Noises and Their Elimination

Radio Record, Volume III, Issue 9, 13 September 1929, Unnumbered Page

Noises and Their Elimination

_ A Useful Summary of Interference Problem from the. point of view of City and Country. Dwellers’

=--, TATIC is often blamed for many 3i rasping roars of which it is not the cause; conversely it is not blamed when it should be-trouble-tracking vans can well testify to this, Diagnosing’ the why and wherefore of inter- , . ference is not the easiest for ‘ the inexperienced listener, but for the old hand who cannot be misled by "Old man Static" this is not a difficult task. Noises may be roughly classified into two groups: noises having their origin -within -the set, and those occurring without it. When the listener is troubled with bad interference there is a definite procedure that he can follow in order to locate its source. , _ Disconnect both aerial and ground, and connect the binding posts with a piece of bare wire. If this eliminates the noise, it can be taken for granted that it is originating from power lines, motors, telephone circuits, or ‘electrical apparatus of any description within or without the home, ° If the noise still persists, look first to the batteries. The "B’s’" and "C’s" develop a crackling noise when the cells are ? -beecoming exhausted, while a badly sul- | phated and old wet battery develops a crackling noise which can be detected by placing a pair of terminals of headphones across them. A normal battery should produce no sound in the. ’phones., "B" batteries that have dropped 20 per cent of their voltage should be discarded. If the "A" battery crackles, and continues to do so after having been charged, consult a battery expert. If all the voltages are correct, see that all terminals are clean and tight. Acid creepage from wet batteries does a great deal of damage, A coating of vaseline will keep corrosion down to a very bare minimum, TEE LEELA ELAURELEAARPLALALLALEBRLELBOELELELILILitrrtri.t.rreetrrTer ee

Corrosion’ is a noise producer in two ways: 1. It makes fer a defective connection; 2. It is quite likely to eat through the insulation of battery cables and eventually: produce a break in the wire. Where "‘B" eliminators are used, the trouble is more difficult to track. If a set of good "B" batteries can be borrowed and substituted, the eliminator can be readily tested. Eliminator outputs cannot be measured by ordinary meters, as the output is so small that most of it is consumed by the meter. A high resistance meter is, however, quite suitable for the job. -Wavering of the needle of the meter would tend to show repairs were needed, If the batteries or eliminators are in © good order, go through their connection with the set, looking for loose terminals, corrosion and broken wires. If all is well so far, look for a de-° fective valve. The easiest method is by substitution. For this test, it is best to reconnect the aerial and the ground, which will indicate the difference in volume level as. well as in noise level. Sockets occasionally cause trouble ‘through corroded, loose, or bent prongs. This may be remedied by sandpaper. UX sockets may be brought back to the normal by the aid of a pair of pliers from underneath but this necessitates its removal and should not be attempted unless definite evidence indicating a defect is present. Examine jack springs, and_ solid connections, touching the parts by a glass rod or piece’ of ebonite. If this results in a grating noise, or a splutter, there is a loose joint. To test variable condensers, and transformers, apply the leads from a 224 "B" battery, one to ‘

one side, the other side to a pair of telephones, and the free terminal of these to the other side of the condenser or transformer. In the case of the condenser there will be one click, and no more, no matter how often the free terminal is replaced. Connect the two terminals of the condenser together.and then repeat the test. Another click should result. With transformers, a click should be heard every time both ends of a continuous winding is touched. Quite often the moving vanes and che fixed vanes of the variable condenser couch at certain portions .of the dial reading. The writer had an example of this in that a set which had just been constructed (the A.C. Browning Drake) gave good results except for the lower frequencies which indicated a great deal of interference by way of spluttering noises. An examination showed that when the upper part of the dial was reached, the moving aand_ fixed vanes. of one of the condensers touched . and caused the set to break out in’ a Succession of noises similar to static.. Another very common trouble is defective grid leak. This manifests itself. by a noisy spluttering. Its cure is obvious, Test speaker cords by shaking , or twisting. If a noise results from this treatment, replace the cords. E now have to take the second part of our supposition, that the set on being isolated from the antenna and the ground functions satisfactorily. |The procedure from now is:-1. Note if any household appliances such as flat-irons, electric fans, violet ray machines, electric vacuum cleaners, or. the lights are turned on. If so, then turn them off and note the result. on" --Continued on page 28.

