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March 1960 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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It is a rare occasion that Barney
bests Mac when it comes to electronics prowess. Good natured back and forth often goes
on between them during troubleshooting sessions and impromptu discussions about business
practices, industry trends, and customer interactions; indeed, John Frye depends on it
to make the stories interesting. This time, underling Barney exploits knowledge gained
from a recently purchased electronics reference book to trip up shop owner Mac over which
of two metals has the lowest resistance. Mac's choice is one many people would instinctively
make - and be wrong as Mac was. Here is a table of
electrical resistivity values for various
metals and substances.
Mac's Service Shop: Two for One
 By John T. Frye
Outside it was a stormy March morning. The howling wind, laced with driving snowflakes,
shook and rattled the heavy "Radio & TV Service" sign over the door of Mac's Service
Shop. Inside things were a little stormy, too, at least with Barney, the red-headed Number
Two man of the service establishment. He stood a couple of feet away from the bench and
bent his lanky body at the hips into an uncomfortable-looking right angle so he could
rest his elbows on the bench and cup his chin in his hands as he scowled fiercely at
the little a.c-d.c. chassis in front of his nose.
"Is all that muttering and sighing and flouncing around your subtle way of sending
up distress signals?" Mac, his employer, asked with a teasing grin.
"Of course not!" Barney retorted.
"Can't a man stop to think without some joker's supposing he's asking for help? The
complaint - well justified, incidentally - on this little fiend is that it picks up all
sorts of code, teletypewriter, and other short-wave signals right along with the broadcast
band it is supposed to tune. The condition is said to be much worse at night than in
the daytime; so wouldn't you know the woman who owns the set is an insomniac who does
most of her listening in the early morning hours!"
"That figures," Mac said with a nod.
"What have you done so far?"
"Changed tubes. Checked the a.v.c. bypass. Checked the loop antenna for short-circuits
or poor connections. Checked the oscillator grid voltage. Checked the oscillator waveform
with the scope. Everything is perfectly OK."
"Why did you check the oscillator waveform?"
"I thought maybe it was putting out exceptionally strong harmonics that were causing
the trouble. In such a case I'd expect the oscillator waveform to depart noticeably from
a sine wave."
"Sounds logical, but I doubt the oscillator is causing the trouble. Almost any oscillator
will have strong enough harmonics well down in the short-wave region to heterodyne strong
signals there to the i.f. frequency if these signals are allowed to reach the converter
signal grid. Normally these high-frequency signals are kept from the grid of the converter
by the selectivity of the antenna tuned circuit. This parallel-resonant circuit between
the input grid and cathode of the converter is tuned to the broadcast band and has a
very high impedance at its resonant frequency. Short-wave signals in the region of several
megacycles should find a very low capacitive reactance through the tuning capacitor and
so be short-circuited to the cathode. Let's radiate a signal from a loop of wire connected
to the output of the signal generator and see if we can find what high frequencies are
getting through."
Barney set up the signal generator and a little tuning quickly established that signals
in the vicinity of thirteen megacycles were the ones causing the trouble. When the chassis
of the a.c.-d.c. receiver was touched with a finger, these interfering signals became
much louder. Mac was studying the diagram of the receiver while Barney was running these
tests.
The input of the converter stage was a little unusual. The tuning capacitor frame
was grounded to the chassis and the antenna loop was connected across the r.f. section
of this capacitor. A small mica capacitor connected the hot side of the tuned circuit
to the grid of the converter tube. The a.v.c. voltage was fed to this signal grid through
a decoupling resistor, the bottom end of which was bypassed to "B-." A coil of wire in
series with a 0.1-μf. capacitor was labeled an "r.f. filter" and was connected between
the chassis and the "B-" point to which the converter cathode was returned.
Mac took the grid-dip oscillator and its set of coils from a cabinet and put one of
the coils in the instrument. Then he turned the receiver chassis over and pointed to
a 0.1 μf. paper capacitor with a few turns of wire wound around one end. "See if you
can find a resonant frequency for this coil," he said as he handed the grid-dipper to
Barney.
"I sure can!" Barney soon announced; "and it's at thirteen megacycles."
"Fine! Now short out that little coil and let's see what happens."
When this was done, leaving only the 0.1 μf. capacitor between chassis and "B-,"
the unwanted signals disappeared. Broadcast reception was just the same as before, but
the annoying birdies were completely gone, even when the full output of the signal generator
was fed into the loop of wire.
"Now how did you know about that?" Barney demanded.
"Because I ran into exactly the same thing on one of these sets not more than three
months ago," Mac confessed with a chuckle; "and I'll admit it had me 'bugged,' as you
would put it, for quite a while. I ran it down with the signal generator, the grid-dipper,
and the diagram, just as you did.
