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1 correction by reubot - Show corrections

WIRELESS.

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Tracing Faults.

A METHOD OF PROCEDURE.

(BY N. M. GODDARD, B.E.)

If a set should completely fall or develop i fault It Is desirable that a systematic method if tracing the trouble be adopted

The first thing to do Is to see that the valves ne getting their proper filament, grid and .Mate voltages Any abnormal condition In wy of these will give a certain Indication of vhere the fault Iles

If these voltages are proved to be In order >r the proper facilities for testing them are lot available pass on and try and locate the failure in the r f or a f stages First of all ittack the audio amplifier Give the detector valve a light tap and listen for a response from the loud speaker which will occur unless the valve ts entirely non-mlcrophonic If the ?et is provided with means for connecting a Dlck-up, place a pair of headphones in this -ircuit and tap the diaphragm and listen for he resulting but enlarged tap from the loud »peaker If the speaker gives no sign from 'ither of these tests the a f amplifier, output /alve, speaker, or power supply to these valves 's at fault and each section should be separ \tely investigated and the components and circuits of each tested for continuity insula- tion, or breakdown by means of a pair of headphones and a small dry battery or some equivalent device

Should the a f amplifier prove satisfactory, the next point to be tested is the detector This may be done by disconnecting the aerial from its usual point of attachment and con- necting it directly to the stator plates of the variable condenser tuning the detector grid circuit It should then -e possible, by mani- pulating the tuning control, to pick up signals from a strong local station provided that the detector is performing its correct functions

After this the rf stages of the receiver may be tested the simplest way of doing this being to touch the aerial on the plate ter- minal of the valve immediately preceding the detector valve If signals are obtained here oass on to the plate terminal of the next preceding valve, and so on, until the normal aerial connection is reached

As far as the detailed search for the fault Is concerned, It is desirable to have rules for guidance but the almost infinite variety of receiving circuits makes it virtually impossible to devise any that will hold good in any but a few cases There Is one condition, however, which is universal, viz, a conducting path between the grid and filament (or cathode), and between the plate and filament (or cathode) of every valve The non-existence of such ade path shows a fault although the existence of such a path does not necessarily mean that everything is in order, because a short circuit which will show continuity may of course, put the circuit out of action

This path, however, is by no means straight- forward In the grid circuits there art the various coils transformer secondai les, grid leaks, bias batteries or their equivalents, while In the plate circuit there are either primary windings or resistances, as well as the B bat- tery or plate supply devices The continuity of any of these circuits may very easily be tested by means of a pair of headphones, a small dry battery and a high resistance of about 100,000 ohms, or more, in series with the headphones, in order to limit the current when testing across high tension batteries or plate supply devices This method of testing can only be carried out when the set _ switched on with batteries or power supply devices in their proper position in the circuit, because, if the batteries are disconnected, or the rectifier is not in use, there can be no conducting path between the plate and the filament or cathode If high plate voltages are used in the set, It is not safe to apply this headphone method, but in such an event suf- ficient Information can be obtained by discon- necting the high tension supply and short circuiting the positive and negatlvr leads or terminals temporarily In these circumstances the short-circuiting connection supplies that part of the conducting path which is normally supplied by the battery or rectifier, and enables

the test to be carried out

It must not be forgotten that the de re- sistance of these circuits varies widely from valve to valve The responsive click in the headphones must, therefore, vary greatly, and due allowance has to be made for this if the -esults are to be interpreted intelligently.

SHORT-WAVE SYSTEMS.

In the single side-band method of tele- phony transmission only that part of the wave which is really useful viz a single side- band is transmitted Certainly the missing carrier has to be replaced at the receiver but definite advantages exist Including the fact that a station operating on this system only occupies about half the space in the frequency band In short-wave work the difficulty of providing accurately the missing carrier at high frequencies (piobablv about l8 million cycles) has held up progress In May communication was successfully estab- lished between Paris and Madrid by the Inter- national Telephone and Telegraph Corpora- tion utilising In conlunction with a single side-band a pilot wave, the object of which was to keep the receiver exactly In step with the transmitter The system is commercially practicable and has been put Into operation on several long-distance circuits

WIRELESS AT SYDNEY HOSPITAL.

