Locating Noise in Receivers

Radio Record, Volume IV, Issue 41, 24 April 1931, Page 16

Locating Noise in Receivers

Probably one of the most frequently recurring questions asked by correspondents to the "Questions and Answers". page deals with noise in receiving sets, Below is published an article which deals comprehensively with the causes of many. phases of noise and outlines a cure for them. Read through the article carefully, then cut it out and keep tt somewhere handy to consult when the set . becomes noisy.-Tec. Ed.

= NTDRNAL noise in a. receiving set is a ‘difficult problem * to the average ‘listener! Usually he can locate ordinary defects’ in' a recéiver such as grounded: cireuits, burnt out ,) by-pass condensers and ‘detfective’: » resistances. But it takes an expert.to locate and overcome the elusive internal" noises that are. often encountered, The purpose of this ‘article is to help the listener locate and overcome these difficult problems. Radio receivers are very sensitive devices and once noise gets into the receiver it is amplified and passed on to the speaker in the same way speech and music are passed on. Some of these noises originate outside the receiver and: some originate inside, -All noises cannot be eliminated or reduced, but a great many can. The causes of noises arising in the receiver are so numerous that it would be futile to attempt to describe each one separately. These noises may be due to poor design, poor construction, or defective parts. Fortunately, there are certain noises which ean be classified into a few major groups. Mechanical and Electrical. Noises. NDER these two broad general classifications of noise in a receiver we have first a noise which reaches the ear by way of the loudspeaker, which is the amplified impulse through the regular amplifying system. Second, there are noises which reach the ear directly without being transmitted electrically through the receiver. These latter noises are frequently as troublesome a source of disturbance as the former, As typical examples of the first class we have intermittent opening and closing of contacts in a circuit. This may bein

the form. of a defective connection or a ilefective condition of one or more of © the parts used in the receiver. As examples of the mechanical noises, — we have noise due to vibration of the | laminations in a power transformer or rattles in a cabinet door caused by — ‘vibration, Some mechanical noises are — heard even though the speaker is disconnected or a resistance network substituted for the speaker. A simple check can be made to determine whether the disturbing noise comes from the speaker or not, Substitute a 4000 to 8000-ohm resistance for the speaker. While the resistance may not match the speaker output exactly it will -place enough load on the apparatus under test to show whether or not the noises are of a mechanical nature. If the noise is still present it does not come from the speaker and must be traced in the set. This test, of course, is not applicable where the noise, although not coming through the speaker, is caused by speaker vibration affecting some part of the receiver, for example, — rattling cabinet doors due to excessive output from the speaker. Here the amateur must use his judgment in applying’ the tests. Noises may further be classified according to the nature of the noise as follows :-

Scratching noises, clicks, crashes, rattles, and grating noises, generally due. to poor construction or defective parts. Whistling, generally due to regenera: tion, poor valves, and oscillation, Ringing. noises, gradually increasing in inténsity. These are called microphonic noises-sometimes due to poor

design — defective parts — defective valves, or..too much volume, Hum-generally due to poor con-struction-defective parts, faulty design, or poor valves, Determine Origin. TN investigating a noisy receiver, the first thing to do is to determine whether the noise comes from. outside or inside. the receiver. In. the case of a battery set this can generally be definitely determined by disconnecting the aerial from the set. If the noise disappears it shows that’ it comes from the outside and that the set is O.K. If the noise still persists, however, the indications are that it originates in the set and It becomes: a matter of diagnosing the trouble in the manner outlined in this article. «./. In the case of electric sets this test is not conclusive.. If disconnecting the aerial causes’ the noisé to stop, its source is: generally external to the receiver.,. If noise is still present, it may still come:in from the outside through the power line and get into the receiver by way of the power pack. ’ A line filter of simple design van" be quickly. and = easily connected . to the line of the ordinary a.c, set to filter out line noises. Figure 1 shows an arrangement which has been used successfully for this purpose Tf the noise: is appreciably reduced after connecting the filter between line and receiver, it indicates that the noise originates outside the set and comes in on the line, If the noise is still present it will be reasonable to assume that its source is in the reeciver and it can be eliminated,

