Mac's Radio Service Shop: The Time Is Now |
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Unlike many electronics enthusiasts including hobbyists, salesmen, servicemen, and commercial and domestic users, Mac McGregor was an early-on believer of the ultimate replacement of vacuum tubes by transistors. Only a decade old and not yet adopted by a lot of manufacturers (to their ultimate demise in some cases), transistors were fighting a major battle to gain acceptance and trust by the public. Not only were transistors still more expensive than an equivalent vacuum tube, but the reliability was not as good - most times due to designers not properly accounting for their special needs for protection against voltage extremes. Once the price of transistorized products fell into parity with their predecessors, consumers quickly adopted the products because of the markedly smaller sizes and lower power consumption. Portability for battery-powered radios and other entertainment-related items was a huge selling point for transistorization, to which magazine advertisements of the era attest. In this episode of Mac's Radio Service Shop, Mac is explaining the utility of his newly acquired Battery Eliminator for use in servicing automobile and household radios that run off of a 6- or 12-volt deep cycle (i.e., lead-acid) battery. Mac's Radio Service Shop: The Time Is Now
Aha!" Barney exclaimed as he came sailing into the service shop out of a gusty March morning and found Mac, his employer, bent over a piece of test equipment on the bench; "new gadget, huh?" "Yep," Mac answered as he plugged the cord of the instrument into a socket. "This is a replacement for our old battery eliminator. I bought it in kit form and put it together last night." "What was the matter with the old one?" "Nothing except that it's outmoded. As you know, we've been hooking a battery in series with the eliminator to work on twelve-volt sets but that arrangement leaves a lot to be desired. For some time I've been intending to buy a new eliminator that would handle both six- and twelve-volt radios, but I was holding out in the hope someone would build in the extra filtering needed in powering hybrid or all-transistor sets. In the past few months several manufacturers have come on the market with just such instruments; so I no longer had an excuse." "I get you. Since the battery voltage furnished a hybrid or transistor receiver is equivalent to the plate voltage in a tube radio, it must be well filtered to prevent hum." "Your little gray cells are working today!" Mac applauded. "As you can see, this eliminator has continuously variable output voltages of either 0-8 or 0-16 volts. Output voltage and current are monitored by the two meters. Notice there are two sets of binding posts, one on each end of the panel. Those on the left provide moderately filtered output, with about 2% ripple, for use in battery charging or with vibrator-type sets. You can take continuous currents of ten amperes at six volts or five amperes at twelve volts from them. The posts on the right furnish five amperes at either voltage with very low ripple." "How is the ripple reduced?" "By passing the moderately filtered output through an extra L-section filter consisting of a heavy choke and a 10,000 μfd. capacitor. This brings the ripple down to less than 0.3%. I am not just taking the manufacturer's word for this; I measured it with the scope; but let me show you the difference." As he said this, Mac connected a high-wattage, low-resistance resistor across the right-hand output terminals and adjusted the eliminator so that it was furnishing about four amperes of current to the resistor. Then he connected a large capacitor to one of the tip leads of a pair of earphones and touched the other earphone lead and the free end of the blocking capacitor to the two output terminals on the left side of the panel. A very noticeable low-pitched hum came from the phones. He transferred the connections to the right-hand terminals. "Hey," Barney exclaimed as he pressed the muff-type earphones tightly against his head, "I can't hear the least trace of hum!" "And you shouldn't," Mac said as he turned off the eliminator. "That ripple level must be very low if you are going to use the eliminator to power all-transistor sets as I expect to find in many cars next year. Of course, when the variable voltage feature is not needed, we can always cut the hum still more and provide very good voltage regulation by floating a battery across the output of the eliminator, but I don't think that will be needed. In a few cases in working with extremely high-gain transistor amplifiers we might want to cut the ripple to an absolute minimum. Such circuits would not require any appreciable current; so it would be very simple to use an outboard filter section consisting of a high-inductance, low-resistance choke and a whopping big capacitor of low voltage between the eliminator and the transistor circuit. About the only time you would need such extreme filtering would be in experimental work, but it's nice to know a simple addition to the eliminator will provide very nearly pure direct current.
