Select Your Local Programme

Radio Record, Volume IV, Issue 45, 22 May 1931, Page 16

Select Your Local Programme

‘The "‘Rejecta" Crystal Set

[-o- HE crystal sets of the Wellington listeners have, for the greater part, become obsolete over-night. With the opening of the new and relatively powerful 2ZW it has become a problem to isolate it from 2YA, and literally hundreds of listeners have communicated with both stations and asked them how it is possible to separate the stations. Right from the onset, let us say that it is no easy task. For some sets it is impossible, and it will need nothing less than rebuilding the set, tor it must be constructed along entirely different lines, It must be made "selective" and in considering selectivity another problem crops up, for when our sets are made selective they usually become less sensitive than they were before. Where we have only a limited amount of signal voltage available and no means of amplifying or strengthening it, we must take care of every atom of power that the aerial collects. We must confess that we have had little time to delve thoroughly into the problem, Within a few days it has been necessary to devise a circuit, and a simple one at that, to differentiate between the stations. "he necessity of providing a suffciently selective circuit was brought home to us forcibly one evening last week. We were listening to 2YA who had .just. announced the speaker for the evening when the familiar tune of "Colonel Bogey": burst in; | It was no good trying to cut him out, he came in at all parts of the dial, for we were using the standard "Radcord"

circuit, with an amplifier, and the set was operated in‘ the "Record" office. which is next door to the new station. The Ideal Set. So we attacked the problem the next day and worked out details of the new circuit. Making up a suitable set from components already on hand we brought it to the office and, lo anil behold, we could separate the stations -right under the shadow of 2ZW at that. It could be tuned out in a few degrees of the dial. On communicating with the trader: n day or so later, it was found thar the parts could not be had. They had all been’ sold out through their being specified for the differential re-ceivers-"Would not something elise do?" ‘So in a very short time we had to make a set, from not what we wauted but from what we could get. However, the set has now been made and we are satisfied with it. From our rather hurried experiments we derived several facts that should help listeners who cannot separate the station. © The first line of defence, or we suppose, attack, is to put a .0008 colidenser in the aerial. These cost only 1/6 and if they do not remedy the trouble will certainly improve matters, and if the "Rejecta" circuit is later made up this component. will alreacty be in the junk box. If a small aerial is used this component will most likely be unnecessary, but as-we work with a long one it is an essential.

Strangely enough the condenser has little or no effect on the volume. Uniess the set is of. the type using a variable condenser. there is little hope of it ever cutting out oné station in. favour of the other, and the set will have to be serapped. © Most un fortunately this dpples to certain type> of commercially-made sets. However do not discard. them, as it is possible to make them more selective with a certain type of wapetrap that we hope soon to describe. But when the parts of the wavetrap have been bought the price of the components necessary fos the ‘Rejecta’" has been paid, so where

is the gain? Furthermore, the parts fx this trap are not in the country at present and will not be for some considerable time, , On investigation we have found thai in Wellington, at least. there is 4 dearth of 2-inch former and .00035 condensers, the. parts we used in the original model. There appears to be plenty of 3in. former and .00025 condensers So we have used these. However, specifications are given for both combinations, just in case anyone happens to have a crystal set of the "Radcord" type on hand and wishes to convert it. : Our experience has been that in this hook-up at least, the .0005 condenser is not suitable. There is not enough separation between the stations unless volume is sacrific ’ to attain selectivity. An undesirable state of affairs. IK : this reason it seems that the best condenser for the job is the smaller 00025, but the coil must be designed with care else one of the stations will be pushed off the dial. With a welldesigned coil, it is possible to put one station at either end of the dial and that is the ideal when stations are to be separated. The first job in commencing the construction of this set is to decide upon the components to be used and obtain what is not on hand. Look over the accompanying lists and fill in the blanks; it will be found that the parts are not expensive. Do not pay fancy prices for extra special quality goods: they are not worth it in a crystal set of this type. The Coil Construction. Tue coil is by far the most important part of the business, so we shall deal with it first. Upon a 4-inch length

