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Mac's Radio Service Shop: Portable Patter
April 1950 Radio & Television News Article

April 1950 Radio & Television News
April 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.

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

Mac's Radio Service Shop: Portable Patter, April 1950 Radio & Televsion News - RF CafeBy 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 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 Electronics 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.

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