"You see that series-resonant circuit made up of the capacitor and the little coil
of wire wrapped around it is supposed to be broadly resonant at the i.f. frequency so
as to offer a very low-impedance path between the chassis and 'B-' at that critical frequency;
but the joker is that the coil and its distributed capacity form another parallel-tuned
circuit resonant at 13 megacycles. This acts as a barrier between any 13 megacycles signal
on the chassis and 'B-.' On the other hand the low impedance the tuning capacitor presents
to this high-frequency signal allows it to go to the converter grid, to heterodyne with
a harmonic of the oscillator, and to pass right on through the i.f. amplifier along with
the broadcast signals being received. Short-circuiting the coil or removing it allows
the 13 megacycle signal to go to ground and so prevents the trouble."
"You know," Barney said slowly, "I should have remembered that it's easy to get two
resonant circuits for one combination of a coil and capacitor - one series-resonant and
the other parallel-resonant. That's at the bottom of a lot of parasitic oscillation troubles
that plague transmitters."
"I am never quite easy about having to make a change in a manufacturer's circuit,"
Mac continued; "so I wrote the service department of the people who made the receiver.
They sent back a service note mailed out to their dealers some months ago in which they
described the self-same symptom and recommended that the coil be removed to correct it.
I didn't recognize the set without its cabinet until I started studying the diagram;
then I decided it would make a stronger impression on you if I had you track down the
cause of the trouble step by step."
"Gee, thanks loads!" Barney said sarcastically. "Now, Doctor, I have a question for
you: of two equal lengths of wire of the same diameter, one of copper and the other of
gold, which will have the least resistance?"
"I suppose the gold one," Mac replied.
"You're wrong!" Barney said triumphantly as he picked up a slender blue book from
the bench, opened it, and pointed to a table of resistances of metals and alloys. "See;
copper has a resistance of 10.37 ohms per circular mil foot as compared with 14.55 ohms
for gold. Silver has 9.796 ohms, and it's the only one listed with less resistance than
copper."
"What's the name of the book and where did you get it?" Mac asked politely.
"It's called 'Handbook of Electronic Tables and Formulas' published by Howard W. Sams &
Co. Inc. and compiled by Donald Herrington and Stanley Meacham of the Same Engineering
Staff. I picked it up at the distributor's when I was over there after parts. For a long
time I've wished someone would come out with a book such as this for folks with feeble
memories, like me. No matter how hard I try, I can't keep formulas for reactance, impedance,
resonance, frequency-and-wavelength, etc., straight in my head. In fact, there's a whole
raft of information involving formulas, wire tables, color codes, miniature lamps, transmission
line characteristics, audio and video carrier frequencies of TV channels, and American
substitutes for European tubes that I have to keep looking up over and over again; and
that has meant going through a half dozen books each time to find what I want. Now, with
this handy-dandy little volume right up here on the shelf where I intend to keep it,
I can put my finger on what I want 'toot-sweet,' as we French say."
"I just can't get over a penny-pincher like you buying a book," Mac muttered.
"It will save me work, man!" Barney explained. "For example, suppose I want to know
how much capacity I need to resonate a one henry choke at 1000 cycles. Do I use a formula
to figure out the reactance of one henry at 1000 cycles and then use another formula
to determine what value of capacity will have equal reactance at this frequency? I do
not! I simply lay a straight-edge across the proper one of these reactance charts in
the book, and I immediately see a capacitor of about 0.025 μf. will do the trick.
Neat, huh? You wanta buy part interest in this book?"
"Let me see it, you Shylock," Mac said as he took the book out of Barney's hands.
"Hm-m-m, I see it has lots of math stuff in it, too. Here are trig, logarithm, and decibel
tables as well as decimal equivalents of fractions, powers of ten, and algebraic operations
and geometric formulas. They should help a fellow working with a formula when his math
is a little rusty. And here is a whole bunch of stuff that used to be printed on the
backs of composition books when I was in school: Measures and Weights and Metric Equivalents.
Boy, does that take me back!"
"Don't overlook that colored foldout showing what services occupy each portion of
the spectrum from 10 kc. to 100,000 mc.," Barney suggested; "and there are lots of little
goodies tucked away here and there in the book such as the amount of power consumed by
various electrical devices used in the home, the proper shunts and resistors to use with
d.c. meters to make them indicate desired voltage, current, or resistance, and a complete
description of the time and frequency signals put out by WWV and WWVH."
"OK, you've sold me," Mac said as he took out his billfold. "Here's half the price
of the book, but let's have one thing clear: it stays right here at the bench where we
both can use it during working hours. I don't want you lugging it home and leaving it
in your ham shack. OK?
"OK," Barney cheerfully agreed as he put the money in his pocket. "And if I ever get
tired of service work I think I'll take up selling. I made a little bet with the boys
down at the distributor's that I could make you pay for half of that book!"
Posted July 5, 2018
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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