of tubing, drill two holes about lin. from the bottom. These are only small holes to accommodate the ends of the Wire and need be of no greater diameter. than 1-82in. On the 3in. former measure off 24in. and drill another two similar holes. This will be for the top end of the wire that goes to the con(ienser. If- you have or ean borruw a coil winder, winding the coil will be simple. If one is not available it will be more difficult, but by no means impossible. Thread the 24 dse. through the hole and commence winding, and for thirt turns allow a space between the Not much, just enough to accommodat} the 30, lying alongside. It will be found that it is better to err by leaving .00 much space than by leaving too little, as the wires can be pushed up tighter | later, but it is a difficult business to pull them out to take in the finer wire. At the 80th turn clean about. an.inch of the Wire and twist it about itself. It will be found easier if a sharp twist is made first at the base of what is to be the tap, and then twist. the wire until all the bared part is twisted together, By making a sharp twist at the bottom, the tap will not pull out when the winding is recommenced. Winding is now recommenced. this time there is no spacing. This is easy and it will take very little time to,complete the coil. If a .00085 condenser is ro be used, take a tapping at the 25th / urn, after the close winding has been/ ‘ommenced, Finish off through the holes provided. If the winding has panned out as it should. this coil will end at the holes provided. If not. finish off through them in any case. To complete the coil, the 30-gauge wire must be wound in the spaces between the turns of the lower half of the coil.. When finished there is to be one thick wire and then one thin, and so on. The lower end of the 30 wire is twisted with the end of the 24 wire, and threaded through the same hole. At the 12th turn make a tap as before, but be careful not to break the wire, as it is very thin. Make another tap at the 22nd turn. This latter will be the one most used, the first heing the most suitable where extreme selectivity is desired. It is unnecessary to use this in most cases. When the tap in main winding is encountered, give tife fine wire a turn round this and cut off from the reel, leaving some 6in. of slack, Now liberally coat the last few turns of both windings with seceotine or other adhesive, and while this is drying set about laying out the remainder of the components. The Lay-out. AbouT this part of the job there is little need to make comment, as the lay-out diagram: shows all clearly, The terminals are arranged on the front panel, and the coil is to be mounted horizontally, This is easily the most satisfactory method. HE crystal may be one of many different types. Experience has shown the variable cartridge type to be the best, although the ecatwhisker’ and galena is slightly more sensitive and the carborundum, less trouble than

\ either. The layout has.provided for a semi-permanent cartridge type.. To the aerial terminal is attached a piece of flexible wire, or for all practical. purposes, a piece of the 24 wire will do. To the other end :is fastened the midget clip, and this is joined to the

upper tapping on the 80 wire. "Lowever, different conditions demand ditferent placement of the clip, and for one station only probably the best results will be obtained by bringing the aerial on to the thirty-fifth turn on the main coil and joining the.top of this coil to the crystal instead of ‘the finer wire. This is the familiar "Radcord™ hook-up. Frankly, we found it not worth. making the’change for results were’ just as good with the set connected up in the intended manner. The coil ends and taps are distributed as follow :-The bottom of both coils goes to earth. The top of the fine "ire goes to the erystal, and that end of the

‘main wire to the fixed. plates of the condenser. The aerial is brought in to the second tapping on the small coil. The others can be tried for results. It is best to obtain a fixed condenser. with grid leak clips and place 2 small length of fin. tubing or rod in the place of the grid leak when only one station is on the air. This will bring up the signals slightly, but will spread them over 2 much greater part of the dial. And so we come to the end of our crystal set. If it has been made to specifications, there is no doubt that it Will eliminate one station in favour of the other. There may be a trace of cross-talk, but that happens in the best of valve sets. By cross-talk we mean that although there is a dead spot be. tween the stations, one can be heard faintly on the background when the other is tuned in. However, it is not nearly enough to cause any worry. For a .00035 Condenser and 2in. Coil. LAER are the specifications for this combination, for which we have more than a sneaking regard, though probably it has no justification. Space out the first 23 turns of 24 gauge wire, putting in a tap, then tight wind for 48 turns. Put in a tap at the twentyfourth turn from the tight wound end. Wind in the 30 gauge tapping at the eighteenth turn for the aerial. This combination will be found very sharp eut in tuning,