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Mac's Service Shop captures here
a moment of technological transition in 1961 where a new "Loud-speaking Telephone"
impresses his right-hand man, Barney. The device utilizes vacuum-tube amplifiers
and a bulky external control box to allow hands-free communication, enabling Mac
to work while handling customer inquiries. Mac, ever the mentor, contrasts this
tube-based unit with the emerging technology of transistorized speakerphones, which
eliminate the need for external control boxes, external power supplies, and warm-up
times. Comparing this to today's equipment highlights a staggering evolution. The
1961 "Loud-speaking" setup, occupying significant space under a workbench, has been
completely replaced by modern smartphones and integrated VoIP systems. These contemporary
devices incorporate superior digital signal processing and advanced acoustic echo
cancellation into a footprint smaller than a deck of cards. What once required vacuum
tubes and complex hybrid coils is now managed by microscopic, power-efficient semiconductors
that deliver vastly higher audio fidelity and seamless, wireless hands-free connectivity.
Mac's Service Shop: Customer Cues
 By John T. Frye
As Barney entered the front door of Mac's Service Shop he could hear his employer
talking back in the service department; but when the youth stepped through the service
department door, no one was in sight. Mac was standing at the bench cleaning a TV
tuner and saying, "All right, Mrs. Carter, I'll send Barney over right after lunch."
"I knew it had to happen; I knew it, I knew it!" Barney said over his shoulder
as he got his shop coat out of the locker. "Sooner or later everyone in this wacky
business flips his wig and starts talking to himself. I do it all the time. I'll
say this, though: you're different. I always talk to the set that's giving me a
hard time, but you talk to imaginary customers. Guess that's because you have business
worries while I just have service headaches."
"So who was talking to himself?" Mac grunted. "I was talking on the telephone."
"Yeah-h-h!" Barney snorted derisively, "with the phone five or six feet away
and with the handset resting on the cradle?"
"That's right. Take a good long look at the telephone. If you don't notice
something different, turn in your Boy Scout badge. You're not observant."
Barney walked over to the telephone resting on the end of the bench.
"Hey, we do have a new phone!" he exclaimed; "and it's a weirdy. What are these
'on' and 'off' buttons down here below the dial for? What's this little knob with
the arrow on it? Is this a dial light? What's behind this little chrome hole plug
in the base? Has that little plastic box that looks like a housing for a three-inch
speaker got anything to do with it?"
"Whoa! Down, boy. Take it easy. This is a Loud-speaking Telephone. It can be
used without lifting the handset from the cradle. To make a call or answer the telephone,
you just push that 'on' button. The sound comes out of the speaker in this little
plastic case. Your voice goes into a dynamic microphone behind that hole plug. The
pilot light tells you the phone is on. The little knob in the upper right-hand corner
adjusts the volume coming out of the speaker. When the conversation is over, you
simply push the 'off' button, and the pilot light goes out. You can pick up the
handset at any time and transfer automatically to normal telephone operation."
"How come you had it put in?"
"The telephone installer talked me into it when he was in here the other day
to pick up a tape recorder I had repaired for him. He says it will be just the thing
when you call in for some information on a set and I have to look it up and give
it to you. This 'no-hands' telephone will allow me to talk to you from anywhere
in the room while I'm leafing through service literature, checking to see if we
have an item in stock, looking through the customer file, and so on. Furthermore,
when a customer calls with a description of symptoms, we can both listen while we
keep right on working. I agreed to give it a trial for a couple of months and see
if it was worth the extra cost."
"Do you know how it works?" "Sure. I told the installer I wouldn't have any electronic
equipment in this room that I didn't savvy; so he let me look at the technical bulletin
on it. If you've got any questions, shoot.
"It must have amplifiers in it. Where are they and how are they powered?"
""It has two printed-circuit vacuum- tube amplifiers. Each amplifier consists
of a CK-512AX hearing-aid-tube voltage amplifier and a 3V4 output tube. Power for
the tubes is taken from the 117-volt line. One amplifier builds up the output of
the microphone before putting it on the line. The other amplifier builds up the
voice currents from the line so they can drive the speaker."
"Surely all that stuff isn't crammed inside the telephone."
"Nope. There's a small 'control unit' about 4" x 7" x 10" fastened up under the
bench. Leads from it go to the telephone, this little speaker, the telephone line,
and to the house wiring."
"Why don't you get feedback from the speaker through the microphone?"
