Mac's Service Shop: Radio Interference
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Although published in a 1972 issue of Popular Electronics magazine, the topic of dealing with radio interference is timeless. Some of the sources of radio interference change over the years, but there is always a need to discover and resolve such problems, and more importantly, use the lessons learned to attempt to prevent it in the first place. The January setting in the Great Lake Midwest region of the U.S. is right on time for when this is being posted. It's snowing here in Erie, Pennsylvania, as I write this, the temperature is about 24°F, and the wind is howling at 15-20 miles per hour as is has been all night. "Someone must have sprayed circuit-cooler around out there," Barney (Mac's able young assistant) complained. "Man, it's cold!" That about sums up today's weather. Unfortunately for Barney, he would be called upon to leave the warmth of the shop to make a service call to figure out why CB (Citizens Band) radio transmissions were being heard on nearby PA (public announcement) systems - known colloquially in the radio realm as BCI (broadcast interference). You can probably imag[e]ine what the cause turns out to be. I still do a lot of OTA (over the air) broadcast listening. A possible solution not mentioned that I have found to be effective for reducing or eliminating RFI and BCI is to relocate the receiver to a different location or plug it into a different wall socket - using an extension cord if necessary. Mac's Service Shop: Radio Interference
A cold, raw, January wind numbed Mr. McGregor's fingers as he unlocked the door of his service shop and stepped inside. There he paused in the cozy warmth to unbutton his coat and survey proudly, as he often did, his place of business. Matilda, the office girl, had left the counter and her desk as neat and clean as always; and through the open door leading into the service department he could see the night light reflecting off the gleaming faces of the service instruments above the bench. The place had a good, solid, responsible look about it, he thought contentedly. There was a sound of running feet outside, and Matilda and Barney, Mac's red-headed Irish assistant, came in together. "Someone must have sprayed circuit-cooler around out there," Barney complained. "Man, it's cold!" "If you'd wear an overcoat as Mac and I do, you wouldn't go around with your teeth chattering," Matilda observed tartly as she removed her stylish floor-length coat to reveal a skirt as short as the coat was long. "Ha! You don't wear that coat to keep you warm," Barney retorted. "You just enjoy the shock effect when you take it off." "Here, here! I'd better separate you two," Mac interrupted. "Barney, you come on back to the service department with me. When you're as old as I, you'll know better than to get into an argument with a woman about clothes. A man hasn't won that argument since Eve picked out the fig leaves. Anyway, I want to brief you on your first service call." "What's unusual about it?" Barney wanted to know, shrugging off his quilted jacket and perching himself on the bench. "CB transmissions are being heard over the PA system in the Sorensen Funeral Home. This amplifier is used to furnish background music during funerals, and it's pretty disturbing to hear a CB operator working skip right in the middle of a service. I've an idea or so that may help clear up the complaint, but perhaps we'd better run over the general subject of interference to electronic equipment caused by the operation of nearby transmitters, be they CB, amateur, special services, or even broadcast." "I'm being paid to listen, so go right ahead," Barney invited, stretching out on his back and closing his eyes. Interference to Broadcast Sets. "That's what I like: a good alert audience," Mac said sarcastically. "Anyway, let's start with interference to broadcast reception. The 'tunable' type is so-called because it's only heard at certain places on the dial. True image response accounts for this kind of interference from amateur stations operating in the 1800-2000-kHz band and for higher frequency broadcast stations that can also be heard on the low-frequency end of the band. "As you know, most present-day superhet receivers use a 455-kHz i-f. To receive a 1000-kHz station, the local oscillator of such a receiver is set to 1455 kHz to beat with the broadcast signal and produce a difference frequency of 455 kHz that will pass through the i-f amplifier. But suppose an amateur station is operating nearby on 1910 kHz. If this signal, which is naturally quite strong in the immediate area, reaches the input of the receiver mixer - and there is only the limited rejection capability of the tuned loop antenna to prevent this - the amateur signal will beat with the local oscillator and also produce a 455-kHz difference signal that will pass through the i-f amplifier, too, and make it seem to the listener the amateur is transmitting squarely on top of the broadcast station, even though the AM amateur is operating properly in his own band. Actually, the receiver is listening where it shouldn't! "As long as the local receiver oscillator operates on the high frequency side of the station being received - and this is usually the case - the true image frequency is always twice the i-f, or 910 kHz higher in frequency than the dial setting of the receiver. This means that images of strong local broadcast stations operating between 1450 and 1630 kHz are sometimes heard between 540 kHz and 720 kHz on the dial, in addition to their proper dial settings." "You keep saying 'true image.' Are there other kinds?" Barney asked. "Yes, the oscillator-harmonic images. The local oscillator has harmonics that can beat with amateur or other signals to produce false responses in the broadcast receiver. For instance, in the example we were talking about, the third harmonic of the local oscillator would be 4365 kHz. This could beat with an amateur station operating on 3910 kHz in the 75-meter band to produce the difference i-f frequency of 455 kHz and again make it