Wireless Broadcast

Star (Christchurch), Issue 16985, 8 March 1923, Page 9

Wireless Broadcast

Contributed by “Aeri»L”

amplifier, troubles, Dozens of amateur and professional operators alike have experienced a good deal of trouble at certain times with audio frequency amplifying apparatus. Sometimes amplifiers work in the reverse direction, that is. they weaken Signals instead of strengthening their, while at other times, bowls squeals rattles and what not all seem to com ‘ bmo to make all possible bother arm ! noise. Tf the job i* tackled system*f.i- | C.ilJy, and each part of the circuit ex- j a mined in turn, the trouble can usunllv oe quickly located. When strength is decreased bv swi tching* enotber tube. it. is well to check the wirmg or the transformers Sec that ike input- from the last valve is connect' i to the primary of the transformer, and that it’s secondarv terminals are joined to the grid and filament- of the next tube. Most transformer? work host with the outside of the primary winding -connected with the plate and the ?. r *ud to the positive H I. Similarly the outside of the secondary should go to grid and the i\ side to the filament negative 1 This arrangement keeps the grid and plate connection as far as possible from ihc iron core A great deal of distortion, so common in audio amplifiers mav be pot. rid of in this way. Also try testmg the windings for open circuits, an i sw that the primary is not shorting in ii-cif. or on to the -secondarv or core. This test also applies to the second v v wi Tiding. When an open circuit i« discoverer!, tlie break is generally found ni tlu point where the terminal wire is con. necterl to the end of the vrindinj... Breaks seldom occur in the middle of tUf eoil The reason Tor this is often dno to th<> soldered connection, if acid or other corrosive flux is used. This kind of flux i? bound to cause trouble ?ooner or later, and a point well worth remembering is to always use resin for soldering in electrical work It makes a neat job. and any resin left on the covering, thus keeping the .joint alirxvs clean. Soldering pastes, such as fluxite leave a greasy film all over the job. and as this film is nothing more than a high resistance leak, trouble is sure to follow. •Sizzling and frying noises may very often be traced to n dud cell in •II ” battery or to a loose or dirtv joint or connection somewhere. Have a good look at the valve socket, and if you are using the English type of valve, open the pins on the base «,f the tube. Does your set work on longwaves with the feed-back coil shorter! <-ut ? If it does, it- is almost a -sure sign of amplifier trouble. Many amplifiers oscillate in themselves, quite- ur. known to the operator, until he finds signals coming in on about 12,000 metres, using a twenty turn honeycomb for a tickler. Obviously if beats are not being &et up in the tuner and detector circuits, they must be beiiUi produced in the amplifier. When this occurs, it will be noticed that .signals are good on one certain wave-length and not nearly so good on others. A howl will probably l>e noticed at one point. The howl or squeal is caused L>. the detector tube oscillating at one frequency and the amplifier at another the two frequencies forming a beat, resulting in the objectionable noise. Good signals on one wave, and poor ones on another may be put down to the fact that the frequency at which the amplifier is oscillating does not change, much a-s the tuning is altered, and as the one frequency is not suitable over the whole scale of wave* j lengths, weak signals are tile results. I The following is a system used ex- ! tensive!y in America- to prevent nmpli- ! fier trouble. Referring to the diagram

Fig. 1. two or three unusual things wiV he noticed. Cl and C2 are air f-irc choke coiL designed to choke out a” - - radio Irequetitv which mav •..-ni.-i i* i the circui . These ma> be mcd“ ii• • winding about iwonty five *o fort > turn? of guage 32 <1 s.c. wire -i lo«*rr.er halt an iuch in diameter. •' ?m.dl piece «?f < b-mite rod make? i -ni'ibh- hubhi:;. liu- coil? marked X are iron cored, imne'lances. Alaximum

