“PROGRESSIVE” RECEIVER

Sun (Auckland), Volume 1, Issue 41, 11 May 1927, Page 10

“PROGRESSIVE” RECEIVER

FROM CRYSTAL TO THREE VALVE

This series of articles on the Roberts Progressive receiver (of which this is the first number) is specially written for the beginner in radio, who wishes to construct his own receiver. The average homebuilder commences in most cases by buying parts for a small set of the crystal or one-valve variety; this, however, soon ceases to satisfy, when the builder finds that to make a larger machine he must buy new apparatus, and discard some of his old instruments.

The builder of the Progressive receiver, however, can build a Crystal set, one valve reflex set, crystal set with valve amplifier, two valve Roberts, three valve Roberts, and when he has built his final machine all he will have left in parts which cannot be used is one crystal detector. With the Progressive there is no need for the constructor to paj r a large amount of money; he buys his material gradually, and gets the full benefit of what he has bought. He learns to walk before he runs, and lastly he knows that every set he builds will work at the highest efficiency of its type, as every set offered to readers of The Sun is constructed by the writer before the article is written. APPARATUS REQUIRED. The crystal set which forms the subject of the first article, is an extremely efficient one, and will do all that any other set of a similar type will do. It will not enable the builder to hear Australia unless he has a neighbour with a howling valve, but it should be possible to hear Dunedin and Auckland, under favourable conditions, with a first-class aerial in a good location, at least 70 feet high. Claims to crystal reception over long distances on a crystal set with the average amateur aerial have not been substantiated. In choosing the apparatus buy only first-class material; a saving of a shill-

ing may mean a poor receiver, and considerable trouble in locating faults. The following apparatus is required:— One Roberts aerial coupler unit. Note .hat if this cannot be purchased without the transformer unit, purchase both as the latter will be required in an early article. One low loss variable condenser to suit unit, with dial. One multiple switch (or set of switch parts) to suit unit. One fixed crystal detector, with mounting. One .005 fixed condenser, Dnbilier, Sangamo, Micamold. Four terminals. Four lengths of busbar. One panel. 7 x 18 inches. (See text Delow) or piece wood, 7 x 8 x 3-16 in. One baseboard 10 x 18 inches or 10 x l inches. Assorted wood screws to fix panel to base, and coupler to board. If the builder intends to build the next model of the Progressive at an early date he is advised to purchase he ebonite panel which will accommoiate the Roberts 2 and 3. The piece of wood can, however, be used as a panel for the crystal set, but should then he discarded. Should the reader lesire to try for distant stations with the crystal set it would probably ) be!st to get an adjustable detector of the Galena or Silicon type, but the fixed detector is more satisfactory for local reception, as it does not require adjusting. Unless the builder has the requisite tools, such as drill, countersink, etc., it will be best for him to ?et the dealer from whom the panel is purchased to do the work, taking care to state that the panel is to be used for a 3 valve Roberts ultimately, and that the condenser and switch are to be mounted in their correct positions. Should the dealer make other suggestions as to modifying the length of panel then his suggestions should he followed, as he has had experience with the units he sells. In most cases 18 inches gives plenty of room for the apparatus. PREPARING THE PANEL. For those who have the tools the drilling will present no difficulty. The sw’tnh can mounted near thr* left

edge of the panel, allowing sufficient room for the cabinet sides, while th® condenser is mounted at such a distance from it that there is room for the coil unit to be mounted in front of the switch (see plan). Care should be taken that the mounting screws for the condenser will register with the holes, and that the shaft does not bind. Some condensers such as Bremer Tully require only a single hole for mounting. MOUNTING THE APPARATUS. When the condenser, switch, and terminals have been mounted on the panel and found to operate in a satisfactory maner, the panel may be screwed to the edge of the baseboard, which should be planed and sandpapered, and given a coat of shellac varnish. The coil unit should now be mounted as shown in plan, except when coils that are intended for panel mounting are purchased, when the unit can be mounted above or below the switch. The detector mounting can now be fastened in position at the rear of the condenser. The set is now ready for wiring. WIRING. The set should be wired with tinned busbar, either resin flux or Fluxite being used; if the latter only very small traces should be used, and care taken to see that it does not run into the instruments, as it will ruin the insulation. Commence by running a wire from

the aerial terminal to the long wire which will be found coiled on unit primary, which is either the inner or outer connection. Next run the remaining wires to the studs of the switch, then the moving arm of switch is connected to the earth terminal. Two wires will be found on the secondary' coil, and from une of these run a wire to the stationary plates of condenser and from thence to one side of crystal detector stand. The remaining terminal of detector goes to one of the telephone terminals. A wire is now run from the second connection on secondary coil to the rotary plates of condenser and from there to the second telephone terminal. The .005 fixed condenser is soldered across telephone terminals, or wires leading to same. This concludes the wiring of set. While the wiring is simple readers will find it worth while to use care in doing the work; bend the wires with a pair of square-nosed pliers, which, while taking a few more minutes, adds greatly to the look of the set, and when the valve set is tried later increases the ease of wiring as well as the efficiency of the set. TESTING AND OPERATING. Connect the receiver to a good aerial and ground, set the aerial switch to one of the middle studs, and slowly rotate variable condenser. As soon as 3YA is heard, adjust both switch and condenser for maximum volume, then take the fixed crystal out of the stand and reverse it, the position which gives the maximum volume being the correct one. If an adjustable detector is being used then its connections should be reversed in the same way. In the latter case take care not to touch the crystal with the fingers. This simple receiver is an efficient one and in building it the reader has completed the first step in the construction of a valve set, while the operation of the crystal is identical with one part of a Roberts 2 or 3. At a later date a further addition in the shape of a 2 valve amplifier for use with the set will be given. And remember that the parts purchased for receivers of this series will not be discarded, but will all be used in the final 3 valve Roberts.