seem the 75-meter station is operating on top of the 1000-kHz broadcast station. Fortunately, as the order of the harmonics increases, their strength ordinarily diminishes. Cross-Modulation & Non-Tunable Interference. "Cross-modulation is another type of BCI identified by the fact that the interference is heard only when a broadcast carrier is tuned in. Between stations the interfering signal is not heard. Cross-modulation is the result of rectification of the strong local signal by one of the early stages in the receiver. The output of this stage carrying the r-f or i-f signal goes up and down with the modulation of the local signal, and so the broadcast carrier is actually modulated by the local signal as well as by the modulation imposed upon it in the studio. Both the broadcast modulation and the modulation of the interfering signal are heard simultaneously. Cross-modulation of CB reception on one channel by the modulation of a powerful signal several channels away is a very common occurrence. "Finally we have the non-tunable type of interference. This is the kind in which the interfering station is heard 'all over the dial.' This happens because the interference is affecting the untuned audio stages rather than the tuned r-f and i-f stages. In fact, this type of interference is often found where there are no tuned stages - just audio stages such as you find in a PA amplifier, electric organ, intercom, etc. Positive swings of the carrier appearing on the grid of the audio stage drive it into the conducting region. Electrons attracted to the grid cannot leak off rapidly enough through the high value of grid resistance, and their accumulation biases the tube to cut-off. This makes a grid-leak detector out of what is supposed to be an audio amplifier, and the detected modulation of the interfering signal is heard in the speakers. This usually takes place in an input stage with a very high value of grid resistance, but it can happen in following stages. Effect of volume control action on the interference will tell you if the trouble is arising ahead or behind the control. The same sort of thing happens with solid-state receivers, too." Getting Rid of It. "Okay, you've told me how the interference happens, but I'm more interested in how I get rid of it." "In all the cases I've mentioned, about all you can do is try to reduce the strength of the interfering signal reaching sensitive areas of the equipment being interfered with by means of shielding, filtering, and bypassing. You can't do much to keep the signal off the unshielded loop antenna of the average broadcast receiver, although changing the position of the receiver's loop antenna may help; but a lot of the signal pickup of such a receiver is through the light lines. The reduction in received signal strength when switching from line to battery operation in a two-way receiver demonstrates this. For that reason, just doubling up the line cord to cut down on its pickup may help. A properly installed line filter will probably be more effective, especially if it is installed inside the set. "If such measures do not clear up the trouble, modification of the circuit may be necessary. This is especially true with 'non-tunable interference to a receiver or interference with an audio amplifier. Incidentally this type of interference usually comes from a transmitter operating above 20 MHz, where a line cord may easily be a resonant quarter- or half-wavelength long, and so poke a lot of unwanted signal into the equipment." "What circuit modifications are you talking about?" "In vacuum-tube equipment grid leads can be shortened and shielded. A high value of grid resistor in an audio stage can be reduced to 2-3 megohms, and the grid may be bypassed with a 250-pF capacitor. Alternately a 75,000-ohm resistor may be connected between the grid and all other grid connections. This resistor, together with the grid capacitance, forms a high-frequency r-f filter. Solid-state equipment can also be treated in the same general way." "You said you had some ideas that may help me with this funeral home job, didn't you?" "Yes. They tell me the interference only started a week ago after we had that freak thunderstorm. Lightning blew a fuse in the amplifier and apparently did no other damage, but the interference from the CB transmitter next door showed up right after that. I have a hunch that lightning may have blown one or both of the 0.001-μF capacitors from each side of the line to ground. If so, this permits the r-f to follow the line cord into the amplifier. Check these first. If they are OK, try bypassing the leads to the speakers with 0.001-μF capacitors to the chassis. Make sure top and bottom shields of the amplifier are in place and are making good connections. If such shields are not furnished with the amplifier or have been misplaced, we may have to make some. Only after you have done all these things should you start modifying the circuit. Hey, what's wrong? You don't seem very keen about this job. Don't you understand what I've been telling you?" "I understand all right. Don't forget I was a ham before I became a service technician. It's just where is the amplifier? The fronts of those places give me the willies, and I don't like to think about what I might run into behind the scenes." "So that's it! Well, relax. The people at the funeral home particularly wanted you to come today because they have no 'customers' in the house and the CB operator will be available for testing. Okay?" "Okay" Barney said, a sheepish grin creasing his freckled face as he slid off the bench and started putting on his jacket. "But suppose we continue this conversation one of these days and talk about television interference, or TVI. Of course I probably know most of what you can tell me, but it won't hurt to talk about it a little." "Of course," Mac said, concealing a grin. "A little brushing up never hurts anybody. We'll do it at the first opportunity."
Posted January 10, 2019 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.
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By John T. Frye, W9EGV, KHD4167