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I have to admit to
not knowing what an "Ouncer"
audio choke was when reading this Mac's Service Shop story from a 1955 issue of
Radio & Television News magazine. Research reveals that United Transformer
Company produced a line of magnetic transformers and RF chokes weighing about an
ounce (lightweight for its day), hence the moniker. They later made Sub-Ouncer products.
Of course like just about everything else, they can still be purchased on
eBay. Mac had been awed by a demonstration of the effectiveness of a new, low
cost type of magnetic shielding that can be placed around a CRT to prevent distortion
due to nearby magnetic fields created either within a chassis or from an external
source. It is especially important for precision test equipment being used in a
field-filled environment like an electronics repair shop. As you might expect, a
bit of theory and manufacturing technique information is included while educating
Barney. In the same issue, a related article entitled "Shielding
in Hi-Fi Equipment" discusses another application of shielding.
Mac's Service Shop: Magnetic Shielding
 By John T. Frye
Barney came storming into the service shop out of the young blizzard whipping
up outside. His spirits always accompanied the weather, and it was evident from
his snapping blue eyes that he was really in high gear this December morning.
"Well what do you know!" he exclaimed as he pounced on a tall gold-colored metal
cone resting on the service bench beside Mac, his employer, and began bellowing
through it directly at the other's head. "You've been practicing up on your old
high school yells. Sizz-boom-bah! Rah-rah-rah!"
"Quit yelling into my ear or I'll shove that thing down over your pointed little
head clear to your shoulders," Mac warned.
"What is it?"
"It's a new magnetic shield for the cathode-ray tube of our old oscilloscope."
"Where did you get it? What's it made of? How come there's another smaller cone
inside this big one?"
With a shrug of resignation Mac laid aside his tools. "I may as well tell you
the whole story for there'll be no rest until I do," he sighed. "Bill Gardner, the
purchasing agent at a local electronic factory, called me the other day and asked
if I wouldn't like to sit in on a demonstration of a new magnetic shielding material
that had been developed by the Perfection Mica Company of Chicago. He said his knowledge
of magnetism was pretty shaky and that he'd like to have my opinion. I told him
it would be a case of the blind leading the blind because I wasn't too hep on magnetism
either; but I went, and I'm darned glad I did, for that salesman put on a real show.
It was so convincing that I ordered this shield for our scope and these small square
sheets of the magnetic shielding material with which to experiment."
"What's the stuff made of?"
"First, there is a base alloy of special formula steel with a low amount of carbon
and manganese. A binder is applied to this and a special combination of ferrous
and ferrite powders is flocked on. Finally a copper-ash coating is applied for electrostatic
shielding."
"Sounds expensive."
"Well it's not - at least in comparison with the nickel alloy materials that
have been used for this purpose. You see it is not only the cost of the material
in the nickel alloys that makes them expensive, but after shields are formed from
these alloys they must be annealed in a hydrogen oven. What's more, they must be
handled with special care after annealing because they are shock sensitive. These
new shields require no such expensive annealing or handling, and their cost is less
than half that of nickel alloy shields."
"What do you mean by 'shock sensitive?' "
"A sharp blow or heavy jar will seriously impair the shielding qualities of a
nickel alloy shield and make re-annealing necessary. Incidentally, so will subjection
to a high-intensity magnetic field. This new material, it is claimed, is entirely
free from these drawbacks."
"But does the new stuff do as good a job of shielding?"
"As far as I could see in the demonstration, it was actually superior on many
counts, especially in shielding from d.c. fields."
"That salesman really did a snow job on you," Barney marveled. "How did he do
it?"
"With a large assortment of shields, a gauss meter, a two-inch unshielded scope
tube, a collection of magnets, devices for generating a.c. fields, a compass-"
"Hold it!" Barney interrupted.
"What's a 'gauss meter'? And shouldn't that word rhyme with 'hoss' instead of
'house'?"
"Nope. I've been mispronouncing it too; but a check with the dictionary proved
the salesman had it right. A gauss meter is a device for measuring the strength
of a magnetic field. I was curious about how this one worked; so I quizzed the man
at some length and found the principle of operation is so simple that even you may
be able to understand it."
"Try me," Barney urged.
"Fundamentally you have a small iron-core inductance, like say an "Ouncer" audio
choke, rotated by a constant-speed electric motor. Slip rings connect the leads
of the inductance to a sensitive a.c. type v.t.v.m. As the inductance spins around
in any magnetic field that may be present, the cutting of the magnetic lines of
force by the turns of the inductance generates a voltage across the ends of the
inductance that produces a reading on the scale of the v.t.v.m. By placing the inductance
'sensing' unit in d.c. magnetic fields of known strength, the scale of the v.t.v.m.
can be calibrated directly in gausses. A voltage divider across the output of the
sensing unit permits the meter to read a wide range of field strengths from a fraction
of a gauss to several thousand gausses. One important point in design, though, is
that the sensing unit must be shielded from the rotating motor. Placing the motor
inside a small box made of the shielding material - which the manufacturer calls
"Frenetic Shielding" - took care of that.
"The salesman," Mac went on, "held a small Alnico magnet near the whirling sensing
unit and got a reading of slightly less than 100 gausses; then, without moving the
magnet, he slipped a small cylindrical shield of the 'Frenetic material over the
sensing unit and the reading dropped to less than 0.5 gauss."
"That's surely chopping it down," Barney observed.
"Next," Mac continued, "he hauled out a large horseshoe magnet. It was a huge
22,000 gauss closed flux job, and it gave him a real wrestle to separate the keeper
from it. When this magnet was held anywhere near the little two-inch CR tube, it
promptly pulled the spot clear off the screen; but when the CR tube was placed inside
two concentric shields and the giant magnet was placed directly against the outside
shield, the spot barely shifted."