a5 Irregular Noises. Bh BH f "si Ds Poe ee ee " U ¢ all sorts of Ing fry! Scratchings, gratings, hiss- : rying noises, The first three" ypes are most frequ . ‘contacts.. These et ane to poor causes, The most important are "poor soldering and poor ‘ oor valve prongs and‘ socket spel oe tacts. con Poor soldering may result in a- a, and therefore « :intermittent which is readily detected and observed by moving the lead to see if it is loose. # The presence ofa -largé amount of resin around & soldered joint is always a sure indication of a poor joint. The continuity tester to be’ described later will help greatly in locating high resistance contacts. The presence of a green substance at a joint is a definite sign of corrosion and indicates a likely source: of noise. The only thing to do in a case of this sort is to re-solder the rly. A poorly soldered joint is a source of noise in ‘any circuit, pow--er pack, audio or radio, since it causes irregular variations of current. >... etn’ TN:AIAG

In battery sets SUCH tees eo hottary arise due to poor: contact at the battery terminals.. To avoid corrosion in stornge battery terminals, it is best to use vaseline on the terminals. Weak batteries are also frequent sources of sputtering and crackling noises. The storage battery should be kept in condition by / keeping it filled with distilled water and not allowing the voltage to fail below 1.8 volts. Frequenty, it is-a single cell in a battery that is the source. of trouble, shown by @ drop in voltage of the battery when it is operating under load. This is especially true -of the storage pattery. © A -hydrometer test may show eorrect gravity but a yolt‘age test will show up a defective cell even ‘if it has correct gravity if the voltage test is made while the battery is under load. Valve and Valve Contacts. GPHECIAL attention should be paid to 7 -these: because. they are’ often @ source of noise. A gassy valve in-

dieaved by a blue. haze in. the valve when working, causes ‘frr ‘egular changes in plate .current, thereby producing noise. The best remedy for this is to use'a new valve even though the old valve may show correct operating / characteristics. However, the aby normal condition causing the blue haze in the old one has more than likely ruined it, Loose elements in valves will cause noise, Tapping the valve with your

j "i ‘(finger will sometimes show wu p a loose ee A slight ringing noise may pe Deodusat on tapping the valves, but is is normal, If tapping a particular yalye ete in ‘crackling or sputterg, examine the socket. contacts examine the prongs. ant : _ Mnior Defects. DP =2FECTIVE socket springs are: another. source of noise. Socket spring contacts are made-.of phosphor, bronze or nickel silver. Such contacts are designed so that they exert pressure against. the valve prongs. Thése springs -Imay make good contact at the start and then later on develop poor contact for various seasons, "Heat from’ a: soldering iron may cause the spring to lose its "tentper"’ and ‘continual pushing and pulling the valve out of its socket may { result in pushing ‘the socket spring so \far away that. it does not make contact with valve prongs, Or the contact may be intermittent and we have another source of noise. Socket con‘tacts and valve prongs should be cleaned if corrosion has formed or if ‘they are dirty. Sand paper prohgs and serape socket prongs with a small penknife, Bend the socket. prong with a pair of small nose pliers so that it will make good contact with the valve prong. Volume Control, [ a scratching ‘noise is heard on any station as the volume control is rotated throughout its range, the noise is due to the volume control and may be in the form of a defective resistance winding or poor contact between the ‘rotating arm and the resistance wind- ‘ ing. There may be an oxidised. contact. Poor mechanical contact may be made by the contact arm being bent away from the winding. It can be bent with a small pair of pliers so that it makes firm contact. A dirty winding or an oxidised winding should be cleaned. First use sandpaper lightly and then apply cigarette-lighter fiuid to the winding by means of a small toothbrush. Defective Insulation. GPUTTERING, hissing and frying ~ noises are frequently caused by poor insulation. If a breakdown oc‘curs between two points;. noise in the receiver results, Poor insulation in a@ power transformer may cause flashovers .and sparks. A frequent offender ys the bakelite terminal strip which carries all. the metal terininals. ' The presence of soldering flux and acid results in leakage between terminals and consequent noise. The *2rminal strip should be thoroughly cleaned and dried. If there is leakage due to the breakdown of the insulating material, parts of the strip have most likely been carbonised or charred. This may be scraped. away. with a knife. Dirt between posts at high potentials eauses the same trouble and should be removed in a similar manner. Power transformers, choke coils, and ‘filter condensers are the -worst offenders in this respect since they are in the high voltage circuits. The rectifier valve socket should also be earefully exam- \ ined for signs of leakage. \ Failure of wire insulation may be caused mechanically. For example, if a high voltage wire is caught under a metal clamp or another wire which is tightened by a screw, the insulation may be broken though to such an ex-