current flow? when the impedance of j the output circuit, exactly equals the J intorqal impedance of the valve. Audio i transformers usually have not enough ! impedance, so these iron-core coils are | placed in series with the transformer -primaries between the plate and the positive high tension Two to three thousand turns of guage 44 enamelled wire. Wound on a bundle of fine iron wires two inches long should be about ! right. The wire core should be anneal- ! od. Put the wire into a fire and lea-.-'" j it there until the fire i? dead out. The j wire is then soft enough for the put- ! pose. A good plan. when winding I such an impedance is to have the wire I core enough to bond back over the cc* ■ when it is won ml. Bend the core wire- j back in such a manner that the whole j thing is enclosed in iron wire. Thi-. I forms a closed core impedance. A j large fixed condenser may be conne d- ] ed from the positive end of the impe l j ances, to the positive of the filament.. The condenser may also be tried be- I tween the positive low tension and th.- j other end of the impedances, in which j case two condenser? will be- necessarv. ; In addition to these precautions, ; each stage, should be shielded, by corn j pletely enclosing both valve and trails j former in a-n iron or still enclosure. | Separate shields for each stage should j be used. Connect all still shields, an-, also the iron cores of both transformers and impedance coils to the positive end of the “B ” battery. Do not place the valves too close together. Three to four inches is the .smallest space ;hat should be left between them. Transformers should be mourned at 'ight angles to each other, and as far apart a? -possible to prevent reaction between them. Wiring should he carried out with guago Hi. bar:: copper wire. This size wire, ensures a good rigid job. Leads through tne shields should, of course. .be taken through insulating bushes. If the. “ B ” battery is too close to the transformers or any part of the wiring, a certain amount of capacity feeds ba nc may get through it. Therefore, ke-jp it well o.xvav from the rest of the gear. Another thing, he sure that the valves are quite suitable for the transform erased. Using one kind of valve. I have seen a certain make of transformer, beat all others, that were tried again?! it, yet. a change of valve, completely upset things, the best transformer for the second tube, being two section? of a Ford eoil mounted side by side on th_ original open core, belonging to the coil. This may seem queer, such a make-shift transformer, proving bet Nr.* than a regular one. but it is neverthe* ! lets? a fact. The experimenter who is responsible for the foregoing idea, says, tha•. * signal strength was increased in i surprising manner, and three stages were used, these working with a lot less noise than two did formerly.” THE IDEAL EXPERIMENTER (Chapter I.) There was once an amateur who owned a beautiful valve receiver. did not use a two-coil circuit, and ho never tapped the aerial terminal when he heard another carrier wave. (Chapter II.) He did NOT live in Christchurch. CLUB NOTES. The Radio Society of Christchurch \ held its usual weekly meeting last Thursday evening. The president Mr E. England, presided over a good attendance. After the usual .routine of business wireless matters were discussj ed, and buzzer practice indulged in. The next meeting will be held on Thursday evening, buzzer practice on Monday evenings, at 7.15 p.m. Any information concerning the club may be -obtained from the secretary. Mr V. Larsen. Tuam Street. Lin wood. Member? of the R.S.C. will be glad to hear of the return of one of our real live wires. Air F. Yixeent, who has. been on a visit to San Francisco. / Tahiti, and R-aratonga. and returned bv the R.M.S. Maunganui last week. Hi? description of wireless conditions j and stations in Frisco will be very in- j NEWS AND NOTES. Up to tlie? present three CliristchuriM j stations have reported hearing signals j from American amateurs. 4wo have j j heard GAY and oZAB'r 1 While the i third (amateur) heard a Ttli distrk d j transmitter. All station? received arc ; CW. There are some spoil sports nroun l ; town. While some serious amateurs are on 200 metres doing their duty listening for American “ Homo.” some others are tapping the aerial terminal signing off with several calls, all pr • tending to be Yanks. You guilty ones, it wont do! You are not worthy to be called radio amateur? if you persist >u tricks like this. Remember that th.vc arc still some loop aerials in Christ church, and if you are caught. v ou stand a good chance of never seeing your “scrap o’ paper ” QUERIES. Questions pertaining to Radio sub jects will be answered in this columr . Inquirers must give lull name and .<0 dress and write questions on one si:!’ j *of paper only and in ink. Sketch-? land diagrams must bo on sopar.i c .'sheets of paper. Not more ihan u-o ■ question? will he answered for any in : qniter in the same edition, i Questions to be must reach ! this office addressed to “ Aerial/ varc ; Editor, “Star,” not later than o p.m. | Saturdays.