudspeaker cord is taken to a cord connector mounted between the set and the "B" battery, on the left-hand side of the cabinet and about 2in. from the bottom. The Set’s Capabilities, ‘OOD loudspeaker strength may be expected from the "Picnic Portable.’ 30 or 40 miles distant from a powerful station, and, when using a femporary aerial slung between two trees and an earth, over much greater distances, which will, of course, dépend upon the efficiency of the aerial.