Our Crystal Corner

Radio Record, Volume II, Issue 27, 18 January 1929, Page 29

Our Crystal Corner

By

Galena

Full Wave Crystal Set. HAVE constructed the full wave crystal set which was described in the " Radio Record," dated December 14, 1928, and I am experiencing a little difficulty in connection with same. I am using carborundum detectors, and find that although I obtain good results (R4 on speaker) with one detector, the volume is considerably reduced when the second detector is inserted. I have tried changing the detectors and have also reversed the ends, but have not been able to make any improvement. The condensers are .0005 semi-circular pattern, and the set can be tuned in with either condenser, the best results being obtained when the moving plates are nearly all out on one condenser, and three-quarters in on the other. I am 2} miles from 2YA, and have a good aerial and earth, the latter being similar to the Pearce system, but on a smaller scale. I shall be greatly obliged if you will advise me how I can obtain an increase in volume when using the second detector, also if better results would be obtained if complete carborundum, stabilizing units were used. You may be interested to hear that the "Low-Loss Crystal Set," described in "New Zealand Radio Handbook No. 6," has given me slightly better results than those obtained from the full wave set.-"PUZZLED" (Wellington).

ANSWERS.-Apparently the crystals are not perfect. Test by the phone and battery method deseribed recently in the "Beginner’ columns. Test by applying the battery lead to each end alternately. Only in one position should there be a click. If there is a click in both directions the crystal is at fault and defeating the object of the two crystals. It appears too, that the coils are not matched. Try a few less turns on the eoil, the condenser ‘connected with which reads nearly 0. Stabilising units would be an advantage but would not cure the trouble at present evident. Full-wave Rectification. N our special issue "Pentode" de-’ scribed a full-wave crystal set by employing two crystal circuits. By this arrangement double-strength signals could be obtained. Some _ correspondents who have made this set seem to have experienced a difficulty. The signals have not been so strong as they had expected, and they have been consequently disappointed. There is one drawback in this type of circuit, and that lies in the difficulty of getting both crystals to act with equal efficiency as far as rectification is concerned. If, for example, one crystal is not set at its most sensitive point, this may mean that some of the high-frequency current is getting through from the wrong direction, thereby nullifying a proportioned part of the current that is passing in the correct "conducting" |

direction, leaving the ’phones, to respond only to the difference or unbalancé® portion of the normal current. The current, however, that is getting through from the wrong direction is evidently shorting out the second crystal’and robbing it of its normal quota of the reversed voltage, so that the signal intensity is much lower than it should be. It is necessary, then, to get a pair of good crystals, equal in sensitivity, which will pass current in one direction. only. A test for this has been described above. Constructors who take this

caution should experience little difficulty in obtaining loudspeaker repro: duction from a nearby local station, Exceptional Results. BOUT a month ago I built a crystal set from which I have had remarkable results, using gauge: 28 for aerial wire. The aerial was put up in the room and, using a wire along: the floor as an earth, I received 2YA x as loud as the set on the outside aerial: with a good earth. In the city I worked it off a frame aerial with fair volume. At Hastbourne I used an aerial 4 feet long with a coil of wire at the end (without insulators) and the guttering of the verandah as an earth. From this I received remarkable results. One Saturday night I used the wire mattress as an aerial and used no earth. The results were fair and I could distinguish every word the announcer said. The set to me seems to be a "freak." The set consists of two spiderweb, coils and the distances between them can be adjusted, a .00025 variable condenser, and a semi-permanent detector. I have also another excellent crystal set, but the new set overshadows it. Hoping you are interested, and wishing you every success with your paper, which I consider the best radio paper for the money in N.Z.-I enjoy every word of it-G. T. KING (Island Bay),

READERS in difficulty, or who feel they are not getting the best from their crystal should send in their difficulties, They will be discussed in the Corner, that is providing they are not too particular to. one case, when they will be replied to by post. If an owner has found a circuit he considers superior to the usual, send it in and if worth publishing it will be passed on to others. Remember, address all inquiries "Technical Editor,’ and mark the letters "Crystal Corner."