February 1949 Radio & TV News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Mac McGregor, owner of Mac's Radio Service Shop, can always be counted on to provide his apprentice technician, Barney, with a lesson from his own life-long attendance at the School of Hard Knocks. Barney is your stereotypical young buck whose level of seriousness needs occasional alignment, just as do the radio and television sets he services. In this episode, I can't find where Mac actually solved the intermittent electrical condition believed to be causing the problem - weird. The "Mac's Radio Service Shop" series ran in Radio & Television News magazine for many years prior to a similar electronics story series called "Carl & Jerry" that appeared in Popular Electronics. Both were created by consummate storyteller John T. Frye.
Intermittents Still Pursue
By John T. Frye
When Barney, the red-headed apprentice of Mac's Radio Service Shop, had gone into the chassis-cleaning booth twenty minutes before, there had not been a single wire recorder in the shop; but when he came out, Mac was busily engaged in taking one out of its case.
"Doggone it!" Barney exclaimed, snapping his fingers, "I wish I'd seen her!" .
"What 'her'?" Mac grunted without looking up.
"Why, the beautiful creature that persuaded you to run her wire recorder around all of these other sets stacked up here! That gal really must have had something."
"For your information, Junior, this wire recorder belongs to the young minister of that church on Ninth Street. I am giving it priority because of the use he puts it to. Each Sunday he makes a recording of the entire church service. Then, during the week, he takes it around to the homes of the sick and aged members who cannot get out and lets them go to church right in their homes. Several of these old people are quite hard of hearing, and he wants me to install a phone jack for their benefit."
"Hey, that's swell!" Barney exclaimed; "that's really using the old noodle."
"It certainly is," Mac agreed, "and speaking of 'using the old noodle,' as you vulgarly put it, I think it is about time we had another chalk-talk on intermittents."
With a flourish Barney whipped out his beat-up notebook and perched himself on the end of the bench. "Yes, sir; whenever you are ready, sir," he invited with mock deference.
"Well," Mac began, still working on the recorder and pausing now and then when what he was doing required a little extra attention, "first I want to impress on your alleged mind that the amount of time that elapses between the time the set is turned on and the time at which the erratic behavior starts is very informative. Most intermittent conditions are caused by expansion of defective parts; and this expansion, in turn, is caused by heating. Since not all of the parts warm up at the same time, but, instead; take on heat according to a definite sequence, you can make a pretty shrewd guess at where the defective part is by how long it takes for the trouble to start."
"For example, the tubes heat up first; then the resistors increase in temperature; gradually the heat spreads through the chassis and warms up the coils and condensers. The proximity of a particular part to a tube or hot resistor must also be taken into account. In general, though, if your trouble begins as soon as the set is turned on, be especially suspicious of tubes and resistors. The filament of an a.c.-d.c. tube that opens up as soon as the filament is hot and then reestablishes contact when it cools down is a well-known example of this. Usually, but not always, this defect will show up in the first five minutes of operation.
"Another case that is fairly common is that of the set that has a lot of noise and abrupt changes of volume when it is first turned on but that settles down and plays faultlessly after about fifteen minutes of operation. In several cases, I have found the trouble to be in a metal-encased resistor riveted to the chassis. The taps on this resistor make poor contact when the resistor is cold; but when the resistor heats and expands, it wedges the metal band of the tap tightly between the element and the case and makes a good contact until the resistor is allowed to cool down again ..
"If the set plays all right for a half-hour or so and then begins to misbehave, it is a good idea to suspect condensers or coils. We talked before about how to locate a condenser that is opening up, but I want to mention, too, that you can often tell in what circuit a condenser is opening by listening to the change in the quality of the signal when it drops in volume. An opening coupling condenser will often cause a severe loss of low frequencies, because the highs will still be passed by the small amount of capacity between leads, etc. A plate, cathode, or a.v.c. bypass that is opening will often cause a characteristic hissing 'near-oscillation' sound to accompany the drop in volume. A defective coupling condenser in the oscillator circuit will often have a detuning effect on the signal just before the set quits playing."
"You told me last month that changes in volume were not the only troubles that came under the heading of 'intermittents,' " Barney said. "What other kinds are there?"