"That's a good question and it worried me, too. You actually can get feedback
if you turn up the speaker volume too high or if you place the speaker too close
to the microphone; but with the speaker three or four feet from the telephone and
properly positioned, there's no problem. A hybrid coil inside the control box does
the trick. This hybrid coil is a clever gadget that connects microphone signals
to the line but not to the loudspeaker amplifier. I haven't time to go into hybrid
coil theory now, but you won't be far off if you think of a hybrid coil as a device
that represents a sort of bridge circuit forcing one signal to buck itself out while
another signal is allowed to pass freely."
"Well, that's the end, the absolute end!" Barney said admiringly as he stroked
the telephone.
"Oh no it's not," Mac disagreed with a grin. "Nothing is ever the end in electronics.
Someone is always coming up with something a little better. Already they have a
Transistor Speaker Phone that has some advantages over this model."
"Such as what?"
"The small power requirements of the transistors are supplied right from the
d.c. current in the telephone line itself; so no connection to the house current
is needed. Miniaturization afforded by transistors permits all the extra equipment
to be housed inside the telephone and the little speaker case, thus doing away with
the need for the 'control box.' That means the Transistorized Speaker Phone is portable
and can be carried about and plugged in at different locations in the house or office.
Transistors, unlike tubes, should last just as long as any other parts of the telephone.
Furthermore, they spring into action instantly and do not need the second or so
of filament warm-up time required by the tubes."
"Those telephone guys are right on the ball," Barney said.
"They did invent the transistor," Mac said quietly. "Don't ever underestimate
telephone engineers. Radio engineers have borrowed very freely from them in the
past and will continue to do so in the future. The telephone boys have done marvelous
research work in the fields of sound, microwave transmission, semiconductors, multiplexing,
and many others. When they talk, I listen."
"Suppose you listen to me a bit," Barney said as he picked up the little a.c.-d.c.
receiver he had been working on at quitting time the day before. "Ain't it funny
how identical troubles come in bunches in service work? This set has exactly the
same difficulty as the last one: a dead oscillator. That one I fixed yesterday had
an open winding in the oscillator coil and I had to replace the coil. This coil
must be bad, too, although the windings show continuity. Guess it must have shorted
turns or something.
"What really bugs me though is that the owner said the pilot lamp went out and
the set quit playing. That makes sense, for the filament of the output tube was
open. But when I put in anew tube the set still wouldn't play and then I found the
oscillator wasn't working. It's kind of a coincidence that the coil and the tube
should go out at the same time. The owner had been into it, for some of the tubes
were in the wrong socket. Maybe he fouled up the oscillator coil by putting the
wrong tube in the socket. Anyway, I've put in a new oscillator coil; so let's hear
it play."
Barney turned on the receiver, but no sound came from the speaker except a low
hum. "What do you know! Guessit wasn't the coil after all," he said in chagrin.
"How about the mixer tube and the oscillator grid capacitor?" Mac asked helpfully.
"Both OK," Barney said promptly. "I checked them before taking out the old coil."
Mac looked down at the receiver on the bench for a minute and then reached over
and gently pulled on a wire that ran from the loop antenna on the back of the receiver
to the ganged tuning capacitor. The end of the wire flipped loose from the tuning
capacitor and the set began to play at once.
"OK, you smart aleck, what did you do?" Barney demanded.
"Just what you asked me to do: I listened to you," Mac said blandly. "What did
I say that gave you a clue?"
"You said the owner had been into the set. I tried to put myself in his place
and look at the receiver. I known on -technical people are always looking for a
'loose wire' in a radio. To such a person, the short external antenna lead on the
loop would seem to be just such a loose wire. When he looked for the point from
which the wire might come, he spied that empty little eyelet connection on one end
of the stator of the oscillator section of the tuning capacitor and he thrust the
bared end of the wire through the hole. That failed to restore the receiver to operation;
so he bundled the set up and brought it to us with the wire still in place."
"And I, like a big dope, never saw it," Barney finished bitterly.
"Don't be too hard on yourself. Your judgment was still fogged by the memory
of the recent dead -oscillator set you had repaired and you couldn't think of anything
but another bad coil. We all tend to expect recent experiences to repeat themselves
in servicing. Some technicians carry this to an extreme and insist on replacement
of certain components in radio and TV sets right off without making sufficient checks
to make sure these components are bad. If this fails to restore proper operation,
then they go ahead to find what actually is wrong. As a result I have known technicians
called 'Resistor Ralph,' Picture Tube Paul,' Filter Capacitor Frank,' Transformer
Tom,' and so on. Just make sure I don't have 'Bad Coil Barney' working for me."
"Oakie doakie, Boss," Barney said with a sheepish smile. "I'll watch it. But
I'm convinced you do a better job of 'no-hands' telephoning than I do of 'no-head'
servicing!"
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. There
are 131 stories as of January 2026.
Color and Monochrome (B&W) Television
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