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Mac's Radio Service Shop: Safety in Servicing |
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It's time to gather 'round for another story about fictional radio service shop owner Mac McGregor and his trusted sidekick technician, Barney. In this episode, an errantly wired bypass capacitor on a chassis from one of the old AC/DC radio sets caused Mac to get a 300-volt wakeup call when his hand brushed against it. After explaining the situation to Barney and apprising him of the danger it poses to an owner who unwittingly sticks his/her hand into the back of the cabinet, Mac lists a few other common dangers to watch for. Radios that ran on either AC or DC power were very common back in the early days because there were homes and businesses that had both type systems wired in to the premises - in part due to the famous battle between Thomas Edison's preferred DC electrical distribution system and Nikola Tesla's preferred AC electrical distribution system. Another reason for DC compatibility was that prior to the Rural Electrification Act of 1936, many homes not located in cities and nearby suburbs had no electric service at all. Farm homes often ran on DC power supplied by banks of lead-acid storage batteries, with wind- or water-powered generators providing the charging current. Radios, aka 'wireless sets,' were powered by those batteries. In order to accommodate both AC and DC supplies, radio designers conjured up power supply circuits that would work on either system, but unfortunately the way it was implemented often caused the metal chassis to have a few hundred volts of potential on them. That meant the metal shafts to volume and tuning controls carried a deadly potential that was isolated only by plastic or other insulative material. Even the external antenna connection could carry the voltage. After many electrocutions, the industry finally standardized a design strategy that provided adequate isolation and safety. Mac's Radio Service Shop: Safety in Servicing By John T. Frye
Mac was aligning a receiver in which his assistant, Barney, had just installed new filter condensers. Suddenly he grunted heavily and jumped back from the chassis. Then, as Barney watched guiltily, he pulled the line plug from the socket and gingerly turned the chassis over. "Now there," he said as he jabbed an accusing finger at the wiring beneath the chassis, "is something I never want to see you do again." "What's wrong with that?" Barney asked. "That old can-type condenser has been cut out of the circuit. See, here's where the lead from the insulated case has been cut loose. The positive terminal has been left wired in merely as a convenient tie-point for the new cartridge-type electrolytic." "And so what happens?" Mac demanded. "The shorted condenser element inside the can conducts the full voltage from the positive lead right to the floating case. To make matters worse, while looking to see what size condenser you would need for replacement, you removed the cardboard covering that was slipped over the can of the electrolytic and failed to replace it. That is why I got a full 300-volt jolt a few minutes ago when my hand brushed against the case of that old electrolytic. The same thing could have happened to a housewife dusting out the chassis when the receiver was turned on; or a little child might have put his hand on the condenser can and received a fatal shock. If this had been a TV chassis, I might have jerked when I was shocked and hit the picture tube with a tool in my hand, causing it to implode. In the future, I want both leads of an electrolytic that sticks above the chassis cut loose, unless the can itself is grounded. "And while we are talking about such matters, it probably will not hurt to touch on two or three other points. For example, the other day I noticed that you used a new 'outboard' one-megohm resistor to replace one that had gone bad inside the socket of a magic-eye tube. You failed to tape the exposed leads of the resistor, although one of those leads was a full 250 volts d.c. above the chassis. Fortunately I noticed this and taped them up before the set got out of the shop. A good general rule to follow is that no on-top-of-the-chassis point should be left exposed to touch if it carries more than ten volts potential." "I saw a dandy along that line the other day," Barney remarked. "Someone had replaced the output transformer mounted on the speaker and through thoughtlessness or neglect the 'B-plus' and output-tube-plate lead junctions were left exposed where even a crawling child could easily reach them. That was dangerous." "Speaking of crawling children, you can't be too careful of line cords," Mac went on. "Quite often these cords will look all right except for a short space just back of the plug, right where the fingers would likely be when the plug is being inserted or removed. The reason, of course, is that the cord at this point takes quite a beating from sweepers, dust mops, and little children. I always try to take a quick gander at the line cord when the customer brings a set in or when I pick it up; and if the line cord looks the least bit suspicious, I suggest right then and there that it be replaced. Few people will turn the suggestion down." "While we're gabbing about safety, maybe we ought not squander all our solicitude on the customer," Barney suggested. "How's about mentioning a few things that may help the service technician reach a mellow age." "At least one technician I know shows promise of becoming mellow, very mellow, long before he becomes ripe," Mac observed, "but your idea has merit. I think we have touched on most shop-safety points at various times in the past, but if these ideas are to do any good, you have to keep harping on them. Anyone who has ever studied how accidents happen knows there is a great difference between knowing that a danger exists and being constantly aware that the danger is present. It is this constant awareness that keeps you from having an accident." "The voltages we constantly deal with are one of our greatest dangers, aren't they?" Barney asked. "That's right, and the dangerous part is that familiarity breeds contempt. We are constantly working on receivers having potentials all the way from 100 volts up to fifteen kilovolts or so. Every now and then we get nipped a little without being seriously hurt, and each time this happens we lose a little more respect for the potentially lethal power that lurks in even a lowly a.c.