As a result of the ABO 's community sing- ing in the Town Hall on Fridays the John Travers ward at Sydney Hospital has been equipped The wiring was voluntarily carried out by 60 members of the Postal Electricians' Union last Saturday week and installation will be formally handed over bv the ABO next Saturday Completed at a cost of £250 the Installation comprises 140 headphone seta with 150 points at which they may be plugged In Three-quarters of a mile of wire which has been entirely concealed, was used to connect tile points

THE SHORT-VALVE NATION W8XAL

Transmitting on 49 5 metres (6060 kilo- cycles) with a power of lOkw the short-wave station W8XAL operates approximately from 0 30 pm to 1 30 am 430 am lo 630 am and 9 30 am to4 30pm, Sydney time The station is located at Cinclnnattt Ohio USA, and its owners the Crossley Radio Corpora- tion ara anxious to receive reports from all listeners

FINANCIAL PROBLEMS

To-monow night at 7 40 o'clock from 2BL Piofessor R O Mills Dean of the Faculty of Economics will deliver the Mist of a series of University Extension Board lectures en- titled "ringer Posts in Finance" The de- tailed subiect for to-morrow night will be "Money " a subiect which in our present troubles is of more than pnsslne Interest

SIDELIGHTS ON OLD SYDNEY

Next Sunday's subiect from 2FD will be "Early Issues of the 'Monitor' Newspaper, Part I ' while on the following Wednesday from 2BL the second part of this subiect will be dealt with

ADDITIONAL NEWCASTLE STATION

Although licensed to use an aerial power of 200 watts, the output of 2KO owned by the Newcastle Broadcasting Co, Ltd, will be re- stricted to 25 watts while it is located at New Lambton Transmitting on 212 metres (1415 kilocycles), it will ultimately operate from Kotara, six miles south of Newcastle, on the Sydney railway line. The station was officially opened last Saturday.

WIRELESS INSTITUTE

At the next meeting of the Wireless Insti- tute, to be held on Monday August 17, In the Conference Hall of the State Theatre Build- ing, the theoretical side of the design and construction of radio frcquencj transformers will be dealt with in a paper to be presented by

Rev R B Dransfleld Th L

FEATURES lrOR lau COMING WEEK

SPG - To-morrow A Woman of no Importunée ' friday Orchestra 3LO relay Saturday Moorefield National Broadcasting Orchostia Sundiy (ami St Andrew» (afternoon» Lvceum (p m ) Pro- fessional Musicians Hind Monday Community singing Tuesday Dance band Wednesday No Nanette (from 3LO)

SBL -To-morrow Mllltiry Band Madrigal Society Prldiy Aeollin Hall Saturday Football popular Items Stadium Sunday (a m 1 Christian Science, (p m I Pitt-street Congregational, light classical concert Monday Extension Board lecture The Lord Mayor (comi.dv> Tuesday Philhar- monie Society Retribution orchestra and songs Wednesday Concert programme

3LO-To-morrow Dance hand Friday Vaude- ville military band Saturday V A T c races old time dances Sunday (am) St Paul's (pul Salvation Army Band The Sundowners Monday ' Sweet Lavender (Pinero) Tuesday Choral light and military band muMo Wednesday No No

Nanette

3AR -To-ninrrow Records Friday Community singing Saturday 1 oothall eonccrt orrhestra and variety Sunday (a m I Australian Church (p m ) Wesley Churrh mia/ from »BL Monday Varloty, old-time dances Tuesdnv The Winters Talo ' Wednesday Brass band and ballads

tQCl-lo morrow The Carat-riin Choir PrMay Records Studium Satuiriav OTC racps planta- tion songs Bri-bnne band Sundas la m 1 St John s Anglican (p m ) Clt\ Tabernacle relay tr m 2FD Monday Ro\a! ahow studio Items Tues- day Community singing Wednesday Industrial Psychology (lecture) records

2NG- Sunday (ami St Andrews Presbyterian (p m I C M M

ÎOW -Sunday u p m The Ring' (Wagner)

Monday Lovltskl records Wednesday Schubert ro

oltal

ANSWERS TO CORRESPONDENTS

OB (Waverley) -The uondenser usui In such i case is of relatively small capacity The efrtcllve capacity of the combined aerial system and oon donser will always he less than 'his lh"ro will bn no disturbing Influence on the ganged circuit pro- vided that the aeilal capacity Is always large com- pared to this condenser

M P lltmpc) -Ih Its usually accepted sen'C power output means the amount of audio fro quenoy or signal energy which can be handed on to the loud speaker Plate dissipation anodo wattage or similar terms refer only * i the output of d o enorgv comumed In tho pijte circuit lo

plate amps by volts This Is always greater than power output For example a valve consuming eight watts In the nnodc circuit rrny onl» bo able to trans'or signal onergy to ths extent of 1S watt

without distortion.

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Digitisation generously supported by
Vincent Fairfax Family Foundation
Digitisation generously supported by

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