tent that there will be mechanical strain and the wire may ground, intermittently. Hxamination of the wiring will show up these causes of noise trouble. Variable condensers are often a source of noise. The plates may touch as the rotor is rotated and produce a clicking noise.. Fine particles of dust between condenser plates will also cause noise and it is often very hard definitely to trace noises to the variable condensers. One should always clean the condenser plates of noisy receivers carefully.. This is accomplished by drawing ordinary smoking pipe cleaners between separate plates of both rotor and stator plates. Noise Due to Feed-Back. Much has been written in regard to feed-back oscillations. We do not intend to go into this subject too deeply here as most constructors are familiar with the general procedure to be followed in correcting for undesirable regeneration, When a set oscillates at radio. frequencies, the characteristic squeal or whistle is heard. The pitch of this whistle, going up and down in scale as the variable condenser is rotated from one end of the dial to the other. In general, if we are investigating a neutrodyne set this means that the receiver must be reneutralised.. This same condition may also be caused by an unbalanced condition of the main tuning condensers in tuned t.r.f. receivers,

Here is an easy method of neutralising. Obtain some drinking. straws, which you can obtain at most sweet shops or chemists, Place a portion of one of the straws on one.of the. valve filament prongs then insert it in the

socket ‘in which it operates, It.is the usual practice to begin at the valve preceding the detector and work’ back toward the aerial stage.’ A broade cast or modulated oscillator ‘signal at (Continued on page 29.)

Locating Receiver’ ~ Noises , (Continued: from page 17.) ibe

about 15UU Kllocycies ls tuned in on the receiver. Due to the valve’s filament being unlighted, it will not amplify and you can adjust. the neutralising capacity for that stage. A correct ad-. justment will be made when minimum signal js heard in the loudspeaker or when a minimum deflection is obtained on an output meter. ’ The balancing condensers.are adjusted in the same manner. An insulated screwdriver or hexagon: wrench is used in balancing these condensers depending on the type of receiver’ in question. If the oscillation is in the radio frequency stages, the following may be the cause: I.. Absence of a good earth on’ a receiver or. imperfect earthing which in most cases increases the tendency to regeneration and oscillation, This applies to the main ground connection and other various parts of the circuit which are supposed to be earthed. II, Too much gain or amplification will also cause excessive regeneration or oscillation, even though the set is neutralised. The better the shielding in a set, the more amplification. we can get out of it and still have the set stable. But there.is a definite limit to thig amplification. Now if there are any conditions which tend to increase this amplification, feed back and oscillations will occur with resultant noise. One of the causes of this is excessive plate voltage, and in case of screen grid valves-excessiv.e screen grid voltage. III. One important object of shielding is to prevent coupling from one coil fo another. ‘Manufactured receivers are supposed to have = satisfactory shielding. Trouble frequently arises

