ANNOYING NOISES

Sun (Auckland), Volume II, Issue 529, 5 December 1928, Page 16

ANNOYING NOISES

HOW TO STOP THEM Of all the faults which occur in wireless receivers the most annoying and often the most difficult to trace are those which present themselves as noises of various kinds in the loud speaker or headphones. Faults of tins kind a.re confined entirely to valve sets, unless it so happens that some outside source of electric current, which might possibly occur from the leaking of an electric bell or lighting circuit, becomes applied by accident to a crystal receiver. Almost without exception noises in valve sets are caused by imperfect contacts, but as such faults on any one of the many conductors or contact points in the receiver can give rise to interfering noises the discovery of the fault is very difficult, unless a systematic search is made. The main cause of noise not directly attributable to a contact fault is the presence of a fault in the high-tension battery. A faulty battery section can give rise to constant frying noises, scratching, and sometimes marked variations of signal intensity which resemble the effects of fading. A voltmeter test on the battery while it is in use will generally confirm the presence of such a fault if it is suspected. CONTACT TROUBLES Among contact troubles the most common are those which occur in the sockets of valves, between the socket contacts and the pins on the base of the valves. The wide use of the new UX type of valve sockets has reduced, but not entirely eliminated faulty socket contacts. If the fault lies on one of the two contacts supplying the valve filament it can usually be detected by turning on the valves in a dark room. Constant changes in the intensity of the glow from the valve affected will reveal the source of the trouble. Faults either to the grid contact or the plate contact, however, occur more often than those on the filament contacts. They cause more serious trouble and are more difficult to detect than those on the filament circuit. They can usually be traced, however, by gently tapping each valve in the set with the finger. When the defective valve is touched the noise in the loud speaker will occur with unusual intensity. The trouble, when found, can be corrected by bending the socket contacts either upwards or together, according to whether they press on to the bottom of the valve pins or grip them by the sides. The implement by which the contacts are bent should be a nonconductor of electricity, as it is easy to short-circuit the positive terminal of the high-tension battery, which is led into to the socket, to the filament battery by probing in the socket with a metallic tool. Next to socket defects the most common cause of receiver noise is that due to faulty rheostats. After constant use the sliding-arm contact, which presses on the resistance wire in a rheostat, loses some of its springiness, and the pressure it exerts on the winding is reduced. Under the influence of the heat which is generated in the rheostat windings when the valves are lighted this contact becomes variable, giving rise to fluctuations in the amount of current supplied to the filaments, and consequently to noises in the loud speaker. The remedy is to remove the contact arm from the rheostat and bend it to increase the pressure on the winding. Although it is difficult to understand how such a thing can happen, screw terminals frequently work loose, releasing the pressure on conductors and giving rise to faulty contact. Due to the natural spring of the wire such faults as this are particularly susceptible to vibration, and the .resulting noises frequently occur if the table on which the receiver is resting receives a jar or even if somebody walks on the the floor near it. The loose terminal can generally be found either by inspection or by gently tapping each conductor in the receiver with a light wooden rod or a lead pencil. To prevent the occurrence of such faults contacts should be soldered wherever possible, or spring washers, which will resist the tendency for a terminal to loosen, should be inserted under each terminal nut. TERMINAL TROUBLES A common source of terminal trouble is the effect of the sulphuric acid electrolyte from the accumulator bat-

tery on the terminals of the battery or the conductors attached to them. Many modern batteries are fitted with acid.resisting terminals, but these terminals will not protect the wires attached to them from the action of any acid solution which may creep from the battery cells, and the formation of a thin coating of copper sulphate between the terminals and the wire attached to them will interrupt the flow of current. Conductors from the low-tension accumulator to the receiver should be examined periodically, and replaced when signs of corrosion occur. The need for replacing a whole conductor can often be avoided by cutting off the damaged ends. If the connection of the negative terminal of the high-tension battery to the receiver is made on to the low-tension battery this lead should not be overlooked, as it is generally more likely than any other to cause receiver noises if it becomes corroded. A fault which often escapes notice is a breakdown in the grid leak. The newer metallised leaks are less liable to failure than the older carbon leaks, but faults, which are specially noticeable when receiving fairly weak signals or distant stations, still occur occasionlly. Once a grid leak shows signs of these defects it is impossible to repair it, and it must be replaced. Blocking condensers, especially the cheaper kinds, with waxed paper as the dielectrio often break down and cause receiver noises. Although mica blocking condensers are slightly more expensive than the paper ones, their use is justified, because the breakdown of a blocking condenser can often result in the destruction of a high-tension battery or even the burning-out of the valves in a receiver. In windy weather faulty joins in the wires in the aerial sometimes give rise to receiver noises. If the aerial wires are soldered this trouble is avoided, but twisted joins become corroded, and when the wires are subject to movement they cause annoying variable contacts. Swinging of the lead-in wire which brings it into contact with spouts or other conductors or semi-conductors in electrical connection with the earth will also set up noises in the loud speaker.