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Can you imagine a time when
you could have your radio, or any electronic device for that matter, diagnosed
for proper operation for only a dollar? That would include not just turning it
on and making certain that it appears to work, but actually doing a cursory
check-out of component function, current draw, tuning, etc. Yes, the good deed
could potentially earn you some customers willing to pay to have repairs and/or
preventative maintenance done, but that's a lot of up-front financial risk. Mac
McGregor, a proven shrewd and honest businessman, evidently thought so when he
ran the "special" at his establishment. Author John Frye nearly always made his
"Mac's Radio Service Shop" techno-dramas about then-current issues and often
for-real equipment. His young and mostly-able assistant Barney often served as
the vicarious substitute apprentice for us, his audience. For comparison, the
BLS'
Inflation Calculator says $1 in 1940 is the equivalent of $18 in 2020 -
still a great deal by any standard.
Mac's Radio Service Shop: Portable Patter
 By John T. Frye
Holy cow! Wha' hoppen?" Barney asked as he stood in the open door of the service
shop and surveyed the sea of portable receivers that almost completely engulfed
Miss Perkins' desk.
"That's the first day's answer to our special 'check-your-portable-for-only-a-dollar'
offer, and don't overlook that TV portable that one joker ran in on us," the office
girl told him. "And let me warn you that your story about being sick Saturday had
better be good. Mac half suspects you of playing an April Fool joke on him."
The apprentice service technician walked over to the service department and cautiously
tossed his battered felt hat through the door.
"Come on in here, you red-headed Irishman!" Mac's voice instantly boomed forth.
"Let me get a good look at you. If you look healthy, you won't long. Of all the
days I ever needed you, Saturday was it, and - Hm-m-m-m," he broke off as he caught
sight of Barney's pale but grinning countenance, "your freckles do look a little
more three-dimensional than usual. I'm not surprised, though. I knew that sooner
or later that billy-goat appetite of yours would cause you to founder yourself."
"The trouble," Barney announced loftily - "and I quote - 'was an attack of migraine,
probably induced by intense cerebration.' "
"I'll buy the migraine part of it," Mac conceded, "but I've got to see a sample
of that cerebration; and come to think of it, where could you find a better place
to demonstrate than on these portables? Before you start, though, perhaps we had
better have us a little chalk talk:
"Naturally, we shall check all of the tubes; but tube-checking in portable sets
can fool you, especially on some kinds of emission checkers. As you know (I hope),
the emission of a filament type of tube depends pretty closely on the temperature
of the filament; but watch the filament of the 1A7 in the checker when I push this
'Test' button."
Barney, with his chin hooked over Mac's shoulder, saw the dull red thread of
the tube filament glow noticeably brighter as the button was depressed and then
return to its former appearance as the button was released.
"The plate current," Mac explained, "flows through that portion of the filament
that lies between the 'B-minus' end and the point where the electrons take off for
the plate. In many tube checkers, this emission current during test is quite heavy,
being a husky percentage of the fifty-milliampere current that is normally supposed
to flow through the filament; and when this emission current is added to the filament
current, it raises the temperature of the filament considerably above normal. The
result is that one of these 50-ma. tubes will frequently show 'Good' in the emission
tester when its emission at the normal filament current is well below what it should
be."
"What do you do about it?"
"If there is any doubt in your mind, try substituting a new tube in the set.
If this makes a marked improvement, a new tube is needed, no matter what the tube-tester
says. With experience you will learn to detect a certain sluggishness or hesitation
in the swing of the meter pointer on one of these low-emission tubes; but for now,
just remember that a playing set and a substitute set of tubes constitute the practical
serviceman's Tester of Final Decision.
"And now let us take up the case of those radios which their owners say are too
hard on batteries. Most of these complaints, while made in all sincerity, are not
deserved. Some people have a most optimistic idea of how long batteries should last;
others simply forget when the batteries were purchased or how many hours the set
has been used since; and finally, never overlook the battery salesman's best friends:
small children who just love to turn on these sets and let them run when Mama and
Papa are not around.
"We cannot, though, dismiss all complaints as being unfounded; so here is what
I want done with everyone of these portables that has a reputation of being a battery-eater:
"Open up the positive filament and plate leads and insert current meters in each.
Then turn the set on and check the currents drawn. Compare these values with what
the set should draw, getting this information either from the service manuals or
by computing it from tube manual data. Then, with the meters still in place, turn
the set off and make sure the currents drop to zero. Finally, in the event the radio
is a three-way portable, see if the currents remain at zero when the set is playing
on a.c.