when the shields are not securely in place, Shields may not be properly earthed. 'The shielding should, there-. fore; be- examined to make’ sure it. makes perfect contact. © IV. Open by-pass condensers. Plate by-pass condensers are employed: for ‘the purpose of eliminating the coupling which is always present in the plate supply, ‘If by-pass condensers of any sort become defective or short circuited, they are likely to cause noise. By-pass condensers are . connected across grid bias resistances, If these by-pass condensers are open, oscillation’ will result. .Condensers suspected. of producing noise should be’ thoroughly checked with a d.c. potential of from 200'to 500 volts. Connect the condenser across any d.c.- potential lying between ‘the above values and then short cireuit its terminals. If not defective, a spark will be produced. This applies only to by-pass condensers having: a capacity sources of noise: — A.F. Regéneration Noises. UNDER this we will consider motor- ; boating ag well as other.. possible sources of noise. : _ The following noises. are frequently encountered in audio.frequency systems :- eae _- 1, Coupling between speaker leads and detector grid circuits..- If. the Speaker, a.c. line or. plate leads are _hear any, part of the detector gri¢ vircuit, they may cause squealing. The remedy is obyious. Keep all leads as far away from the detector grid circuit as possible. : ' 2, An open plate circuit in the de ‘tector will. cause a howling. noise, Tracing back over the plate circuit will ‘of course establish the fault. 3. Open grid. circuits will cause squealing noises. Open grid circuits may be caused by open secondaries of , audio transformers,.open grid bias bate

ant teries, open grid bias or. open grid return: connections. condition is generally indicated by an absence of grid bias on the radio tte quency valves, 4, Ifthe trouble-is. due to motor-boat: ing, it manifests itself by a steady "put put’ gound.resembling:a motor-boat in action. This is due to a low oscillatory feed-back in the eliminator: or ‘power pack cireuit. To eliminate this, connect large by-pass condensers across each of the voltage dividers to ground. By-pass. condensers.of about two microfarad capacity can be connected between the plate circuit and filament: eircult to reduce motor-boating. Noises Due to‘ Valve Howling. MiICROPHONIC valves often cause a ringing sound in the loudspeaker gradually increasing in intensity, which is generally caused by the vibration of the valve.elements inthe. detector or audio frequency stages. This vibration causes the plate current. to change very rapidly, and the. change in plate eurrent is transmitted through the set resulting in the ringing sound. It is usually sound waves coming from the speaker that Set the valve into vibra-. tion. Special heary weights can sometimes be placed on microphonic valves to prevent them from vibrating. An applica-. tion of several turns of ‘friction tape around the top of a valve will sometimes reduce a microphonice condition. In. most cases it is best to rearrange the valves until you find one that is ‘not microphonic. In the modern receiver, microphonics are rarely encountered, and are nearly always due to a defective valve. ; Further Notes on Mechanical Noises. 1. Spluttering, snapping noisesthese noises are caused by electric . sparks due to the breakdown of insulation in some: part of the circuit. They can be heard even with the loudspeaker disconnected and: are invariably due to defective insulation either on wires or the terminal strip. Where this occurs, separating the wires or: cleaning terminal strips is the only remedy. A visual inspection while the set is operating will reveal where the breakdown occurs. 2. If there are any loose parts in the receiver such as loose screws and nuts, these parts .may be set into vibration when. the loudspeaker is in operation. Noises thus developed are clicking, rattling, metallic noises. The remedy,

=- of course,’ "ig to . tighten. up. these paris. 8. If the power transformer vibrates: or hums . excessively, tighten. the clausps which hold ° the transformer core, If-the: windings are loose, wedge them ‘by "means of bakelite or: dry wood. spacers." ' . Speaker’ Noises, "ARSE: "noises are .mechanical ‘in origin. and consist:of rattles, clicks and scratches. .In most cases the defect ean-be established by visual inspection. The two main’ types: of:speakers.in use are the magnetic: and dynamic, Most troubles will be. found to be in dynamics, as more dynamics: than magnetics are in-use. In the case of .a magnetic. speaker, we generally have more moving paris, such as springs, driving pins, and more soldered ‘connections. There are always two or three soldered connections on a magnetic speaker and they are likely to. develop noise. ~ Corinections should’ be. examined. carefully and. the, scone

and driving pin moved back and forth to see that the driving pin does not slip through the solder. _. The most common trouble in dynamic Speakers is caused by the voice coil dragging against pole piece. This produces scratching and distorted sounds, and is easily recognised. Some dynamic speakers have an adjusting screw attached to the centre of the electromagnet, -which in.turn is mechanically connected to the voice coil of the speaker by means of .a bakelite strip which is called the spider. By adjusting the screw that is attached to the pole piece, the position of’ the voice coil can be varied in relation to the pole piecé. This screw: should be adjusted until the voice coil does not touch the pole piece at any point. A thin piece of paper inserted between pole .piece .and voice coil can be used to gauge the distance between the two. Sometimes dust and foreign particles get between the pole piece aud the voice coil, and these in turn cause certain noises. One effective way of removing these is to use a blower from an ordinary vacuum cleaner. Before using the blower, however, turn it on

