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July 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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These Mac's Service Shop techno-stories tutorially mix real-world entities with
tips on troubleshooting, repair, alignment, customer service, industry news, and
other relevant topics. Although not directly related to this "Oscillating
Canine" episode in a 1960 issue of Electronics World magazine, mention is made
of the Bon Ton Department Store. It could possibly be a reference to the
modern-day The Bon-Ton
store since according to Wikipedia the company was founded in 1898 (renamed
Bon-Ton Millinery
in 1902). The reference is actually to the store owner, who engaged Mac to
convert a radio receiver to pick up weather station broadcasts from his
airplane. The story's title has to do with another subject - a
hard-to-get-rid-of oscillation in a transistorized radio. It is one of the first
instances of solid state components and the printed circuit boards to which they
are attached. Barney laments the difficulty of troubleshooting the configuration
as opposed to the good 'ole vacuum tube circuits that used point-to-point wiring
in the chassis. It was an era of major transitions in the electronics industry.
Mac's Service Shop: Oscillating Canine
 Barney stopped short in the door of the service department as he heard a droning
voice, mixed in with slow-speed code, emanating from a three-way portable sitting
on the end of the service bench.
"What station's that?" he demanded of Mac, his employer, who was looking down
at the receiver with a smile of satisfaction.
"It's the aviation range station at Center City operating on a frequency of 266
kilocycles," Mac explained. "This station puts out a continuous weather report for
planes in this general area. The reports are on tapes that are changed at frequent
intervals. That 'dit-dah, dit-dah, dit-dah' in the background shows we're not in
the center of the beam."
"Just a minute, just a doggoned minute!" Barney interrupted. "That's a broadcast
receiver, pure and simple. How come it's pulling in that low-frequency station -
and why is it?"
"The receiver belongs to Mr. Smith, owner of the Bon Ton Department Store. He
is a flying enthusiast with his own plane. As such, he likes to keep an eye on the
weather, even when he is down at the office. When conditions are particularly good,
he can sneak off for an hour or so of flying. With this in mind, he asked me if
I could convert one of the many broadcast receivers he had lying around the house
so that it would pick up this weather report.
"At first I shied away from the idea. I was thinking in terms of trying to make
the broadcast receiver tune the whole low-frequency band from 200 to 550 kilocycles;
and you know what a complete re-doing of the front end that would take; then, too,
I was concerned with the idea an aviation weather receiver should be very, very
dependable because the flier's life might depend on it. But Mr. Smith explained
he only wanted to receive just this one station and that the receiver would be used
only in his office or at his home. His plane was fully equipped with all sorts of
standard aviation radio equipment. This put a different light on the project, and
I agreed to see what I could do.
"Actually the solution turned out to be ridiculously simple. After putting the
receiver into good operating condition-it needed new filter capacitors - I had only
to pad the oscillator and antenna circuits down to where the 266 kc. station could
be picked up with the receiver dial sitting on 600 kc. I picked this dial setting
so as to have a little tuning leeway and yet be able to use most of the capacity
in the receiver's tuning capacitor.
"For padders I used the double compression trimmers out of a discarded i.f. transformer.
These had a measured maximum capacity of about 200 μμf. each; and one was
more than enough to lower the oscillator frequency from 1056, where it sat to tune
in 600 kc., down to 722 kc., where it had to be to produce a 456 difference beat
with 266 kc. However, since you have to quadruple the capacity of a parallel tuned
circuit to halve the resonant frequency, I knew I'd have to use considerably more
extra capacity than the 200 μμf. available in the other trimmer to lower the
frequency of the antenna circuit from 600 to 266 kc. A little cut-and-try procedure,
using the grid-dip oscillator to keep track of resonance, revealed that a 0.001-μf.
silver mica in parallel with the 200-μμf. trimmer and the set's tuning capacitor
did the job in fine shape."
"Having to listen to that program very long would drive me nuts," Barney said
as the taped voice, punctuated with the slow code, droned on and on. "It would be
easy, though, to use a d.p.s.t. switch to cut out the extra capacities and restore
ordinary broadcast reception."
"Yes, and I mentioned that to Mr. Smith; but he said it wasn't worth the trouble
to him. He already had lots of broadcast receivers," Mac answered.
"Well," Barney said with a deep sigh, "you've solved your problem, but I've still
got mine."
"You mean you're still working on that little transistor set you started on this
time yesterday?"
"Yes, and at the rate I'm going, I'll be still working on it a week from now.
That little monster is a real live dog."
"The only trouble is that it oscillates, isn't it?"
"Oh yes, that's the only trouble," Barney answered sarcastically; "but I've got news for you: running down oscillation
in a printed circuit transistor receiver is a lot different than hunting the same
difficulty in a tube receiver."
"From what I've heard, it's an r.f. or i.f. rather than an audio oscillation."
"True. You get a heterodyne whistle on every station."
"I suppose you started by changing the battery."
