OCR Text |
Show DO NOT DESPISE CRYSTAL DETECTOR It Is Dependable and Cheap and Every Amateur Receiving Set Should Have One. There is considerable to know about tic oijstnl, and a little knowledge will t-.:a!.le III.,- operator to get better re-nuts re-nuts .mil briug Increased respect for tins instrument. There are many reu-mw'.h reu-mw'.h for IIjo longing of tlio owner of a crystal receiving set for the vacuum outfit. However, he bus a great v.: :n.v advantages which the owner of II:;' i:i-.re M"'iisive outiit does not pos- l-'or insluiitv. tho crystal reprodue-lioii reprodue-lioii of the radlophono broadcasting is of I lie lio.it pos.-iibie quality. The ae-i! ae-i! '!i being only rectifying there la no . ! nice of distortion. The tube re- vier nearly ulways distorts to sonie-e sonie-e -v t i ; r . If maximum regeneration la ""I. oven though the tube Is not ul-lo.-.ed to oscillate, distortion is bound to iVMilr. Transformer or choice coil . . ; i ; t i ; u ? ; between amplifiers, no grid ten or wrong grid bias, poorly n.alokod tubed und transformers or bail loud talker will all cause distortion. A combination of several of these of ten causes very groat distortion. When tho cryshil receiver Is out of commission It does not take a genius to locate the trouble. The circuit and the adjustments are simple. A burned out tube means Iffi.OO or more for another. an-other. A bad crystal means a little I S" e uM L wi Tumer C 4- 1-- I rouble to clean It up or at the worst a dime for a new one. Tho crystal set woes not go out of commission periodically period-ically as the tube set does, due to a worn out battery. The crystal set Is quiet in action, adding no nolso of Itself, as do many of the detector tubes. Reception through static Is nearly always more easily accomplished with a crystal. The atmospherics seem to be brought In all out of proportion to the signal by the tube detectors, while the crystal seems to damp out the high voltage peaks of the static or strays to some oxlent, leaving the signals, which are strong enough for It to rectify audibly, Tfii'y clear. The crystal set has numerous disadvantages, dis-advantages, of course, but so has the vacuum tube, and the good points of the former greatly overbalance its f units. It is not very stable, a slight jar flon being sufficient to "knock out" a good point. There is no way in which a crystal may be made to amplify and it does not give suflicient signal strength to use with a loud talker. Tt usually causes broad tuning, which makes It dillicult to tune out interference. inter-ference. It is not as sensitive to faint signals as the vacuum tube, although it Is surprisingly sensitive when properly prop-erly adjusted. Adjusting the crystal is a tedious tusk, und it has become especially espe-cially so since the labor-suving vacuum tubes have become so numerous. A crystal detector should be included in every receiving set. It is dependable, depend-able, cheap, and takes up little space. Even (bough you have a two-step amplifier am-plifier and a loud talker with all the trimmings, you will admit that there ere numerous times when your receiver re-ceiver is out of commission because of a bad A or I'. batlery, dead tube or sotne trouble In one of a dozen piece of your apparatus. During these periodic peri-odic lapses there is usually some talk that you would Ilka to have heard or a particular concert. A crystal detector will fill In these blanks very well, and you will be surprised at the quality of the music our broadcasting stations are sending out when you hear it via the crystal Instead of at the noisy end of a two-step amplifier. Many of the so-called disadvantages of the crystal detector can be partially overcome. Crystal receivers usually tune broadly broad-ly ; that is, a small change in tuning does not cause a very great change in signal strength, and a station can be heard over quite a range of luning. This means that if two stations are sending and their wave lengths are not very far apart it will be Impossible (o tune one in and exclude the other, whereas if the receiver tunes sharply, a station will be beard over a very narrow band of wave lengths. In other words, a very few degrees either way from the point where the best audibility audi-bility Is obtained will tune Uie station out. Then some selectivity can be had between two stations even if separated only by a few meters with a sharply tuned receiving set. The crystal in the usual hookups Introduces a resistance into the oscillating oscil-lating circuit, causing that circuit to be somewhat aperiodic. That is, it will respond to more than one wave length. It is rather hard to tune sharply with a crystal and n single tuning coil. In this case the crystal must be shunted across at least a purt of the indu-tance and there is no way of keeping it entirely out of any of the circuits which may be used. In order lo keep it as sharply tuned as possible, it is best to reduce the number of turns shunted by the detector circuit us low us possible without sacrificing too much signal strength. If the circuit cir-cuit shown in Fig. 1 is used, the number of turns between slider 2 and ground should be kept less tluui a third of the number between slider 1 and ground. This will allow the antenna an-tenna circuit to oscillate much more freely at its natural period. If using a single variometer better tuning may he had by tapping off the detector circuit, cir-cuit, as In Fig. 2, about three-fourths of the stationary coll of the variometer, instead of hooking It across the entire variometer. The loose coupler Is very much to be preferred over either of these two tuning tun-ing schemes. The two circuits are loosely coupled to each other by induction induc-tion and both circuits will have a hlgrfer resistance to any but the wave length to which they are tuned, each tending to damp out the undesired signal. The crystal is causing rather broad tuning in the secondary circuit, but the antenna circuit with It is rativr broad in low resistance, is left to oscillate freely, and will be sharply tuned. This will exclude to a great extent signals on any but the desired wave length. These signals, if In duced into the secondary ct all, will be greatly weakened. The secondary, though broadly tuned, will still have a greater resistance to any signals not on Its own natural wave length. The secondary circuit can be matte still more selective by carrying out the same scheme as in the lirst two figures ; that is. by connecting the crystal across only a part of the circuit. Sharp tuning cannot be obtained with any ordinary outfit between local station even with a vacuum tube detec tor; it is almost Impossible with o crystal, but can be done with a tubt outfit by using very loose coupling to the antenna circuit or by using a loo; to pick up the signals. |