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BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...
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|January 1949 Radio & Television News|
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged.
Don't let the title fool you. This is not a "bees-birds-and-flowers routine" being provided to Barney by his boss, Mac. It turns out to be a brief introduction into the fine art of troubleshooting intermittent problems in radio and television circuits. As is usually the case, while the specifics of the scenarios Mac describes might not apply to your challenge at hand, the general philosophy always does. It is basically the old process of elimination where after rapping components mechanically and/or heating or cooling them in hopes of observing a tell-tale change in performance, the next step is to divide the suspected circuit portion in half (electrically, but sometimes also physically) and look in one direction. If the problem isn't there, then divide the circuit in the other direction in half and go there. Repeat until the problem is found.
One of my personal favorite first steps is to verify all mechanical connector interfaces (if any) are contacting properly. Clean with alcohol if possible, and burnish with sandpaper if appropriate, then plug and unplug the connections a few times, just to make sure proper seating is occurring. It is no exaggeration that up to 50% of the problems I have encountered with everything from wall-mounted light switches to computer hard drives have been remedied by cleaning connector interfaces.
By John T. Frye
A sudden January thaw had melted the snow into a slushy mess; and as Barney returned to work at Mac's Radio Service Shop after his lunch hour, he had to pick his way carefully around the soot-covered puddles standing on the sidewalks. The dazzling sunshine and the bright blue sky overhead, though, were the things that struck a responsive chord in Barney's naturally cheerful nature; and he was doing his whistling best to recall last summer's hit of "Woody Woodpecker" as he stepped inside the shop.
Mac was sitting on Miss Perkins' desk talking to her, and Barney had the uncomfortable feeling that they had been talking about him. What led him to this intuitive conclusion was the fact that they were still doing it. Mac was saying:
"But he has to know sometime. He can't go on being just a big, innocent, trusting lummox all his life. He is a big boy now, and it is better that he should hear things from me and get them straight than to pick them up goodness knows where."
Miss Perkins looked wistfully over her glasses at Barney standing pop-eyed in the door and said in her best soap-opera imitation" "Yes, I suppose you are right, but I can't bear to see that innocent, carefree expression go out of those lovely, lovely blue eyes. But go ahead. Take him into the back room and tell him all, but please, please close the door behind you!"
"Hey, look, you two -" Barney began to protest as Mac took him gently by the hand and led him into the service department. "I'm not so - I mean I know -"
"Yes?" Mac encouraged as he shut the service room door and leaned against it.
"Well, doggone it," Barney spluttered, "why are you giving me this bees-birds-and-flowers routine?"
"Whatever are you talking about, Barney my boy? I was just telling Miss Perkins that I thought it was high time you learned something about the seamier side of servicing, or intermittent radios," Mac explained blandly.
"Is that all!" Barney said with a sigh of relief as he stretched his lanky frame out on the service bench and waited with a stubby pencil poised over his dog-eared notebook.
"Don't dismiss the subject of 'intermittents' so lightly," Mac told him. "Radio men spend more time 'cussing' and discussing such sets than they do on any other subject. The discussing is fine, but I think the 'cussing' is rather shortsighted and foolish."
"Why 'shortsighted'?" Barney wanted to know.
"Well, if the servicemen would stop and think a bit, they would be grateful for the 'intermittents' Since these sets are the most difficult to repair and take the most experience and good equipment to solve, they separate the men from the boys in the service game. If the only troubles that radios ever had were burned out tubes and blown bypass condensers, anyone could learn to be a serviceman in a month, and all the equipment he would need would be an ohmmeter. It is the tough sets, such as the intermittents, that serve as a challenge and give a fellow a chance to prove just how good a serviceman he really is- or isn't!"
"What do you call an 'intermittent', Boss ?"
"That depends upon how long I have been working on it," Mac said with a slow grin; "but seriously, an intermittent is any set that suffers abrupt changes in the quality of reception at irregular and unpredictable time intervals. That phrase 'change in the quality of reception' may cover everything from slight changes in volume or tone-quality to the set's going completely dead; or it may be a noise that comes and goes along with normal reception. The time interval may be every few seconds or once or twice a week. The significant point is that the objectionable feature is only present part of the time."
"What's the most common type?" Barney asked.
"I'd say that the set that changes volume abruptly shows up the most often. This is the radio that the customer usually brings in with the remark, 'This set will be playing along just fine until someone flips on a light in the house or the refrigerator starts up, and then it will blare out and scare you half to death. You turn it down, and then a few minutes later the volume drops so low you can't hear it.'''
"I can't see what makes such sets so hard to fix."
