A Crystal Set with Unusual Volume

Radio Record, Volume II, Issue 22, 14 December 1928, Page 26

A Crystal Set with Unusual Volume

(By

Pentode

ITH the advent of the commencement of the "Crystal Corner" in this issue of the "Radio Record," the writer will endeavour to interest readers with the description of one or two crystal reecivers, that may be justly claimed to be a little out of the ordinary. These sets are a little more expensive to build, but no doubt there will be many cases where readers have » "junk" box containing sundry components which will find good use if built into one of the following circuits. These sets are of special use to listeners situated just too far away from a _ broadcast station to be able to comfortably hear the broadcast on the usual type of crystal set. Also, for those constructors who reside close to a powerful t .nsmitter and wish to reecive on the loudspeaker without the aid of any amplifier. No guarantee, however, for speaker reception from any crystal set can be made, but the \-riter can tune in the local station any evening, quite loud enough on the Speaker for domestic entertainment. Situation, length and height of aerial, efficiency of earth connection are important factors in the performance of crystal receivers. The only energy available to operate a crystal receiver ‘is that given out by the transmitter, and it behoves one to collect as much energy as possible and not waste any by using poor insulation and absorbtion by having the aerial or lead-in too close to buildings, trees, ete. Crystal receivers are essentially all the same. There are hundreds of crystal sets described in the various periodicals and journals, but when analysed. they can all be reduced down to some form of tuning system and a detector. The tuning system may consist of coil and condenser. variometer, or

merely a solonoid coil arranged with a slider. The circuit may include two or more coils loosely coupled. The chief advantage of this is to increase selectivity. Or an untuned aerial coil may be arranged more or less tightly coupled to a tuned secondary: coil in which circuit is included the crystal detector and phones. Rectification. VARIOUS types of crystal detectors can be employed, but each attributes its action to the fact that it will allow a current of electricity to pass in one direction and retard the flow in the opposite direction. Thus the incoming signals being alternating current are rectific by the detector to pulsation of direct current to operate the reproducing mechanism, whether phones or speaker. Now, in receivers using only one detector, one half: of the cycle is suppressed and is, as it were, wasted. The other half-cycle is used only to operate the phones. Therefore, in the usual single detector circuit there is a certain amount of energy that is not being used. A more detailed description of the theory of reception will be given in later articles in the ‘Radio Record" and the reader is advised to read them over, as it explains in a simple way just what happens when you are tuned into a station and the reason that audible sounds ean be heard miles from their origin. A brief outline was given last week in "Our Crystal. Corner," Returning tu the subject under discussion. If this waste energy could be turned round to flow in the sume direction as the rectified current, then the signal strength would be increused

with the increase of current fed into the phones. . Dual Tuning System. ‘THE most satisfactory way to do this is to employ two tuning systems, two erystal detectors and either arrange the circuit so that... available energy from both sets operate a single pair of telephones, or feed both the outputs into a transformer which has two primaries and one secondary. Below is given a circuit diagram showing the former. method. although instructions are given for using with the extra output transformer.

As will be seen, two separate crystal sets are employed. These must be kept quite separate by placin= the coils at right angles. Although it is quite possible to only use one tuning coil, the results are not | .ite so good as when using the arrangement shown. Two separate condensers are shown, but two gauged together will do quite well, but care must be exercised to see that both -un: . coils are identical . with regard to number and spacing of turns, guage of wire, diameter of coil former, ete., otherwise one of the sets will be constantly out of tune and no advantage will be gained. Although two fixed cryst: re used by the writer, two cat’s whisker types will do quite well and can be mounted on the front panel. The Coils. ‘THE coils are the only components needing much attention. In the ebonite or cardboard former, drill two small holes, 1-8in. from each end, on the same side to fix down to the baseboard. Two small wooden blocks can be made with holes drilled down the centre so that when a screw is passed through the former and this block, the

former will stand about an inch. from the baseboard. Both coil formers are treated in the same way and are wound with exactly the same. number of turns in each case. Stargng from one end, wind on 60 turns.0f the 22 s.w.g. DCC wire and fasten the wire by passing in and out of two small holes drilled in the former. Note which is the beginning and end of each coil, and over the wire at the begjnning end wrap a strip of brown paper 1l3in. wide and about 2ft. 6in. long. This will give three layers and can ve given a liberal coating of shellac vere nish or celluloid solution. Over thése layers of brown paper wind 20 turns of the 22 .DCC wire, fastening each end by tying with cotton, Leave about 9 inches free wire and this can be used to fasten direct on to the terminals. Mount these two finished coils on a baseboard 11lin. x Tin., toward the back

and at right angles to each other. The ebonite panel is drilled to take the two ) condensers and terminals for aerial, earth kind *phones. If square busbar is used to wire up the receiver, the two erystals ean be held in position by this wire. If not, then two clips will have to be devised to hold the detector cartridges away from the baseboard. Do not screw metal clips direct to the wooden baseboard, but.mount on a small sti®o of ebonite. Wiring. HE wiring is a comparatively simple matter, and no error can be made if the diagram is followed closely. The tw aerial coils are joined in series and the two free ends joined to aerial and earth terminal respectively. ‘The to sets of moving plates of the condeasers are joined together and also run to the earth end of each coil, that is the end of the secondary under the aerial coil. This wire also joins to one end of ’phone terminal, which ter-

mindjs are bridged by the .001 fixed condenser. The other ’phone terminal joins to one end of both of the crystal detectors. The free end of detectors joins in each ease to fixed vanes of the condenser and to the end of each coil opposite to the aerial primary coil. This finishes the wiring and a test will have to be made to determine

whether the erystal detectors are connected the correct way round. This can be determined only by actual test. Listen to the strength from the local station and reverse either, one, not both, of the crystal detectors and note signal strength again. It will be either louder or softer and the constructor will soon know which is the right way round. If a gauged condenser is used and signal strength does not seem up to seratch, a midget balancing condenser will be found useful to adjust both coils to resonance with the transmitting station. Using an Output Transformer. TF the reader has a push-pull output transformer it would be "quite worth while to include it in the set as a trial. To connect up the transformer break the three wires marked with a small cross. Connect the two outside ones to the centre tapped prim-

ary and the middle one to the centre tap. Short circuit the ’phone terminals and connect the two ’phone leads to the secondary of the transformer. It is not worth the expense of buying a special transformer for the job and the arrangement described will be an improvement over the usual erystal set.

Two Variable Condensers, .0005 m.f.d. (separate or gauged ). Two Crystal Detectors. | Two Cardboard or Ebonite Formers, 3in. dia., 4in. long. Panel.-Wood or Ebonite. ) a ' 4 Terminals. : Fixed Condenser, .001 m.f.d.°