Mac's Service Shop: Magnetic Shielding |
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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
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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