Mac's Service Shop: Two for One
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It is a rare occasion that Barney bests Mac when it comes to electronics prowess. Good natured back and forth often goes on between them during troubleshooting sessions and impromptu discussions about business practices, industry trends, and customer interactions; indeed, John Frye depends on it to make the stories interesting. This time, underling Barney exploits knowledge gained from a recently purchased electronics reference book to trip up shop owner Mac over which of two metals has the lowest resistance. Mac's choice is one many people would instinctively make - and be wrong as Mac was. Here is a table of electrical resistivity values for various metals and substances. Mac's Service Shop: Two for One
Outside it was a stormy March morning. The howling wind, laced with driving snowflakes, shook and rattled the heavy "Radio & TV Service" sign over the door of Mac's Service Shop. Inside things were a little stormy, too, at least with Barney, the red-headed Number Two man of the service establishment. He stood a couple of feet away from the bench and bent his lanky body at the hips into an uncomfortable-looking right angle so he could rest his elbows on the bench and cup his chin in his hands as he scowled fiercely at the little a.c-d.c. chassis in front of his nose. "Is all that muttering and sighing and flouncing around your subtle way of sending up distress signals?" Mac, his employer, asked with a teasing grin. "Of course not!" Barney retorted. "Can't a man stop to think without some joker's supposing he's asking for help? The complaint - well justified, incidentally - on this little fiend is that it picks up all sorts of code, teletypewriter, and other short-wave signals right along with the broadcast band it is supposed to tune. The condition is said to be much worse at night than in the daytime; so wouldn't you know the woman who owns the set is an insomniac who does most of her listening in the early morning hours!" "That figures," Mac said with a nod. "What have you done so far?" "Changed tubes. Checked the a.v.c. bypass. Checked the loop antenna for short-circuits or poor connections. Checked the oscillator grid voltage. Checked the oscillator waveform with the scope. Everything is perfectly OK." "Why did you check the oscillator waveform?" "I thought maybe it was putting out exceptionally strong harmonics that were causing the trouble. In such a case I'd expect the oscillator waveform to depart noticeably from a sine wave." "Sounds logical, but I doubt the oscillator is causing the trouble. Almost any oscillator will have strong enough harmonics well down in the short-wave region to heterodyne strong signals there to the i.f. frequency if these signals are allowed to reach the converter signal grid. Normally these high-frequency signals are kept from the grid of the converter by the selectivity of the antenna tuned circuit. This parallel-resonant circuit between the input grid and cathode of the converter is tuned to the broadcast band and has a very high impedance at its resonant frequency. Short-wave signals in the region of several megacycles should find a very low capacitive reactance through the tuning capacitor and so be short-circuited to the cathode. Let's radiate a signal from a loop of wire connected to the output of the signal generator and see if we can find what high frequencies are getting through." Barney set up the signal generator and a little tuning quickly established that signals in the vicinity of thirteen megacycles were the ones causing the trouble. When the chassis of the a.c.-d.c. receiver was touched with a finger, these interfering signals became much louder. Mac was studying the diagram of the receiver while Barney was running these tests. The input of the converter stage was a little unusual. The tuning capacitor frame was grounded to the chassis and the antenna loop was connected across the r.f. section of this capacitor. A small mica capacitor connected the hot side of the tuned circuit to the grid of the converter tube. The a.v.c. voltage was fed to this signal grid through a decoupling resistor, the bottom end of which was bypassed to "B-." A coil of wire in series with a 0.1-μf. capacitor was labeled an "r.f. filter" and was connected between the chassis and the "B-" point to which the converter cathode was returned. Mac took the grid-dip oscillator and its set of coils from a cabinet and put one of the coils in the instrument. Then he turned the receiver chassis over and pointed to a 0.1 μf. paper capacitor with a few turns of wire wound around one end. "See if you can find a resonant frequency for this coil," he said as he handed the grid-dipper to Barney. "I sure can!" Barney soon announced; "and it's at thirteen megacycles." "Fine! Now short out that little coil and let's see what happens." When this was done, leaving only the 0.1 μf. capacitor between chassis and "B-," the unwanted signals disappeared. Broadcast reception was just the same as before, but the annoying birdies were completely gone, even when the full output of the signal generator was fed into the loop of wire. "Now how did you know about that?" Barney demanded. "Because I ran into exactly the same thing on one of these sets not more than three months ago," Mac confessed with a chuckle; "and I'll admit it had me 'bugged,' as you would put it, for quite a while. I ran it down with the signal generator, the grid-dipper, and the diagram, just as you did. "You see that series-resonant circuit made up of the capacitor and the little coil of wire wrapped around it is supposed to be