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Mac's Radio Service Shop: Barney Turns Inventor
February 1950 Radio & Television News

February 1950 Radio & TV News
February 1950 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

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. BTW, the reference to "John's Other Wife" is a radio soap opera that ran in the mid-to-late 1930's.

Mac's Radio Service Shop: Barney Turns Inventor

Mac's Radio Service Shop: Barney Turns Inventor, February 1950 Radio & Television News - RF CafeBy 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:


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?" Mac hazarded.

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?" Mac challenged.

"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 the sides."

"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 of tubes."

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 speaker.

"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?"

Vacuum tube filaments in series - RF Cafe

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 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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