"You will, my boy, you will!" Mac promised. "For one thing, almost any part that is used in a radio can cause just such a symptom: a bad tube, a defective coil winding, a poor resistor, or almost any condenser in the circuit can be the culprit. It is like having an oleomargarine salesman sandbagged at a dairymen's convention; there are lots of suspects!"
"Another thing that complicates matters," Mac went on, "is that the intermittent is touchier than a sunburned rattlesnake. Often the slightest jar, the smallest voltage surge, or the least circuit-loading, such as is caused by using a voltmeter or a signal tracer, will cause the low-volume condition to disappear for the rest of the day."
"What is the best instrument with which to tackle such a set?" Barney wanted to know as he scribbled away at his notebook.
"I use every instrument in the shop on some of them," Mac replied, "but I honestly believe that the best service instrument of all is good old-fashioned horse sense. The best way to find the cause of an intermittent condition is to corner it. By that I mean that you must use a process of elimination. Let's take one of those sets that hop up and down in volume as an example:
"The first thing to do is to determine whether the trouble lies in the r.f. or the audio section. A signal tracer, a v.t.v.m., a scope, or even a pair of headphones in series with a condenser across the detector output will tell us this. If the signal drops at this point when it does in the speaker, our trouble lies in the r.f., oscillator, or i.f. circuits; if not, we know it is in the audio section. In the latter event, we can move our testing equipment from point to point through the audio amplifier until we reach a place where the signal voltage rises and falls in step with the volume issuing from the speaker. Then we work backward from this point at which the volume does change and forward from that at which it does not - just like two baseball players closing in on a runner caught off base - until we have narrowed down the separation of these two points to the smallest possible circuit distance - say the opposite ends of a coupling condenser. Then that condenser has to be the cause of the trouble; and incidentally, coupling condensers which have poor connections between leads and foil are one of the most common causes of intermittent sets."
"What if the trouble is ahead of the detector?"
"Use the same 'closing-in' tactics, but your instruments have to indicate r.f. and i.f. voltages. That means you must use an r.f. probe on the v.t.v.m. or scope, or you can employ the signal tracer. Plate, screen, cathode, and a.v.c. bypass condensers; defective windings in r.f., oscillator, or i.f. coils; and defective tubes are some of the most common causes of abrupt changes in the amount of signal voltage delivered to the detector tube."
"I can see how that would work if the set kept cutting in and out quite often, but what are you going to do if it won't cut out when you have it on the bench?"
"The only thing you can do is try to make it cut out," Mac said. "I usually start by tapping the tubes, especially the ones with grid caps, such as the 6A7's, 6A8's, 6Q7's and 75's. Lots of times striking the caps of these tubes smartly on top with a bakelite rod will cause the volume to change with each blow; but you want to make sure that the change is not caused by the jarring of some other defective unit instead of the tube itself. If tapping the cap of a new tube has no such effect, you can be pretty sure you have found the villain.
"If the tubes seem OK, I next try to find a bad condenser. I feed a strong unmodulated signal into the antenna post from the oscillator, tune this signal in on the receiver so that I hear the characteristic rushing sound, and then gently tap the various bypass and coupling condensers. Notice I say 'gently.' If you go yanking the condensers around roughly, you will have a lot of little intermittent conditions instead of just one. When you tap a condenser that has a poor connection between lead and foil and allow it to vibrate on its leads, you can usually notice a chopping-up of the rushing sound coming from the speaker. A good condenser will give forth nothing except possibly a slight microphonic sound."
"Do you have any other third-degree methods to make these intermittents sing?"
"Yes, most of them are affected by heat; so I use an infrared lamp to warm up suspected parts. These lamps have a small intense heating area that is fine for this job. Also, I try both low and high line voltages on the set, using the tapped isolation transformer for accomplishing this. Many a time I have first struck the trail of an intermittent condition by simply 'wracking' the chassis a little with my hands. Thumping the chassis in various spots with a little rubber hammer such as a doctor uses for testing reflexes is also helpful."
Mac was interrupted by the bellow of the one o'clock whistle at the laundry across the street.
"Well," he said, "the subject of intermittents is far too big for us to cover at one session; so suppose you work on the sets I think will fall under what we have talked about today for the rest of this month; then we will renew the discussion and talk about some other kinds of intermittents and some other methods of tracking down the causes."
"That's oke with me," Barney agreed as he closed his notebook and slid from the bench; "and in return, if there is ever anything you want to know about the bees or the birds or the flowers, just ask old Uncle Barney here!"
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of no 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.
Posted November 12, 2015