broadly resonant at the i.f. frequency so as to offer a very low-impedance path between the chassis and 'B-' at that critical frequency; but the joker is that the coil and its distributed capacity form another parallel-tuned circuit resonant at 13 megacycles. This acts as a barrier between any 13 megacycles signal on the chassis and 'B-.' On the other hand the low impedance the tuning capacitor presents to this high-frequency signal allows it to go to the converter grid, to heterodyne with a harmonic of the oscillator, and to pass right on through the i.f. amplifier along with the broadcast signals being received. Short-circuiting the coil or removing it allows the 13 megacycle signal to go to ground and so prevents the trouble." "You know," Barney said slowly, "I should have remembered that it's easy to get two resonant circuits for one combination of a coil and capacitor - one series-resonant and the other parallel-resonant. That's at the bottom of a lot of parasitic oscillation troubles that plague transmitters." "I am never quite easy about having to make a change in a manufacturer's circuit," Mac continued; "so I wrote the service department of the people who made the receiver. They sent back a service note mailed out to their dealers some months ago in which they described the self-same symptom and recommended that the coil be removed to correct it. I didn't recognize the set without its cabinet until I started studying the diagram; then I decided it would make a stronger impression on you if I had you track down the cause of the trouble step by step." "Gee, thanks loads!" Barney said sarcastically. "Now, Doctor, I have a question for you: of two equal lengths of wire of the same diameter, one of copper and the other of gold, which will have the least resistance?" "I suppose the gold one," Mac replied. "You're wrong!" Barney said triumphantly as he picked up a slender blue book from the bench, opened it, and pointed to a table of resistances of metals and alloys. "See; copper has a resistance of 10.37 ohms per circular mil foot as compared with 14.55 ohms for gold. Silver has 9.796 ohms, and it's the only one listed with less resistance than copper." "What's the name of the book and where did you get it?" Mac asked politely. "It's called 'Handbook of Electronic Tables and Formulas' published by Howard W. Sams & Co. Inc. and compiled by Donald Herrington and Stanley Meacham of the Same Engineering Staff. I picked it up at the distributor's when I was over there after parts. For a long time I've wished someone would come out with a book such as this for folks with feeble memories, like me. No matter how hard I try, I can't keep formulas for reactance, impedance, resonance, frequency-and-wavelength, etc., straight in my head. In fact, there's a whole raft of information involving formulas, wire tables, color codes, miniature lamps, transmission line characteristics, audio and video carrier frequencies of TV channels, and American substitutes for European tubes that I have to keep looking up over and over again; and that has meant going through a half dozen books each time to find what I want. Now, with this handy-dandy little volume right up here on the shelf where I intend to keep it, I can put my finger on what I want 'toot-sweet,' as we French say." "I just can't get over a penny-pincher like you buying a book," Mac muttered. "It will save me work, man!" Barney explained. "For example, suppose I want to know how much capacity I need to resonate a one henry choke at 1000 cycles. Do I use a formula to figure out the reactance of one henry at 1000 cycles and then use another formula to determine what value of capacity will have equal reactance at this frequency? I do not! I simply lay a straight-edge across the proper one of these reactance charts in the book, and I immediately see a capacitor of about 0.025 μf. will do the trick. Neat, huh? You wanta buy part interest in this book?" "Let me see it, you Shylock," Mac said as he took the book out of Barney's hands. "Hm-m-m, I see it has lots of math stuff in it, too. Here are trig, logarithm, and decibel tables as well as decimal equivalents of fractions, powers of ten, and algebraic operations and geometric formulas. They should help a fellow working with a formula when his math is a little rusty. And here is a whole bunch of stuff that used to be printed on the backs of composition books when I was in school: Measures and Weights and Metric Equivalents. Boy, does that take me back!" "Don't overlook that colored foldout showing what services occupy each portion of the spectrum from 10 kc. to 100,000 mc.," Barney suggested; "and there are lots of little goodies tucked away here and there in the book such as the amount of power consumed by various electrical devices used in the home, the proper shunts and resistors to use with d.c. meters to make them indicate desired voltage, current, or resistance, and a complete description of the time and frequency signals put out by WWV and WWVH." "OK, you've sold me," Mac said as he took out his billfold. "Here's half the price of the book, but let's have one thing clear: it stays right here at the bench where we both can use it during working hours. I don't want you lugging it home and leaving it in your ham shack. OK? "OK," Barney cheerfully agreed as he put the money in his pocket. "And if I ever get tired of service work I think I'll take up selling. I made a little bet with the boys down at the distributor's that I could make you pay for half of that book!"
Posted July 5, 2018 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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