"Why two shields?" Barney wanted to know.
"I asked that, and the man explained that no one shield will do a good job of
shielding against both high intensity and low intensity magnetic fields. A shield
to be effective at low intensity must have high permeability; but at high intensity
such a shield saturates and loses its effectiveness. In Frenetic Shielding the base
material is designed to provide shielding against one intensity and the ferrous
and ferrite coatings handle the other intensity - up to a point that is far beyond
the scope of a single-material shield; but for maximum attenuation of an extremely
strong field, such as that of the powerful magnet, double shielding provides six
to eight times more attenuation. The high-intensity outer shield knocks down that
22,000 gauss field to one of only a few gauss; then the low intensity inner shield
takes over and reduces this to a small fraction of a gauss."
"Some of these sheets are coated with large coarse particles while others have
a much finer grain," Barney remarked.
"That's right. The ferrous and ferrite powders are frequency sensitive. For low
frequency and magnetostatic fields, large particle sizes are used with a mesh of
20 to 50. As the frequency increases, the particle size supplying the most effective
shielding decreases until for some purposes particles of 2000 mesh are used. By
combining different mixtures and different particle sizes, shields can be tailored
for maximum attenuation of any frequency from d.c. to two hundred megacycles."
"Then it's really important to know exactly what sort of magnetic fields you
are trying to shield against in selecting your shields."
"The salesman was very emphatic about that. While general purpose shields will
do a perfectly satisfactory job in many applications, maximum attenuation of a particular
field can be had only when that field is measured and identified and the shield
designed for it."
"Well, let's put the shield on the scope and see what happens," Barney urged.
"OK, but first let's take a couple of readings. With the vertical and horizontal
gain controls of the scope turned entirely off, I'm going to hold this speaker magnet
right against the side of the case at the point where it has the most influence
on the spot position and see how far we can displace the spot. Hm-m-m, it looks
like we can move the spot a full inch up or down from center simply by turning the
magnet around. Now I'll hold the solder gun - which the salesman said was the most
vicious generator of an a.c. field he had found - in the same place and pull the
trigger. That produces a line slightly more than four inches long. Remember these
figures."
In a few minutes Mac had slid the scope from its case, installed the shield over
the CR tube, and put the instrument back in its housing. Once more he held the speaker
magnet against the side of the case.
"Golly, that spot can't be moving more than a sixty-fourth of an inch if it moves
at all," Barney marveled.
Next Mac pushed the solder gun housing against the case of the scope and pulled
the trigger. Instead of a four-inch-long line, the spot traced out a segment only
about a fourth of an inch in length.
"Something else has changed, too," Mac remarked. "These center-tapped positioning
controls have a small bit of knob travel at the center of rotation where the slider
is moving across the junction of the tap and the resistance element in which no
effect on spot positioning is had. Before we put on the shield, this 'dead spot'
was clear over to one side of center; now it appears when the spot is right in the
middle of the five-inch screen, proving that the influence of a d.c. field has been
removed."
"You think a CR tube shield is an absolute necessity on a scope, huh?"
"No, I don't think that. In many instances, especially where the scope is operated
in a location comparatively free from strong magnetic fields, the shield will make
little essential difference. Modern scope manufacturers use power transformers especially
designed to restrict any influence on the beam from that source, and they carefully
orient these transformers so the critical area of the beam path is in a magnetic
null of the transformer field. On the other hand, if the scope is to be used around
strong fields, a shield is a real necessity; furthermore, if the scope owner is
a darned crank, as I cheerfully admit I am, who does not want anything influencing
the motion of that spot except the signal fed into the amplifiers, a shield is worth
its cost in personal satisfaction. For most applications the large outer shield
would probably be adequate; but I thought while I was at it I might as well go whole
hog and get the maximum attenuation provided by the double shield, since this only
adds about twenty-five per-cent to the cost."
"I'd think scope shields would be a rather small market."
"Don't ever imagine magnetic shielding is used only on service and laboratory
scopes," Mac exclaimed. "Magnetic shielding is becoming more important every day.
Take tape recorders, for instance. On a tape deck in which the sensitive heads are
mounted above the deck and the field-producing transformer and motor are mounted
below, making the deck out of magnetic shielding material like this would establish
a magnetic barrier between the fields and the heads. Recorded tapes stored in cans
of this material would be safe from damage by magnetic fields. Radar equipment must
be carefully shielded from magnetic fields if it is to be reliable. Airplane instruments
containing magnets can be shielded so they may be mounted on the panel of the plane
without affecting the compass.
"The salesman told about one interesting use of the material," Mac related. "Magnetrons
have a terrific field, and when shipped by plane they formerly had to be stowed
in the tail as far away from the sensitive instruments on the control panel as possible.
Now, however, they can be encased in a double box of Frenetic Shielding and stowed
wherever convenient without concern. What's more, since this shielding material
does not retain any magnetism, the same shipping container can be used over and
over.
"But probably a more important use lies in the aid this magnetic shielding material
gives the modern trend toward more compact electronic equipment. Transformers encased
in this material can be mounted side by side without coupling between them. No longer
must we depend upon separation and careful orientation to prevent such coupling.
"But there's no point in my trying to list all the possible uses of magnetic
shielding. Now, with magnetrons, magnetic amplifiers, and a whole host of similar
magnetically-operated gadgets coming into daily use, it's of growing importance
that we be able to confine the fields surrounding these pieces of equipment. That's
why it seems almost like fate that this new lower-cost shielding material should
appear on the scene just in the nick of time. And speaking of time, let's quit wasting
it and get to work. If we get a bunch of these sets out in a hurry, I'll show you
some tests I've worked out with this shielding stuff that'll make your eyeballs
stick out like bubblegum bubbles!"
Posted September 16, 2020
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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