Mac's Radio Service Shop: The TVI Sleuth |
||
Tracking down the source of TVI (television interference) or any kind of RFI (radio frequency interference) can be a challenge. In the days when most people received their audio and visual entertainment (radio and TV) via over-the-air broadcasts*, any form of interference was usually picked up by multiple people, who promptly reported the issue to the power company or the FCC. Arcing power lines, transformers, and switches were prime generators of RFI, particularly during wet and high wind conditions. However, much more prevalent was interference from industrial and domestic equipment. Neon (see The Neon Interference Problem) and fluorescent light, heating pads (see Hazel: Stop Rockin' Our Reception episode), ignition systems (automotive and oil burners), diathermy (heat therapy popular in the day), industrial RF heating, kitchen appliances, and tool motors were among the likeliest offenders. Of course amateur radio operators were guilty of spewing out interfering EM crap, but contrary to popular belief, they were (are) one of the least offenders of all groups. RF Cafe visitor Denny Condron (K0LGI) provided a short article entitled, "Tracking Down a Mystery Signal," describing his effort to locate a particularly annoying interference instance with a local repeater station. * If you ask what other kind of entertainment was there?, the answer is records and magnetic tapes (maybe a few wire and wax cylinders, too, in 1954). See also Interference Analysis (Radio-Craft), Mac's Service Shop: Radio Interference, Canada Puts Limit on R.F. Interference (Electronics World), Television Interference (TVI) and the Novice (QST). Mac's Radio Service Shop: The TVI Sleuth
Barney considered returning late from lunch his own peculiar prerogative; therefore he was considerably annoyed when Mac, his employer, did not show up until the youth had been back at the service bench a full hour. "Have something especially good for dessert?" he inquired sarcastically as the older man entered the service department. "Nope," Mac replied with a teasing grin. "I simply ran into Joe Smith, radio communications man for a public service company, in the restaurant, and he wanted to show me the new mobile unit he has just built up for running down radio and television interference charged to their lines. I've been riding around with him while he demonstrated it." "What's it like?" Barney capitulated, deciding to give up being mad in order to satisfy his curiosity. "The whole installation is housed in a metal-body station wagon that has had ignition noise reduced to a minimum by the use of resistor-type spark plugs, suppressors, heavy bonding to the frame, and a reduced gap in the distributor. A heavy-duty 800 watt alternator is driven off the engine. The ten-volt a.c. output of this is rectified and used to charge the battery. The same output is stepped up by a transformer to 117 volts for powering the receiving equipment. In addition, for operating the equipment when the car's engine is turned off, there is a motor generator that puts out 250 watts at 117 volts a.c. Both of these power sources are voltage regulated. "What kind of noise-detecting equipment does he use?" "First, there's a broadcast receiver with the a.v.c. disabled and with a meter on the output. Next is a communications receiver that tunes from 540 kilocycles to 109 megacycles and can receive both AM and FM signals. Then there are two television receivers with signal strength meters connected to their video detectors. Finally there is a calibrated field strength meter with inputs for either twin-lead or coaxial line. In addition, of course, any special service receiver, such as police, taxicab, airways, etc., can be set in for use if needed. He also has two-way communication with company radio stations." "What antennas does he use on the receivers?" "A whip is used for general coverage reception and for picking up strong TV interference. There is a rotating mounting on the rear of the car on which he can place any of his single-channel yagi antennas for determining the direction from which noise is coming on a particular channel." "Is that all Joe does, ride around and listen for noise?" "Not by a long shot. As radio communications man, he has to take care of the company's microwave, mobile, carrier telephone, carrier relay, and carrier telemetering equipment. In addition, however, he is on call by any district in which the company operates if the district people are not able to clear up a case of interference by themselves. Since Joe has a full ten years of experience at this interference-sleuthing business, he is darned good at it and gets all the really tough cases. He also runs a complete noise check on any new lines or other equipment as soon as it is put into service. Then he makes periodic noise surveys over the transmission lines and substations in this half of the state. All the time he is driving around - and he drives about 4000 miles a month - the noise-detecting equipment in that station wagon is running; and if he notices anything unusual, he reports it, together with the pole number where it seems to peak, to the local