"Well, intermittent noisy conditions are quite common. We already have spoken about noisy resistors. Coils that open up are very common, especially in circuits that carry d.c. current. This takes in the .primaries of r.f., i.f., oscillator coils, and audio transformers, and it also includes speaker field windings. An experi\enced serviceman can usually spot one of these 'opening coil' cases by the distinctive sound they make. In addition to an intermittent rustling sound, there is often a kind of high-pitched squeaking sound, like this:"
Mac drew his breath in between tightly-pursed lips to produce the sound that many people use to call a near-by dog. Barney dropped his notebook, clapped both hands over his heart, and rolled his eyes blissfully toward the ceiling. "Ah!" he exclaimed rapturously, "that reminds me of how Margie says goodnight:"
"You keep your mind on what. I'm saying, or I'll 'goodnight' you," Mac warned trying to scowl fiercely. "You can often show up which winding is going out by increasing the current through it. To do this, connect a resistor of around 5000 ohms between the point fed through the coil and the ground for two or three seconds. If the coil is OK, it will pass this temporary overload without harm; but if it is defective, the noise will become much worse or the coil will open up completely.
"Another very common noisy condition, especially with a.c.-d.c. sets, is caused by a defective filter condenser. I am not sure as to exactly what happens inside the condenser, but the effect is a loud scratching sound that may or may not be accompanied by a noticeable increase in hum. Usually, if you bridge the defective condenser with a good one, the surge that takes place will cause the noise to stop abruptly; and ordinarily the noise will not start immediately when the good condenser is removed. Sometimes it will not commence again for several days. Often this noise will be radiated and can be picked up by other sets in the same room, which will fool you if you leave the defective set on while you try another set to see if it is noisy, too.
"If you suspect this condition, turn the set off and clip a good condenser across the suspected unit; then, after the set is playing, gently remove the good condenser and see if the noise begins. In this way, you will not 'cover up' the condition you are trying to locate - as you are almost certain to do if you employ the usual method of bridging filter condensers while the set is playing.
"Another very common noisy condition with a.c.-d.c. sets is caused by noisy rectifier tubes such as the 35Z5's, 35Z3's, and 35Y4's. Something happens inside these tubes so that an annoying scratching sound is heard every time they are jarred ever so slightly. This sound is not present when the volume is turned off or when the r.f. and i.f. tubes are disabled; so it must be picked up by the antenna. In fact, I have noticed that the noise is much worse in sets in which the loop antenna is near the rectifier tube. Just the vibration caused by the sound from the speaker will make the rectifier give forth with this annoying sound. The cure, of course, is a new tube."
By this time Mac had the phone jack installed, and he began to replace the recorder in its cabinet.
"An intermittent hum," he went on, "naturally causes you to suspect the filter condensers first, and that is right; but the filter condensers are not always at fault. Cathodes that develop partial or complete shorts to the filaments as the tubes reach a certain critical temperature are fairly common, especially in the a.c.-d.c. sets. A grid that is left floating by a defective coil or resistor will introduce a hum that can be spotted by observing that as the hand is brought near the floating grid the hum will increase greatly. A hard-to-locate hum will occasionally show up in a.c.-d.c. sets that do not normally connect one side of the line to the chassis if the line becomes shorted to the chassis, say through poor insulation in a dial-lamp socket."
Mac paused briefly to tryout a pair of phones in the new jack. Then he continued:
"Finally we come to those sets that start playing quite well at first but gradually develop a progressively worse distortion. The first thing to suspect is a leaky coupling condenser that is lowering the bias on an audio tube. The leakage of a condenser is often dependent on its temperature, and that is why it may take some time for the distortion to show up. Measuring the grid bias with a v.t.v.m. is the quickest way to check on a leaky coupling condenser. There are times, though, when you can cut the coupling condenser entirely loose from the grid and the grid will still read positive. What is more, no positive voltage will be found at the cold end of the coupling condenser. In that case, you have a tube that is suffering from 'secondary emission.' Such a tube will gradually draw more and more plate current and will cause more and more distortion. If the grid resistor of such a tube is discovered to be at its rated value, the only thing to do is to replace the tube. A too-high grid resistor will aggravate or even cause this condition."
Mac paused, and Barney broke in hopefully:
"Is that all there is to know about intermittents?"
"Not by a long shot!" Mac said. "We have just hit the high spots of what I know on the subject, and I am still learning something new about intermittents nearly every day."
Barney heaved a big sigh as he put away his notebook and slid from the bench. "The nice thing about you, Mr. McGregor, is that you make radio servicing sound so-o-o-o easy!'; he said bitterly.
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
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 January 8, 2020 (original 4/30/2015)