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Mac's Service Shop: Oscillating Canine
July 1960 Electronics World

July 1960 Electronics World

July 1960 Electronics World Cover - RF Cafe  Table of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

These Mac's Service Shop techno-stories tutorially mix real-world entities with tips on troubleshooting, repair, alignment, customer service, industry news, and other relevant topics. Although not directly related to this "Oscillating Canine" episode in a 1960 issue of Electronics World magazine, mention is made of the Bon Ton Department Store. It could possibly be a reference to the modern-day The Bon-Ton store since according to Wikipedia the company was founded in 1898 (renamed Bon-Ton Millinery in 1902). The reference is actually to the store owner, who engaged Mac to convert a radio receiver to pick up weather station broadcasts from his airplane. The story's title has to do with another subject - a hard-to-get-rid-of oscillation in a transistorized radio. It is one of the first instances of solid state components and the printed circuit boards to which they are attached. Barney laments the difficulty of troubleshooting the configuration as opposed to the good 'ole vacuum tube circuits that used point-to-point wiring in the chassis. It was an era of major transitions in the electronics industry.

Mac's Service Shop: Oscillating Canine

Mac's Service Shop: Oscillating Canine, July 1960 Electronics World - RF CafeBarney stopped short in the door of the service department as he heard a droning voice, mixed in with slow-speed code, emanating from a three-way portable sitting on the end of the service bench.

"What station's that?" he demanded of Mac, his employer, who was looking down at the receiver with a smile of satisfaction.

"It's the aviation range station at Center City operating on a frequency of 266 kilocycles," Mac explained. "This station puts out a continuous weather report for planes in this general area. The reports are on tapes that are changed at frequent intervals. That 'dit-dah, dit-dah, dit-dah' in the background shows we're not in the center of the beam."

"Just a minute, just a doggoned minute!" Barney interrupted. "That's a broadcast receiver, pure and simple. How come it's pulling in that low-frequency station - and why is it?"

"The receiver belongs to Mr. Smith, owner of the Bon Ton Department Store. He is a flying enthusiast with his own plane. As such, he likes to keep an eye on the weather, even when he is down at the office. When conditions are particularly good, he can sneak off for an hour or so of flying. With this in mind, he asked me if I could convert one of the many broadcast receivers he had lying around the house so that it would pick up this weather report.

"At first I shied away from the idea. I was thinking in terms of trying to make the broadcast receiver tune the whole low-frequency band from 200 to 550 kilocycles; and you know what a complete re-doing of the front end that would take; then, too, I was concerned with the idea an aviation weather receiver should be very, very dependable because the flier's life might depend on it. But Mr. Smith explained he only wanted to receive just this one station and that the receiver would be used only in his office or at his home. His plane was fully equipped with all sorts of standard aviation radio equipment. This put a different light on the project, and I agreed to see what I could do.

"Actually the solution turned out to be ridiculously simple. After putting the receiver into good operating condition-it needed new filter capacitors - I had only to pad the oscillator and antenna circuits down to where the 266 kc. station could be picked up with the receiver dial sitting on 600 kc. I picked this dial setting so as to have a little tuning leeway and yet be able to use most of the capacity in the receiver's tuning capacitor.

"For padders I used the double compression trimmers out of a discarded i.f. transformer. These had a measured maximum capacity of about 200 μμf. each; and one was more than enough to lower the oscillator frequency from 1056, where it sat to tune in 600 kc., down to 722 kc., where it had to be to produce a 456 difference beat with 266 kc. However, since you have to quadruple the capacity of a parallel tuned circuit to halve the resonant frequency, I knew I'd have to use considerably more extra capacity than the 200 μμf. available in the other trimmer to lower the frequency of the antenna circuit from 600 to 266 kc. A little cut-and-try procedure, using the grid-dip oscillator to keep track of resonance, revealed that a 0.001-μf. silver mica in parallel with the 200-μμf. trimmer and the set's tuning capacitor did the job in fine shape."

"Having to listen to that program very long would drive me nuts," Barney said as the taped voice, punctuated with the slow code, droned on and on. "It would be easy, though, to use a d.p.s.t. switch to cut out the extra capacities and restore ordinary broadcast reception."

"Yes, and I mentioned that to Mr. Smith; but he said it wasn't worth the trouble to him. He already had lots of broadcast receivers," Mac answered.

"Well," Barney said with a deep sigh, "you've solved your problem, but I've still got mine."

"You mean you're still working on that little transistor set you started on this time yesterday?"

"Yes, and at the rate I'm going, I'll be still working on it a week from now.

That little monster is a real live dog."

"The only trouble is that it oscillates, isn't it?"

"Oh yes, that's the only trouble," Barney answered sarcastically; "but I've got news for you: running down oscillation in a printed circuit transistor receiver is a lot different than hunting the same difficulty in a tube receiver."

"From what I've heard, it's an r.f. or i.f. rather than an audio oscillation."