"If any of these tests reveal anything funny, find out why. If not, when you
give the set back to the customer, remind him that batteries are like human beings
in that they last much longer if they are given time to rest and recuperate between
periods of activity than they do if they are kept going steadily. If the set is
a three-way portable, strongly suggest that it be used on batteries only when an
a.c. outlet is not available. As a clincher, remind him always to be sure and get
fresh batteries, like the kind he always gets at Mac's Radio Service Shop!"
"Gotcha!" Barney grinned. "Tell me more!"
"Well, one thing you want to watch is to see that the chassis and batteries are
in place in the cabinet when you align the r.f. trimmer that is across the loop;
otherwise the loop will be seriously detuned when these items are placed inside
its field. This is especially true where the loop is wound on the inside of the
cabinet. If the loop is fastened to the back of the cabinet, this should be in its
normal position before adjusting the trimmer.
"In most sets, provision is made for doing this; but there are a few that offer
no porthole for reaching this trimmer with the chassis and back in their normal
operating positions. When the cabinet is of wood covered with airplane cloth, I
usually drill a small hole in the cabinet that allows me to adjust the trimmer and
then I close this opening with a snap button hole plug."
"And if the cabinet is made of a plastic material?"
"Sometimes there is room to use a screwdriver with a flexible shaft; but usually
I slide the chassis out until I can reach the trimmer, give it an eighth of a turn,
and slide the chassis back in. I keep doing this until the output meter shows the
maximum reading with everything in place. In order to keep my temper during the
tedious process, I play a little game in which I imagine various accidents befalling
the muscle-head who designed the set. Falling into a vat of boiling transformer
oil is one of the less-gruesome of these pictures."
"How's about those storage battery portables over there in the corner? Do they
have any peculiarities I ought to know?"
"Plenty of them; and not the least important is never to do what the owner did
to that one on the end of the bench: put in a heavier-than-recommended fuse. These
sets have a small voltage - dropping transformer and a couple of copper-oxide rectifier
assemblies hooked in a full-wave rectifying circuit to keep the two-volt storage
battery charged. Occasionally one of the rectifiers shorts out, and then the quarter-ampere
fuse in the primary of the transformer is supposed to blow and prevent damage.
"This man found the fuse blown, and a new 1/4 ampere fuse went out, too; so he
simply put in a one-ampere fuse and put the set on charge. By the time he saw the
smoke curling out of the cabinet the damage was done. Now he needs a new rectifier
and a new transformer."
"And a new 1/4 ampere fuse," Barney added; "but how can you check one of these
rectifier assemblies for short?"
"After you disconnect them, an ohm-meter will do the trick. In the conducting
direction, they will show almost a dead short; but they should show a resistance
of at least 300 ohms in the opposite direction. If they are shorted, of course,
they show a very low resistance in either direction. Incidentally, I never replace
just one of the rectifier assemblies at a time. It is good insurance to replace
them both when one goes bad.
"Finally, there is the matter of the two-volt vibrators. As you know, I am ordinarily
opposed to tinkering with vibrators, for as you read in that MYE Technical Manual
I told you to take home, these gadgets are precision-made and carefully adjusted
at the factory with special equipment that the service technician does not have.
However, on several occasions I have run into these two-volt vibrators that only
operated a few days and then stopped vibrating. By experimenting, I found that the
points that were normally supposed to be closed when the reed was at rest were not
quite making contact. A slight clockwise adjustment of a screw found at the base
of the reed restored the vibrator to action. Careful checks over periods of three
and four years showed no later failures of these adjusted units; so I have no hesitancy
about making this adjustment on vibrators that do not show excessive wear, burned
and pitted points, etc.
"I made up that little adapter there on the bench that allows the vibrator to
be in action while it is raised up out of its shield can until I can attach the
scope leads to it and also reach the adjusting screw. As you see, the adapter is
just an old vibrator base with a vibrator socket mounted on pillars about six inches
directly above it and with heavy leads connecting the lower pins to the corresponding
upper socket connections.
"The scope should always be used, for it allows you to set the adjusting screw
for the optimum pattern as shown in the MYE Technical Manual; on top of that, it
will show up any other troubles that may be present, such as a leaky buffer condenser."
"Hey, Boss," Barney interrupted, "I think I feel another attack coming on. Maybe
I had better go home now."
"Oh no you don't!" Mac exclaimed. "You just grab your soldering iron and see
what wonderful curative powers a little hard work has. It will surprise a fellow
like you who has never tried it!"
Posted June 29, 2020
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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