It has been a long
time since I heard this saying: "Well, they always say that if you want to find
out the best and easiest way of doing something, just put a lazy man at the
job." Mac McGregor offered that line to his service shop technician Barney - in
jest of course - when Barney explains his million dollar invention idea for a
fool-proof vacuum tube tester that can be used by just about anyone. Mac's Radio
Service Shop creator John Frye often used the monthly techno-drama to introduce
some good ideas for new inventions and/or new methods for troubleshooting
problems. Somewhere along the line I think I have seen an advertisement for a
tube tester that used the automation concept dreamed up by Barney. Amazingly, I
discovered a typo error in the schematic referenced in the article.
Mac's Radio Service Shop: Barney Turns Inventor
By John T. Frye
The icy February wind carried little invitation to loiter in the great out-of-doors;
and Mac, back from lunch, stepped right briskly through the door of his radio service
shop. He was greeted by Miss Perkins, the office girl, with an admonishing finger
raised to her lips in what was definitely a "shushing" gesture. With the other hand
she pointed dramatically to a crudely scrawled placard fastened with Scotch Tape
to the closed door of the service department. It read:
QUIET! INVENTOR AT WORK!
Wondering what devilment his red-headed apprentice was up to now, Mac tiptoed
across the room and soundlessly inched the door open. There at the bench sat Barney,
his elbows planted on each side of a diagram-covered sheet of paper in front of
him. Both of his bony hands were tightly clenching handfuls of his sorrel thatch,
and his freckled face was screwed up in a look of agonized concentration. Upside
down on the bench at his left was the chassis of the tube checker which had been
removed from its case.
"I hate to disturb you, Mr. Inventor," Mac said softly; "but why is the tube
checker lying there with its inner workings so immodestly exposed to the vulgar
public gaze? Something the matter with it?"
Barney slowly turned around to confront Mac with the glazed eyes of a sleepwalker.
"Oh no," he said dreamily; "I was just looking to see - Say!" he suddenly exploded
as his eyes focused on Mac's face, "I've got it! I've got it!"
"Yes, I rather suspected all, along that you had it," Mac said soothingly; "but
we will keep it a secret between just us two. No one else need ever know. Most of
the time you act perfectly normal - "
"I mean I have just discovered a marvelous invention," Barney interrupted impatiently.
"A plastic coating on an all-day sucker to make it last two whole days, perhaps?"
Before replying Barney carefully shut the door of the service room and thrust
a twisted bit of paper into the keyhole. Then he approached Mac and triumphantly
announced in a hoarse conspiratorial whisper that could be heard out in the street:
"A self-service tube checker!"
"Oh no! Not that!" Mac cried in quick alarm. "We have enough trouble now patching
up the sets that customers have tried to fix themselves without being forced to
stand here helplessly and watch them burn out their own tubes."
"But that is just the point. My tube checker is foolproof."
"Even against the cute helpless little woman who simply can't understand why
she can't get John's Other Wife when the bandswitch is in the short-wave position?"
"Even against her," Barney boasted. "This checker has no switches to throw, no
dials to turn. All you do is take a stiff cardboard card that has the number of
the tube you want to test printed on it in big letters and push that card into a
slot in the checker. When the card is pushed clear in, a pilot lamp lights up behind
the right socket. You simply put the tube into that socket and watch the meter hand
to see if the tube is 'good,' 'bad,' or 'doubtful.' "
"Sounds wonderful - too wonderful," Mac said skeptically. "How does it work,
with atomic power?"
"Nope; the secret of the whole thing lies in several little holes punched in
exactly the right places in the card. When this card is pushed home in the slot,
several rows of spring-actuated 'fingers' rest against it. The holes in the card
allow the rounded ends of certain of these fingers to drop into them. The movement
of these fingers opens or closes contacts that do the same things you do on an ordinary
checker by throwing switches and twisting knobs."
"Hm-m-m," Mac hm-m-m-ed, beginning to show some genuine interest. "How are you
going to replace the variable resistors?"
"By using a multi-tapped resistor with the taps being connected to one row of
the fingers," Barney answered promptly.
"But the wrong fingers will be dropping into the holes as the card is slid in
or pulled out. Won't that cause trouble?"
"No, because only the very last thirty-second of an inch of travel of the inserted
card turns the checker on. The instant you start to pull the card out the instrument
is automatically turned off.
"Just think of the advantages!" Barney rushed on. "When the customer can test
his own tubes, he will feel confident he is getting an honest check. You get his
business without having to lose time checking his tubes. Keeping the checker up
to date is as easy as pie. When a new tube comes out, all you need is a new card.
