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May 1930 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
Manmade electrical noise (QRM) and natural electrical noise (QRN) has been the nemesis of communications - both wired and wireless - since the first signals were sent. While it is true that over the last century the amount of 'background' noise has increased significantly, the ability of modern circuits to deal with (reject) it and/or accommodate (error correction) it has pretty much kept up with the advancement. You might be tempted to think that 'back in the good old days' such problems did not exist, but operators were plagued by poorly designed and inadequately filtered transmitters as well as really deficient electrical service installation that spewed noise from transformers, inadequately grounded transmission lines, lousy connections, and arcing motor brushes. This 1930 article from Radio-Craft was written by a serviceman who troubleshot and solved many of the issues prevalent in his day - a very interesting read.
A Trouble-Shooter tells of some sources of interference which he has found in his regular work of checking radio noises
By J. E. Deines, W9CU
Much has been said about electrical apparatus that interferes with radio reception, about methods of location, the kind of set to use in this work, and all that - but still there is that puzzling case that makes you scratch your head and wonder what it is all about. Perhaps some of these ideas will help you.
We all agree that the noise travels back on the electric line, much in the manner of "wired radio"; and that the way to look for it is to keep the loop parallel and directly under the line, then reduce the volume and try - first in one direction and then the other - until the loudest spot is found. This is usually a pole. If it is secondary distribution that you are working on, the trouble is probably in some one's home. There are two things that you can do in this case. Either walk under the "services" (lighting lead-ins to houses) one at a time, and pick the loudest; or ask everyone connected to that pole what they are using and, if it sounds suspicious, have them shut it off to prove your case.
In this day and age of powerful and sensitive receivers, interference seems to be on the increase. A check of the electric light companies' records shows that about 65% of the trouble located is in consumer's appliances; while the owners of these do not seem to realize the importance of applying filters, and usually make the statement that they use the appliances only for a few minutes.
But consider a number of these appliances used at alternate intervals, and we have a chain of interference that will last for hours.
Radio has a peculiar place in the electrical industry, due to its rapid growth in the last ten years. It has grown to a giant ranking next to the automobile, and we have only. "scratched the surface." Too much time has been spent selling radio and not enough spent in making a place in which to use it.
We are now facing the problem of working over and filtering all our old equipment (which is, otherwise, operating normally) to make the world, speaking from a radio standpoint, a better place in which to live.
Some noises which can be heard on the electric sets can not be heard on the trouble shooter's set, even under the house "service"; - that is, interference which is not a major noise, that would spoil reception entirely, can be heard on a sensitive A.C.-operated set when a station is tuned in. It creates a background roar and spoils the tone quality. The reason for this is that the electric set is more closely coupled to the line than the portable; and a careful inspection of the electric lines in the vicinity will soon get you on the right track. It can readily be seen from Fig. 1 that interference set up in one secondary line will in turn set up an interference in a parallel line; the intensity of the transfer depending on the length of exposure. The noise will be weaker, to be sure; but nevertheless it is there and can be found if looked for in the proper manner.
Troubles in House Wiring
We all know that any arc or spark causes radio interference, and we can no longer tolerate loose connections. An easy way to find troubles from this source in house wiring is to turn on the radio set at full volume, shake all fixtures and pound all the wall switches, listening for cracks and pops that you will no doubt hear. Many of the older houses throughout the country were once piped for gas lights and, in some cases, combination gas and electric fixtures are still in use. Others have the pipes capped off under the new light and fixtures. Here is a place for a lot of trouble. In an installation of this kind it is very seldom that the fixture is free from grounds. (See Fig. 2.)
When lightning strikes in the vicinity of the electric line, the induced current usually runs into the house and jumps off at the most likely spot - the gas-pipe ground - and the result is damaged insulation. If it is on the live side a fuse goes out; but, if it is on the ground side of the line, nothing happens until the fuse (X) goes out. Then the fun begins. The current flow is now from X to the transformer ground in the alley and, because contact is poor in the fixture, an arc is the result. Several cases of this kind were found where a loud buzz was set up with the set turned on only about sixty watts. The greater the load, the louder the buzz.
It seems to be a habit with the electricians, when they cannot find a ground in the wiring, to reverse the circuit; thus putting the grounded wire on the neutral or ground side of the electric line. This is all right where there are no neutral fuses but, if there happens to be one and it blows, then the noise starts. Therefore, if in doubt as to the origin of the noise look at the neutral fuse.
In a fixture of the type shown in Fig. 2, where the wire is woven through the chain, a static charge is set up in this chain and, as long as everything is quiet, there is no trouble. But walk across the floor, or otherwise move or jar the chain, and a crackling or popping noise will be set up. The cure here is to tape the eyelet (Z) and thus insulate the chain from the canopy.
Another spot in house wiring that will bear watching is the entrance switch at the meter; here is a likely place for loose connections. (Fig. 3). All places marked X are likely places and, if the meter switch and fuse box happen to be located near a door, the vibration due to constant opening and closing of the door will loosen all screws and fuses. These loose connections can be found by the method used above.
Even the lowly electric lamp comes in for its share of the blame. Investigation of one complaint showed that the noise was coming from a neighbor's home; but only a 100-watt lamp was turned on that time. Turning it off stopped the noise, and it was found that the lamp filament had parted and was holding an arc that did not go out until the lamp was turned off.
Troubles in Receivers
One day we received a complaint of a humming noise which came in at one spot on the dial. During the course of the evening it would move from place to place. Upon investigation it was found that a neighbor was using a superheterodyne he had built from a kit. By a mistake in wiring, the antenna was coupled to the oscillator, and it would radiate at double the frequency the set was tuned to.
The heater-type tube causes a number of complaints, for it sometimes emits noises that imitate most any interference. All are caused by a static discharge from heater to cathode. Many sets are found with defective power packs. Small arcs in the condensers, due to loose connections or high-resistance short circuits, cause many of the unusual growls heard in the listener's sets. Also some voltage-divider resistance units have a broken wire caused either by corrosion or by breakage due to contraction or expansion. This will show up only when the set gets good and warm; and many other complaints of this type, that appear after the set has been for use for hours, will account for the large number of cases found clear at the time of inspection.
Strict attention should be paid to the ground wire and its connections. When it· is connected to the antenna post trouble starts. With such a connection, the light line acts as an antenna and, since interference travels on the line, we can readily see what will happen. In districts where street cars are used or direct-current lines exist such connection makes the noise about 30% louder.
It always pays to put up a good antenna.
Loose connections in ground wires always cause trouble; because any number of electric receivers use by-pass condensers on the line side of the power transformer. Since even a small condenser will pass alternating current, and since the electric company's lines have a grounded center or neutral wire, a small arc will result at the point of poor connection. (Fig. 4.)
Another condition that will produce a loud hum is a lamp sitting on top of the set over the detector tube or cord stuffed inside the set too near the tube.
Key click from telegraph offices some times causes severe interference in the form of a loud popping or thumping noise and, when other lines parallel the circuit, it will spread over a large area. This is not so hard to find, but the cure may be a little harder. Use a 1/2-mf. condenser with about 200 ohms in series across the key. The resistance should be variable, and different sizes of condensers may be tried until the noise is stopped and the key does not arc too much at the contact. (Fig. 5.)
One piece of electrical apparatus emits a noise that sounds like the ticking of a clock; it is licensed under the Abrams patents and used for electrical treatments. The same filter will apply to this.
The trouble shooter's life is not all roses. He is usually a much cussed and discussed man and, if every owner of an interfering device would apply a filter, it would save him many a gray hair and many a cold ride.
Posted August 27, 2015