office of the company." "He must be a busy cuss," Barney remarked. "How does the company receive interference complaints?" "A few come through the Public Service Commission. More of them are turned in to the various district offices of the company. Quite often the first report of noise comes from company employees using the carrier telephone for talking from one plant to another over the transmission lines. Since the receivers pick the signal right off the wire, many unusual noisy conditions are immediately noticed in these receivers." "Is power line noise all he runs down?" "I should say not!" Mac chuckled. "While it is true that he only goes out on a complaint when someone thinks a power line is causing the trouble, he finds the line guilty only one time in ten. That checks closely with a national survey that placed line noise ninth on the list of interference producers. The other nine times Joe finds the interference coming from an oscillating booster, TV set oscillator, neon sign, thermostat, heating pad, oil burner, ignition system, old-fashioned lamp bulb, diathermy, food mixer, blower motor, germicidal ozone or fluorescent lamp, light bulb loose in its socket, defective switch, or what have you." "Does he always pinpoint the real cause of trouble?" "No. When he is sure the company lines are not at fault, he usually stops right there. There are good reasons for not identifying sources of interference outside the company property. In the first place, many people owning interference-producing equipment do not take kindly to having this fact spotlighted by the power company. To do so is to run the risk of gaining ill will with a service primarily intended to promote and maintain good will. Secondly, if the power company cleared up every case of interference, no matter what the cause, it would soon become a clearing house for all complaints. It could not possibly take care of all the complaints that would then pour in, nor is there any valid reason why it should try. Quite often. however, it is necessary to find out what is causing the interference to prove the lines are not. "A good example of that," Mac went on, "is contained in an incident that happened one Sunday morning a few weeks ago. Before noon on that day the power company logged between three and four hundred complaints of interference on channel 6. The district manager called Joe and asked him to see if he could find the trouble. One glance at the TV set in the station wagon was all that was needed to know the interference was not coming from a power line, for it displayed typical r.f. cross-hatching. Nevertheless, since three-hundred-odd persons believed it might be line noise, Joe set out to prove it was not. This was the routine. "First he looked at a city map and discovered that the outline of the area from which complaints were coming strongly resembled the response pattern of a yagi antenna pointing north with the intersection of the major and minor lobes on the south side of town. Next he took a couple of bearings on the interference with his channel 6 yagi and noted these lines also intersected on the south side. Connecting the signal strength meter to the whip antenna, he drove to that vicinity. By driving back and forth in a criss-crossing pattern while watching the meter, he soon spotted a house that gave a maximum signal of 500 microvolts directly in front of it. No one was home at the house; so Joe briefly opened the line switch at the meter. The interference promptly disappeared. "This information was relayed to the police, and they managed to find the owner of the house and summon him home. Joe went inside with him and found his TV set turned off, but a one-tube booster on top of the set had been left on. When the TV set was turned on, the booster stopped oscillating; but it promptly started again when the set was turned off. That booster was putting out twenty microvolts of signal strength two miles away!" "That's one for the book!" Barney exclaimed. "Here's another Joe calls The Case of the Methodical Maid," Mac said. "This case of interference looked like diathermy and came on promptly at eight o'clock each evening and stopped just as promptly at nine. The chief complainant was an elderly doctor. The best Joe could do was pin the interference source down to the single block in which the doctor lived. Signal strength was about the same anywhere inside the block, and the yagi antenna would not give a conclusive directional reading. One evening Joe was parked in the alley of that block, elbows stuck through the spokes of the steering wheel, chin resting on the palms of his hands, staring, vacantly up into space while the TV set behind him gave out with the characteristic rasping sound of the interference. He had just decided the only thing left to do was to call out a crew of men and cut off the drops to the houses one at a time, when a little rectangle of light in his line of vision winked out. At the same instant the noise stopped, and a glance at the TV screen revealed the broad band of interference was gone. "In a matter of seconds Joe was ringing the doorbell of the house in which the light had been turned off - the house of