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February 1949 Radio & TV News
 [Table
of Contents]
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
acknowledged.
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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 sus-pect 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.
Posted January 8, 2020
(updated from original post on 4/30/2015)
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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