The Selective Crystal Set

Radio Record, Volume I, Issue 26, 13 January 1928, Page 12

The Selective Crystal Set

CONSTRUCTING AS A WAYVE-TRAP

The R.R. selective crystal set described last week answers quite well as a wave-trap for valve sets, rendering them just as selective as the crystal set itself at the most selective setting, which used as a trap on the four-valve Browning-Drake at two miles from 2YA, cuts out that station with a movement of the dial of two degrees either way. Many constructors near a main station will prefer to construct the set complete with crystal as shown. This can then be used as a waye-trap, and in case of trouble with the set or during alterations is always available as a stand-by receiver. Jor use as a wavetrap the ’phones are not required, the aerial is connected to its usual terminal at the right, and the earth terminal is connected to the aerial terminal of the valve receiver. ‘The crystal need not be plugged in, and earth is plugged into socket 6, and aerial usually into socket 5. ‘hen tune in the unwanted station on the valve set, and with the trap condenser tune the unwanted station to minimum strength possible. The trap is then left permanenily set if only one interfering station lias to be dealt with. Any wanted station can then be tuned in on the valye set without interference, except upon wave-lengths very close to that of the unwanted station.

CONSTRUCTING AS A WAVE-TRAP ONLY, Construction as a wave-trap only dispenses with the telephone and crystal contections, and for most situations a permanent aerial tap at the 10th turn of the coil will be all that is required. It would be a good plan when installing the trap to test the aerial connection to the 18th turn also, as although tather less selective at this setting, it might be more suitable for certain conditions. Once this has been decided, the connection can be permanently soldered to either position. The 10th turn will be the best where ihe distance from a main station is not great. Tested on a very unselective | one-valver that brought in 2YA all over the dial at two miles, this trap gave it selectivity that cut 2YA out at four degrees cither way. Constructors will please themselves in the method of fitting up the wave-trap, but a neat accessory will be the result if it is housed in the cover as shown, with panel minus sockets and *phone clips. ‘The aerial terminal would then be permanently connected behind the panel to the coil tap and the "earth" terminal to the bottom of coil. This terminal then connects to the aerial terminal of valve set.

MAKING THE COVER. Tfor the assistance of constructors making the cover, the description of a simple method is given here, with sizes that will serve as a good guide, thonglh care must he taken to allow for any deviation from size in patel or -base-board. ‘The cover sizes will fit a Gin. by 7in. panel, and 6in. by..8}in. baseboard. ‘he top and sides are made preferably of Oregon 8-ply, and the back

of fin. rimu. ‘The joining of two edges of 8-ply at the top corners is effected by screwing each piece to a strip of wood jin. square, placed inside the corners and short enough to allow for the thickness of the panel. ‘Iwo strips jin. by tin. shortened to allow the width of the front batten of baseboard, are screwed to the inside of each side as shown, and into these the receiver slides. A coat of shellac gives a suitable finish. If a strut is placed at the side of the panel it is to be set in sufficiently far to clear the running strips inside the cover. This is quite a useful wave-trap for New’ Zealand conditions, and will be found a tremendous improvement on an ‘anselective receiver. Three dozen 3in. No. 4 brass screws will be required.