"True. You get a heterodyne whistle on every station."

"I suppose you started by changing the battery."

"Right. I'm hep to the fact that some of the cheaper receivers depend on the low resistance of a good battery to tie signal-carrying return circuits down to ground. When the battery is partially exhausted, its internal resistance goes up and permits coupling between some of the return circuits that can cause oscillation. A new battery made no difference."

"Then I suppose you paralleled all the decoupling capacitors. one at a time, with a unit you knew to be good."

"Yes, and since this is a well-designed receiver, there were several of those capacitors. I had to remove the speaker to get at some of them, but I never missed a one. I'll stake my reputation as a hot-shot electronic technician that there are no open capacitors."

"At the moment you wouldn't be putting up much," Mac said with a wry grin. "About next, I imagine you checked the alignment."

"Your crystal ball is clear today! I went over the alignment two or three times, and there is nothing wrong in that department."

"How about the possibility that the printed circuit board was cracked so as to produce a break in the grounding lead? That would leave part of the grounded circuit floating."

"I thought of that, too," Barney said triumphantly, "and I jumpered across various portions of the ground bus. It's OK."

"H-m-m-m-m, how did a careful voltage check turn out? Maybe some of the bias resistors have changed value and altered stage gain."

"All voltages are right on the button, but I did not stop there. I pulled all the transistors from their sockets and did a complete resistance check. The resistances at all check points provided in the service data are well within five per-cent of values specified."

"This is getting interesting," Mac said as he reached over and picked up the circuit diagram. "Looking at this, I can see one very nasty possibility: the i.f. stages are neutralized, and if one of those neutralizing capacitors opened up, or if this resistor in series with a neutralizing capacitor opened up, the stage would oscillate quite merrily."

"You've got to do better than that," Barney gloated. "I used a little variable trimmer capacitor in place of each of those little ceramic neutralizing capacitors and varied it back and forth without helping the oscillation a bit. And there's nothing wrong with that resistor, either. Unsoldering those neutralizing capacitors and soldering them back is quite a tedious job, I might mention."

"I can imagine," Mac muttered as he continued studying the diagram. "Of course we could be up against a defective transistor, but I doubt it."

"The transistor checker doubts it, too," Barney said. "They all check good with normal gain and no excessive leakage."

"Is there any way you can stop oscillation and still get reception?"

"Yes, if I put my finger on the collector or base of the mixer stage the oscillation stops and the local station can be heard."

Mac was silent for several minutes as he scrutinized the diagram. Then he put it down and picked up the little chassis and looked it over closely. "Hand me a 2.2k resistor," he finally said to Barney.

Carefully he shaped the resistor leads until he could bridge the resistor across a couple of points on the printed circuit board; then, holding the resistor in place, he turned on the receiver. It played perfectly, without a sign of oscillation, until he removed the resistor. When he did that, the oscillation returned.  

"Well I'll be!" Barney exclaimed with a mixture of relief and disgust in his voice. "I was so sure those resistance checks were all correct."

"They probably were; but this open resistor - if it is open - would not show on those checks," Mac answered as he plugged in the pencil soldering iron.

"Take a look at the diagram. The set has an r.f. stage ahead of the mixer, but only the antenna circuit and the oscillator circuit are tuned with the tuning capacitor. The collector of the r.f. stage is coupled to the base of the mixer through a capacitor. Now notice this parallel-tuned circuit in the collector lead of the r.f. stage. See the coil resistance is given as 19 ohms. But notice, too, that this 2.2k resistor is actually connected directly across the tuned circuit. It 'loads' the circuit. lowers the 'Q,' and broadbands the response. With the resistor in place, the r.f. stage is probably designed to deliver uniform response across the whole broadcast band; but when the resistor opens, the 'Q' of the tuned circuit and the impedance at the input of the mixer stage shoots up sharply while the frequency response narrows - which makes things dandy for the mixer stage to oscillate at the i.f. frequency." As he finished speaking, Mac lifted the unsoldered resistor from the printed circuit board and placed the ohmmeter test leads across it. The meter pointer did not even flicker. "It's wide open all right," he said as he started soldering the new resistor in place.

"An open 2.2k resistor would never show across 19 ohms," Barney mused; "but what ever made you suspect that resistor might be open?"

"Your excellent, methodical procedure had just about eliminated everything else; and when you said you could kill the oscillation by putting your finger on the base or collector of the mixer, I started looking for something that would normally load a portion of the circuit as the presence of your finger did. That line of reasoning pointed squarely at the quarter-watt resistor. Incidentally, I've noticed quarter-watt resistors are more prone to open-circuiting than are half-watt units, even when they carry no current."

"Well. I wish I'd found the trouble myself, but I'm happy it's been found." Barney announced. "Ain't it a great feeling when a problem that's been deviling you for a long time is finally whipped? I feel exactly like a cork that's been held under water for a long time and has finally been released so it can pop to the surface."



Posted January 19, 2023

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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