The checker will be fine insurance against the mistakes that even servicemen make
now and then in operating tube testers. Think what a boon it will be to busy clerks
in radio stores. Why it will -"
"Whoa there, Nelly! Slow down!" Mac commanded. Then he went on more gently: "Red,
it could easily be that you have yourself a good idea there; but before you get
too excited, try sleeping on it. Wait and see how it looks in the morning. Then,
if it still looks good, go ahead. I'll help you all I can. But what ever started
you on this inventing binge in the first place ?"
"You know old man Porter, the retired railroader who lives on Bethel Street?"
"You mean old 'Packrat' Porter who boasts that he never throws anything away?"
"The same! Well, he brought down a market-basket full of old tubes for me to
test right after you left. There were exactly thirty-nine of them, and most were
of the old-fashioned slow-heating type that take forever and a day to warm up. Boy!
did he have some oldies in that mess! He got real annoyed because there were a couple
I could not test. They seemed to have the filament leads brought out to pins on
"Old Kellogg tubes!" Mac exclaimed in the tone of fond reminiscence that a man
usually reserves for speaking of an old flame. "He must have had some relics."
"By the time I waded through that basket, I decided there ought to be an easier
way. That is when I started in venting."
"Well, they always say that if you want to find out the best and easiest way
of doing something, just put a lazy man at the job," Mac gently jibed; "but what
did you find out about that new customer's radio?"
"That's it playing there on the end of the bench. A 50L6 was out. She says that
in the last two months she has put in three 35Z5's, and this makes the second 50L6.
Yet most of the time the radio plays OK. Once in a while, though, she says it will
kind of die away for a few seconds and then come back. She noticed, too, that when
this happens the dial lamp flickers. Probably the 50L6 filament was intermittent
for a while before it went clear dead. The set sounds perfectly all right now."
"Sounds logical except for one thing," Mac said with a frown. "That does not
account for so many tubes going out in such a short time - especially the same kind
He picked up a little rubber hammer such as doctors use to test muscular reflexes
and struck each of the tubes sharply from several different angles. When he struck
the 12SK7, the radio developed a sudden hum that slowly died away - along with the
music. At the same time the dial lamp and the filaments of the 50L6 and the 35Z5
grew much brighter. A second sharp rap on top of the 12SK7 returned the dial lamp
to normal brilliance, and a few seconds later music started coming again from the
"That 12SK7 cathode is shorting out to the heater," Mac said in answer to the
mute question of Barney's arched eyebrows.
"That explains the hum," Barney agreed; "but what causes the filaments of the
glass tubes to brighten up?"
Fig. 1 - Vacuum tube filaments in series. Note that this
drawing incorrectly labels two points as "X." Per Mac's explanation, the "X"
between the 12SA7 and the 12SK7 should be labeled as "Y."
Before answering, Mac sketched the diagram of Fig. 1 on the blackboard at the
end of the bench.
"As you know, the tube filaments are all in series. Notice that the 12SK7 is
in the middle of the string. In this set, the 12SK7 cathode goes directly to the
chassis, as does one side of the line. When the filament of the 12SK7 shorts to
the cathode, it is just the same as though you placed a jumper from point 'X' to
point 'Y.' Instead of the line current going through all of the filaments, it just
goes through the 50L6 and the 35Z5. As the short first occurs, it causes a hum to
be fed through to the speaker; but as the filament of the bypassed 12SQ7 cools down,
both the hum and the music die away.
"Ordinarily, this short is brought about by the expansion of the hot 12SK7 cathode
and filament. It is probably close to the 50L6 end of the filament; so this allows
the remainder of the 12SK7 filament to cool down after the short has happened. The
contraction that accompanies this cooling relieves the short. That is why she said
the set would die away and then come back by itself."
"And I suppose the extra current that goes through the glass tubes when the short
happens is what accounts for their short life. Get it? 'short happens' 'short life!'"
"Yes, I get it," Mac said, holding his nose, "and it ought to be buried. You
had better go outside and air off a while after that pun."
"You'll be sorry you talked to me like that when I am wallowing in the government
lettuce I will get for my invention," Barney warned.
"Yes, and you will be sorry if I catch you forgetting to check all the tubes
carefully for shorts in an a.c.-d.c. receiver that seems to be exceptionally hard
on filaments," Mac countered as he replaced the 12SK7 with a new tube from the bin.
Posted July 31, 2020
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive 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 Radio & Television News
magazine (which itself started as simply Radio News), and then changed
its name to Mac's Service Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney
is his eager, if not somewhat naive, technician assistant. 'Lessons' are taught
in story format with dialogs between Mac and Barney.