"Make sure, too, that the output transistors are working into a proper load at all times. Running a signal into them with the speaker disconnected is a fine way to ruin them. And you've got to be careful what you do with your test prods when you're working on one of these sets. They are as touchy in that respect as battery portables, but for a different reason. With the portables, you have to be careful that you do not accidently short the high plate voltage across the fragile filaments and blow them. With the transistor sets, it's not the voltage as much as the almost unlimited current provided by the battery that you have to treat with respect. A test prod that accidentally grounds the base of an output transistor can ruin the transistor while you are batting your eyes. That's why you must never try to use the old circuit-disturbance type of troubleshooting in which you ground a tube grid and listen for a click in the speaker in working on transistor sets. Ground a base and the click you hear will probably be the last." "I'd think that a battery-eliminator, being fused and providing only a limited amount of current, might provide some measure of insurance against that kind of damage." "It does, but don't depend on it. A charged 10,000 μfd. capacitor can put out a lot of current for a few milliseconds and a fuse takes an appreciable time to melt. Let's just say you'd be less likely to ruin a transistor when the set is being powered by an eliminator than you would when a battery was furnishing the current." "Sounds to me as though we're getting pretty transistor-minded all at once." "That we are and I think this is precisely the time we should. We have to keep in mind that the attitude of a practical service technician toward new electronic developments has to be considerably different from that of an experimenter or hobbyist. The latter start working with the new device as soon as they can get their hands on it, but the wise service technician knows that he cannot do this and still take care of his bread-and-butter job the way he should. He has just about all he can do to keep abreast of the new circuits that are constantly being thrown at him by radio, TV, and hi-fi manufacturers. When and if the new development proves itself and is incorporated in the equipment the technician must service, then it is high time he becomes thoroughly familiar with its possibilities, its limitations, and its general behavior. "As far as transistors are concerned, I believe that time is right now. We both, of course, have kept abreast of the theory and general development of transistor circuits through the fine magazine articles that have been published on the subject, and we have had no trouble taking care of the few transistor radios that have drifted into the shop so far; but from here on in we can expect to meet transistors more and more often. Now is the time to get any special equipment needed to service transistorized equipment, and now is the time to start getting all the practical experience we can with transistors. What's happened to your beagle instincts? Over there on the end of the bench is a new transistor-tester you haven't even noticed yet!" "Well so it is! I have read, though, that transistors fail very seldom as long as they are not abused. In fact, it is said that in a tube set that won't work, the first thing you suspect is a bad tube; but in a transistor set, the transistors themselves are the last thing to test." "That's probably true, but I expect to use the transistor tester in a lot of ways. For one thing, it is often necessary to use matched transistors and this tester will allow us to select such matched pairs. Also it will permit us to sort through our stock of transistors - pretty limited, I'll admit, at present - and select the best transistors for particular jobs. For example, we can sort out low-noise units for the front end of amplifier circuits, save the high-gain transistors for places where gain is important, and so on." "The whole discussion makes pretty good sense," Barney admitted. "I was reading the other day where a Mr. Fancher of General Electric said that while transistors are used in only about 12% of new electronic equipment built this year, they will go into 80% of the equipment built in 1967. Looks like it's time we got on the bandwagon." "Check! I'm also collecting all the transistor data I possibly can from the various manufacturers," Mac said as he waved at a new shelf on the wall. "Everyone of them puts out some printed information on their products and various applications. These vary from simple characteristic leaflets to complete booklets on the theory of transistors, suggested circuits, etc. Every time I see something new is published I order it." "I see some books on that shelf that are not concerned with transistors," Barney observed. "That's right. I'm trying to select a bunch of books that will help us with the 'dogs.' Each book covers just a specialized portion of a circuit or piece of equipment. You'll find them on servicing sweep circuits, video circuits, a.g.c. systems, sync circuits, TV tuners, hi-fi systems, tape recorders, and record changers. When we get a real tough problem in the shop and know that the trouble must lie in a particular portion of the equipment, we need exhaustive information on the problem to make sure that we are not overlooking a possibility. That is where the books are well worth their cost. They provide a concentrated review of everything we should know about that particular circuitry or mechanism. I know from experience that leafing through one of these books can often spotlight a source of trouble I was completely overlooking. I intend to keep adding to that shelf. For example, I intend to get a book on color convergence procedures and another on industrial electronics as soon as I can." "Good," Barney applauded. "I can see where reading through one of these books will be just like having someone say, 'Did you try this?' and 'Did you try that?' But where was your imagination when you built that shelf? Why didn't you build it in the shape of a dog house?"
Posted February 14, 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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