Noises and Their Elimination

(Continued from Front Page)

[TF this test reveals that any of this apparatus is giving trouble, the supply must be filtered, Wiltering methods should be considered under the following headings: (a) Filtering directly at the device by the use of a 1 mfd. condenser shunted across the terminals can eliminate 90: to 100 per cent. disturbance. (b) By filtering at the meter from 80 to 100 per cent. may be eliminated. (ec) Filtering at both points, A.and B, may be necessary in bad cases. Filtering directly at the receiving set can eliminate 25 to 90 per cent. of the disturbance. Further types of filtering devices for stopping electrical disturbances are as follow: (1) Single condensers. from ¢.mfd. to 1 mfd. or more capacity haying a D.C, test of 400 volts or over. Simply connect a condensér across the spark or are direct or connect a noninductive resistance of 1000 to 30,000 ohms in series with a condenser and place this combination across the are. If this interferes with the functioning of the spark, see method 2. Another cure is tO connect one leg of a small condenser, say, 4+ mfd. capacity, to ground, and the other leg to each side of the power lines in turn. When connected to the positive side, much noise will be prevented. If the plug to the electric device is reversed the noise will again be bad. In this case reverse the plug or connect the device in figure A to the line. (b) Double condenser filters. These ate very effective for small motors such as sewing machines, yacuum cleaners and electric gramophones. Where one condenser bridged across a spark gap in seriés with a resistance interferes with the spark use two condensers in series. with a centre resistor of 1000-2000 ohms bhetween the centre taps of the condensers and the ground connection. Combined Choke and Condenser Filters. 2. Switch.off the light current at the point where it enters the house. If, after the opening of the master switch, the noise continues it: is safe to assume that the trouble jis outside the home, and the authorities, whoever they might be, should be communicated with. 3. Aerial and earth. Note the con-. dition and position of these. Where an electric appliance is grounded through the earth connection of the wireless set it is possible for a highfrequency interference to feed back through this source. Likewise. any other metal in the building may carry disturbances into the set. The remedy is to have a separate and independent eatth contact with the set. Keep the ground lead-in wires as short as possible. If the aerial runs close or parallel to power lines it should be turned round at right angles to it and made as short as possible. A counterpoise earth ‘will quite frequently help to reduce interference. THIS concludes the search for the city dweller who has at his hand power companies and sympathetic post and telegraph officials. In the case of Wellington listeners a very wise plan has heen adopted by the Post and Telegraph Department,

and listeners who have failed to locate interference by the above tests, should communicate with them. . Upon receipt of this communication, the Post and Telegraph Department forward a form, a copy of which appears on this page. They aré asked to make their observa-

‘tions carefully and by doing this they will materially aid the Department’s officials. HERD are a very large number who are outside the cities, and who cannot call upon the assistance of a trouble-seeking van. A large proportion of these have recourse to power boards, and providing the trouble is located in their power lines, the individual listener need worry no furthers.

> The Listener Outback. THERE exist a’ proportion of the listening public who are so remote from city and town that they have to rely on the power derived from motor or water generators for their

power. These are more often than not the cause of interference. Motors in all their special forms cause a great deal of noise to the adjacent receiving set. A motor consists of an armature revolving in a strong magnetic field. The driving current passes first through the armature whence it is used to excite ‘the field magnets. It then passes to the windings of a revolving spool, and this can be done only. by sliding contacts called brushes. ‘The pbrushes press down on a drum of copper, and

