A Corner for Beginners

Radio Record, Volume II, Issue 33, 1 March 1929, Page 11

A Corner for Beginners

A Unique Exponential Horn. I INTENDED to try my hand at making the exponential horn, but was unable to get any wall-board in Te Kuiti, so rather than be without one, I decided to make one out of paper. I pasted 12 thicknesses of newspaper together and this made a fairly stiff board of the right size. This can be cut with a sharp knife. I found that this makes a fairly good speaker, although it needs a strip of thin board about jin. wide run round the mouth of it. to stiffen it. Hach board must be dry before it is ent, and even then the speaker is a little bit limp until it is dried. on account of having the paste strips of paper inside and outside the joins. It has one advantage over others, that it is very light; also it can either be painted or cloth can be pasted over it. I find an annoying rattling develops in the speaker unit. This does not appear in the small horn, but spoils reception in the big horn. I have altered the screw at the bottom, but this does not stop the rattle until the music vanishes to almost nothing. yet when I return the unit to the little horn the rattle is not noticeable. Can you snggest a cure? I find that the set does not deliver enough power to drive the large horn, although the small one has about three times the volume.-F.M. (ing Country). The innovation is a novel one, but it apnears that the paper is absorbing

much of the volume. It should have been shellaced thoroughly and left to dry. Several coats would have made it more impermiable to soundwaves. The rattle is probably due to something being loose. Wither some of the sheets or ends are not properly glued together, Bad Connections. A GOOD cirenit and a good receiver may be utterly spoiled by bad connections. These may appear neat enough, but may be so arranged as to set up hand-eapacity and occasion other evils. Lead should not run in absolute parallel, but should cross and travel at an angle. In most circuits the components are arranged so that this system will be easy to adopt. It is unwise to use very thin wire possessing a naturally high resistance. The other extreme should also be avoided-the use of very thick wire, difficult to manipulate, and impossible to accommodate on a terminal shank when two connections have to join there. A safe medium is No. 20, a gauge frequently advised when cireuits are detailed. This is sufficiently pliable to permit easily wiring, but stout enough to retain its position without sagging and making contact with a neighbouring wire. If this appears possible, however, take the precaution to insulate one of the adjacent connections with "sleeving." Too much of this insulating material causes internal capacity, which will probably upset tuning, especially that of the weaker signals. These may disappear when the hand is removed from the condenser knob. Set Refuses to Oscillate. T sometimes happens that the set works at remarkable volume on a near station, and yet will not bring in distant transmissions. A set which functions thus, is, to all intents and purposes, a "local station only" receiver. Its range is about 30 miles. It will not receive over longer distances because it refuses to oscillate and is therefore lacking in sensitivity. ~ Tack of oscillation is a trouble which arises from any of several possible

-_-- awd eauses. A high resistance rheostat in series with the filament of the detector valve is one of them, and the most likely. Another is a falling-off in the voltage of the accumulator due to deterioration or to a "short" between the plates. A reaction coil connected "the wrong way round will absolutely prevent oscillation, and if the reversal does not correct matters, a faulty erid Jeak may be looked for. When the Set Howls. WHEN a set is worked in a state. of oscillation and transmits intei+ ference a wave ig radiated . which varies in length as and when the. funing of the receiver is varied. . This explains the variation in téné of an interfering note. The transmitted wave is nearly the length’ of a wave the careless operator is. trying to receive, and therefore approximate to the wave his victims are actually receiving with distressing interference, It is impossible for the victims to’ tune out interference, for the smple reason that the culprit, in varying his tuning’ crosses and recrosses the wave- -length upon which or to which sets in: the vicinity are tuned. When Volume Decreases. | Gi VE the A battery a good charge when a sudden decrease in volume is experienced. Test the B batteries with a voltmeter. Operate the set for about. fifteen minutes, and while the set is turned on apply the voltmeter to the B batteries. If they are rested after: the set is idle for a period, the batteries will recuperate and show an increased reading. When the batteries are ail in good working order, look for a defective valve. If the valves have been in operation for more than a year, it is possible they need replacing. Position of the Speaker. XO get the best results from a loudspeaker, it should be shifted about the room and even the entire home. Furthermore, the volume should be regulated according to the size and the nature of the room. A large room can take more volume than a small room, while a sparsely furnished room will take less than a well-furnished and heavily draped room. It is well to remember that the loudspeaker is not necessarily chained alongside the receiver. Move it about with an extension cord or wiring, so as to be able to get the best results