the doctor, no less! It was quickly revealed the window from which the light had disappeared was in the maid's room on the third floor. Subsequent investigation revealed that her room was lighted by an ancient tungsten-filament bulb that was the best little interference transmitter you ever saw. "The thing that threw Joe was the precise timing of the interference. He had been thinking all the time of some mechanical device that was turned on and off with a time clock, or something like that. Talking with the neighbors revealed that the doctor always ate dinner at exactly the same time. The maid cleared off the dishes and retired to her room at exactly eight o'clock. Then she read for precisely an hour before turning off the light and going to sleep." "Hey, this is more fun than ghost stories," Barney exclaimed. "Tell me another!" "Okay," Mac agreed, "but it'll have to be a short-short. We've got to get to work. One day a report came in of interference along a brand new 230,000 volt line that had just been erected. Incidentally, Joe tells me the higher the voltage of the transmission line, the less noise you have. It is a lot harder to keep 12,000 and 33,000 volt lines quiet than it is 60,000, 132,000, and 230,000 volt jobs. Anyway, he did not believe the complaint could be justified for he had driven the full length of the line when it was first activated, and there was nary a sound. In fact, he had parked directly under this line and pulled in a TV station forty miles away with just his truck-mounted yagi, and there was not the first trace of interference. "When he checked the line this time, however, he found there was bad interference peaked on channel 6. Using his field strength meter, he was able to drive right to the cause of the noise: a piece of bailing wire tossed over one of the wires. When that wire was removed and measured, it was found to be exactly one-half wavelength long on channel 6." "Is it unusual for line interference to be peaked on one TV channel?" "That's right. Power line interference is of the random noise type and normally covers a broad band of frequencies. Occasionally this band will extend from the broadcast band through the TV channels. In a very few cases it will peak up on the higher frequencies and not be heard on the broadcast band. In the great majority of instances, however, it will be quite strong on frequencies up to about three or four megacycles but cannot be detected at all above thirty megacycles. "Joe made several other points," Mac said as he ticked them off on his fingers. "For one thing, he wished people could learn interference that shows up on just one channel is practically never from a power line. Neither is interference that produces cross-hatching of the picture or a reversal of the black and white portions. That comes from some r.f. producing gadget, such as an oscillating booster. Secondly, he wants people to know power companies are sincerely glad to know of any unusual noise along their lines, for such noise is often a warning something is going wrong. If they find out what it is promptly, this may save them thousands of dollars in equipment. People who believe power companies do not seriously hunt noise sources are just not being realistic. "Third, Joe says no transmission line can be made absolutely noise free any more than a gasoline motor can be so completely muffled it cannot be heard running in a quiet room. A good line will not cause any interference to a TV set operating within a few yards of its poles if that set has any kind of a normal signal; but when you try to receive a TV signal of five or ten microvolts right under the line, you will probably see, mixed in with the thick snowflakes produced by the set's own noise, a few sparkles of light that represent noise from the line. Power companies are engaged in constant research and rebuilding to make their lines more and more nearly noise free, but they do not hope to achieve 100% success in this effort. "Finally, it must be understood that while power companies are doing everything they can to kill their own snakes, they simply cannot use their special noise-locating equipment to run down general complaints of interference not connected with their systems. That is a job for service organizations, TV viewer groups, radio clubs, city governments, or whoever else is willing and able to take on a chore that can be thankless and frustrating, but never dull - at least that's what Joe says!"
Posted March 15, 2021 Mac's Radio Service Shop Episodes on RF Cafe This series of instructive technodrama™ stories was the brainchild of none other than John T. Frye, creator of the Carl and Jerry series that ran in Popular Electronics for many years. "Mac's Radio Service Shop" began life in April 1948 in Radio News magazine (which later became Radio & Television News, then Electronics World), and changed its name to simply "Mac's Service Shop" until the final episode was published in a 1977 Popular Electronics magazine. "Mac" is electronics repair shop owner Mac McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant. "Lessons" are taught in story format with dialogs between Mac and Barney.
|
||
By John T. Frye