ia when both are clean function without sparking or interruption. Dirt and corrosion alter the whole. performance, sparking occurs and the surface becomes pitted with the result that more sparking occurs. The first and obvious treatment of such a case is to clean it, Cleanliness saves the upkeep and service.and is an ecohomical procedure... Even a/ sparking commutator will; not cause 2‘ great deal of trouble if the iron shell of the motor were grounded,. but surveys have shown that old motor in-" Stallations were frequently made on wooden supports insulated from the ground. Grounding must therefore be listed as a-second step. If theginterference still persists connect'a 2 mfd. condenser tested to stand 1000 volts D.C. across the brushes. The design of the filter varies somewhat if the line is less than 600 volts and ungrounded. In that event, two condensers ground at their common lead may be used as shown in upper diagram. When a source has been found and the foregoing treatment is not applic- ° able, the interference is most effectively suppressed by connecting a filter in the line which supplies the current to the electric device which is causing the trouble. If it-is not possible, such a filter may be connected in the line at the main switch, or even between the plug socket and the radio set, though in this case, less noise will eliminated than if the filter is installed at the source. ) Suitable filters are in n general of low east type, i.e, they will readily pass the 60-cycle current and block the. higher audio frequencies which-constitute the noise. In some cases, it is sufficient to connect two fixed condensers between the two sides of the line and the ground. Then simply by-passing the audio frequencies. In more persistent cases, it is necessary to insert choke coils so as to block the high frequencies, and allow them to be by-passed to the ground, as in lewer diagram, This circuit is applicable to four-fifths. of interference trouble. — A suitable filter can be made by anyone who is at all handy with tools. The chokes may consist of 100 turns of wire each banked round on a double wooden bobbin or fibre cylinder. They may even be wound as an‘ordinary ‘radio coil on a fairly large former. No, 14 D.C.C. wire should be used for ? amp. load. No, 12 for 6 amps., andro, 10 for 9 amps. Each layer should be separated by a covering of empire cloth. The condenser should be tested or have a working voltage higher than that to be handled. They should be installed in metal boxes, .their capacities being 1 or 2m.f.d.’s It should be borne in mind when installing any of these: devices that large capacity condensers contain wax, and therefore, should be. placed where heat generated by the motor will not affect them. Bad contacts in power lines as well as in the radio set itself are a very common source of interferehce. ad Loose fuses, loose connections, Kae secure light sockets, cause a great deal of trouble, besides which they constitute a fire risk, Most of these have fallen under the heading of defective household apparatus.

ae ~ @« © i ia ia HERE is another classification. Static brought about by belts and printing: presses, although this interference is highly local and of little or no importance except where sets are operated in industrial areas, and qitite a large number are. Condensers and chokes are of no use in the solution of such problems. Grounding the frames of the machinery is the first step. The second, to provide some forn’ ,of brush: collector that will remove the static charge from the belt or paper without sparking. A lead from the. connector to the gtound is. the only _ requirement, Investigatiors where farm lighting plants are in use shows that a large proportion of interference arises from poor grounds. In sandy districts, such as near: some of our coasts, the problem is Serious, as the soil is a reason-

ably good insulator. Moistening of the earth about the ground connections has little effect. It is recommended for such localities that the ground be attached to a pipe driven down into the soil at the bottom of a well. It might be’ .@tached to an artesian bore, or a windmill, Interference from farm lighting equipment is sometimes difficult to trace because of the multiplicity of spark producers that may be found in such an installation. All of these plants consist of some driving mechan_ism, a dynamo, and a storage. battery bank either active or floating in’ the system. The driving mechanism is usually a gasolene motor with spark plugs and magneto. In other cases, it is a water wheel. The elimination of trouble in such _systems will require a variety of treatment, A small condenser usually about .25 mfds. connected across the sparking contacts, in the ignition system, should end the trouble from that source. It may also be necessary to’ shield the mugneto and ground the shield. Commutator troubles sueh as

have been dealt with are quite common. If there is a make and break contact device, it will have to be shunted. by a condenser similar to Diagram 1. It has been found that .25 mfd. condensers are almost standard for all filtes affecting small motors. For larger motors a condenser of from 2 to 8 mfds. may be necessary. but these are not common, and usually fall under the supervision of the power boards concerned. Recapitulation. NOISES: in'a radio receiver may be classified into two broad groups. Noise within the set, and noise eXtraneous to it. 2. Noise within a set may be tracked down by systematic search. Noise outside the set, either by a complaint to a power board or other influential body, or by an examination of electric motors and other likely sources. 8.. Most interference of this type is the result of defective contacts which result in high-frequency current. 4. This interference may usually be efded by the. application of a filter. 5. A filter may consist of a simple fixed condenser,, two condensers in series, with a ground lead from the point of their connection, or two con- densers so connected and used in conjunction with a radio frequency choke. e