--_ to be sure that no dirt: is in the pipe. Then allow the full force of air to blow between the. voice coil and pole piece, moving the diaphragm up and down at the same time with your hand. This generally : Temoves : all. -dust ‘from this. point; . Examine the-cone both at the voice coil and- outer. edge. If it is torn: or uncemented, use’, ambroid : cement, which ‘can be obtained from several radio dealers.. If. the leather strip or chamois skin is hard in spots, rub it with your fingers'on each side until it becomes soft and pliable. Traeing Noise. | HE best instrument’ which can be employed ‘as an indicator of noise is a good speaker or pair of headphones, in the last anaylsis, what you hear is what counts.. It is of the utmost importance, however, that the speaker or ‘phones be in good mechanical. condition. The most frequent sources of noise are varying resistors, intermittent contacts or shorts. An ohmmeter and continuity tester are frequently of great assistance in detecting these. Intermittent contacts can be quickly established by using the continuity tester shown schematically in Fig 2. This consists of a voltmeter connected in series with a 44-volt C battery. When the two test. leads are connected across the terminals of the resistor or the. apparatus under test it will read less than 43 volts. If the meter needle flickers between values it indicates a varying resistance and that part of the circuit should be most carefully checked. In checking the condition of resistors do not bear down on the resistance, but pull up with your test leads. This will serve. to strain the connection, and if dn intermittent contact is being made it will show up when making this test. Stage by Stage Elimination. [F it is determined that the noise is internal it is necessary to locate the particular circuit which is causing the trouble, namely, where it is in the audio or radio frequency system, the detector or power pack, and in which particular stage the trouble arises. The best method to follow. here is the stage by stage elimination process. . In this method one stage is added .at a time and observation made when the noise comes in. All valves should be kept in operation. This method of test assumes that the aerial and earth system is in perfect mechanical condition and that it is not itself a source of noise. Assume now that the noise is internal. Therefore, -disconnect the aerial: and earth. The noise should still be. present.. Then, short circuit the grid or grids of the output valves. This immediately disconnects all valves ahead of the output valve or: valves. If noise is still present, it means that the source of trouble is in the output stage or in the voltage supply or the speaker, .and these should be examined carefully. If noise is not present, the output stage and voltage supply for these valves is O.K. Now add the first audio frequency stage by disconnecting the short on the grid of the output valves. Short circuit the grid of the first r.f. valve. If noise comes in now it is in the first audio frequency stage or voltage supply to this stage. These circuits should be critically ex-' amined. If noise is still. absent, both these stages and their associated voltage supplies are O.K: Next remove. A

the short-circuit from. the first audio,’ frequency stage and short: circuit detector grid. Proceed this-way unti the source of trouble is definitely locat-ed-in a particular circuit. Knowing the circuit in. which the trouble ' is present, it is a simple matter to: make corrections. When each radio frequency stage is checked and- the grid of each radio frequency: stage is shorted, rotate the variable condenser ‘to see whether this introduces noise. Frequently the ‘variable’ condenser plates may short at some point’ and ‘cause noise. This test should-be made with each ‘radio frequency stage shorted so as to determine. which particular variable condenser is causing the‘trouble, if ‘any. : ; me . Visual Inspéction. HEN visual inspection is one-of the simplest and most effective means available for use ‘in locating ordinar sources of noise. In this method i instruments other’ than the listener’&’ eyes are needed, but he must. exercise his powers of observation. A visual examination of the wiring of the set will show connections which are loose or unsoldered, soldered joints which are corroded or resinous, insulation which is punctured or mechanically injured, valve ‘socket, prongs ‘which. are bent. and make imperfect contact: and soon. The possible sources of trouble causing noise which can be detected by the observant eye are many. One of the first steps. to. be taken therefore is: to examine critically the wiring and construction to see what defects can be found. =