"Right. I'm hep to the fact that some of the cheaper receivers depend on the
low resistance of a good battery to tie signal-carrying return circuits down to
ground. When the battery is partially exhausted, its internal resistance goes up
and permits coupling between some of the return circuits that can cause oscillation.
A new battery made no difference."
"Then I suppose you paralleled all the decoupling capacitors. one at a time,
with a unit you knew to be good."
"Yes, and since this is a well-designed receiver, there were several of those
capacitors. I had to remove the speaker to get at some of them, but I never missed
a one. I'll stake my reputation as a hot-shot electronic technician that there are
no open capacitors."
"At the moment you wouldn't be putting up much," Mac said with a wry grin. "About
next, I imagine you checked the alignment."
"Your crystal ball is clear today! I went over the alignment two or three times,
and there is nothing wrong in that department."
"How about the possibility that the printed circuit board was cracked so as to
produce a break in the grounding lead? That would leave part of the grounded circuit
floating."
"I thought of that, too," Barney said triumphantly, "and I jumpered across various
portions of the ground bus. It's OK."
"H-m-m-m-m, how did a careful voltage check turn out? Maybe some of the bias
resistors have changed value and altered stage gain."
"All voltages are right on the button, but I did not stop there. I pulled all
the transistors from their sockets and did a complete resistance check. The resistances
at all check points provided in the service data are well within five per-cent of
values specified."
"This is getting interesting," Mac said as he reached over and picked up the
circuit diagram. "Looking at this, I can see one very nasty possibility: the i.f.
stages are neutralized, and if one of those neutralizing capacitors opened up, or
if this resistor in series with a neutralizing capacitor opened up, the stage would
oscillate quite merrily."
"You've got to do better than that," Barney gloated. "I used a little variable
trimmer capacitor in place of each of those little ceramic neutralizing capacitors
and varied it back and forth without helping the oscillation a bit. And there's
nothing wrong with that resistor, either. Unsoldering those neutralizing capacitors
and soldering them back is quite a tedious job, I might mention."
"I can imagine," Mac muttered as he continued studying the diagram. "Of course
we could be up against a defective transistor, but I doubt it."
"The transistor checker doubts it, too," Barney said. "They all check good with
normal gain and no excessive leakage."
"Is there any way you can stop oscillation and still get reception?"
"Yes, if I put my finger on the collector or base of the mixer stage the oscillation
stops and the local station can be heard."
Mac was silent for several minutes as he scrutinized the diagram. Then he put
it down and picked up the little chassis and looked it over closely. "Hand me a
2.2k resistor," he finally said to Barney.
Carefully he shaped the resistor leads until he could bridge the resistor across
a couple of points on the printed circuit board; then, holding the resistor in place,
he turned on the receiver. It played perfectly, without a sign of oscillation, until
he removed the resistor. When he did that, the oscillation returned.
"Well I'll be!" Barney exclaimed with a mixture of relief and disgust in his
voice. "I was so sure those resistance checks were all correct."
"They probably were; but this open resistor - if it is open - would not show
on those checks," Mac answered as he plugged in the pencil soldering iron.
"Take a look at the diagram. The set has an r.f. stage ahead of the mixer, but
only the antenna circuit and the oscillator circuit are tuned with the tuning capacitor.
The collector of the r.f. stage is coupled to the base of the mixer through a capacitor.
Now notice this parallel-tuned circuit in the collector lead of the r.f. stage.
See the coil resistance is given as 19 ohms. But notice, too, that this 2.2k resistor
is actually connected directly across the tuned circuit. It 'loads' the circuit.
lowers the 'Q,' and broadbands the response. With the resistor in place, the r.f.
stage is probably designed to deliver uniform response across the whole broadcast
band; but when the resistor opens, the 'Q' of the tuned circuit and the impedance
at the input of the mixer stage shoots up sharply while the frequency response narrows
- which makes things dandy for the mixer stage to oscillate at the i.f. frequency."
As he finished speaking, Mac lifted the unsoldered resistor from the printed circuit
board and placed the ohmmeter test leads across it. The meter pointer did not even
flicker. "It's wide open all right," he said as he started soldering the new resistor
in place.
"An open 2.2k resistor would never show across 19 ohms," Barney mused; "but what
ever made you suspect that resistor might be open?"
"Your excellent, methodical procedure had just about eliminated everything else;
and when you said you could kill the oscillation by putting your finger on the base
or collector of the mixer, I started looking for something that would normally load
a portion of the circuit as the presence of your finger did. That line of reasoning
pointed squarely at the quarter-watt resistor. Incidentally, I've noticed quarter-watt
resistors are more prone to open-circuiting than are half-watt units, even when
they carry no current."
"Well. I wish I'd found the trouble myself, but I'm happy it's been found." Barney
announced. "Ain't it a great feeling when a problem that's been deviling you for
a long time is finally whipped? I feel exactly like a cork that's been held under
water for a long time and has finally been released so it can pop to the surface."
Posted January 19, 2023
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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