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January 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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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
 By John T. Frye, W9EGV, KHD4167
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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