-d.c. set. In fact, there are some pumpkin-headed technicians I know who boast about how they take no precautions against being shocked because they 'can take it.' Their undertaker will be doing the taking one of these days. "A simple precaution that should be standard procedure in any shop is to plug all sets, both a.c.-d.c. and transformer type, into isolation transformers while they are being serviced. Most technicians realize the danger present when working on hot-chassis a.c.-d.c. receivers; but they overlook the fact that quite often lightning will cause the condenser that goes from one side of the line to the chassis of a transformer set to become shorted. Such a set can be a real death-trap, because you do not expect the chassis to be hot. "Another point I had vividly demonstrated one time is that you should never try to pick up a heavy chassis and move it while the cord is plugged in. I saw a person pick up such a chassis and start to slide it into the cabinet. Suddenly he began to shake all over and to make a peculiar noise. I jerked the cord from the wall socket, and he dropped the chassis to the floor and tottered to a chair. A strong odor of burned flesh was in the room. His fingers, curling beneath the edge of the chassis, had come into contact with the tie-points for the 117-volt line cord. The current froze him so that he could not let go and made his contracting muscles force his fingers tighter and tighter against the searing wires. Grooves were burned into those fingers that took months to heal." "Now look, let's not try to win Oscar with this safety talk," Barney said with a grin. "You're waxing pretty dramatic, you know. I suppose we ought to mention, too, that you should use an insulated rod instead of your fingers for probing around the wiring of a live set." "That's right," Mac agreed. "On the whole, though, I imagine that a service technician would be likely to be more aware of the danger of electrical shock than he would of other possible sources of injury. For example, we know how important it is to avoid breathing the fumes of carbon tetra-chloride or of getting the stuff into your eyes or a break in the skin; yet I still see a lot of fellows handling the solvent as carelessly as though it were water. They ought to read some of the articles that have appeared in radio magazines telling how familiar carbon tet can cause serious illness or even death if mishandled. "In the same way the average technician has been pretty well impressed with the fact that he should handle a TV picture tube with extreme caution to avoid an explosion, but he does not hesitate to grab hold of a receiving tube with his bare hand and yank with all his might in an attempt pull it from a stubborn loctal socket. If one of these tubes breaks and cuts a quarter-inch-deep gash in his thumb, as it did in mine, he will not need be reminded after that to use a tube-puller, leather glove, or some other means of protection when taking out ordinary receiving tubes." "Even if the tube does not break, you can often get a nasty burn from trying to handle a too-hot tube," Barney remarked. "Right again," Mac applauded, "and a burn is always dangerous because affords a good place for infection to start. This is one reason we should be careful not to be burned with a hot soldering iron or hot solder. Parts being soldered should always be held in place with pliers instead of the fingers. Even though the part can be gripped in such a manner that heat from the joint cannot be conducted to the fingers there is always the danger that the point of the iron will slip off the joint and dab against them or that a blow of excess solder will' roll from the joint or the iron on to the fingers and burn a blister before you can say 'ouch'! "What really chills me, though. Mac continued, "is the sight of a guy pulling on a wire with all his might while he softens the solder joint with a hot iron. When the wire finally does break loose, it invariably throws a spray of melted solder, usually directly toward the person pulling on it. A particle of hot solder striking the eye ball could easily cause permanent blindness. Never, never do that. Use a soldering tool to free the end of the wire and work it loose while the iron keeps the joint hot; but never pull on the wire while you are doing this so that it becomes a slingshot for throwing molten solder into your face." "You keep on," Barney warned, "and you're going to convince me that a radio or TV chassis is nothing but a booby-trap. I'll be afraid to touch one of the things." "You'll be safe enough unless you really are a booby," Mac said with a grin. "I'm not saying, however, that even a careful person can always avoid a minor scratch or burn; but such a person will take care of the minor injury promptly and so insure that it remains just that. Whenever you break the skin, even with the smallest scratch, I want you to go to the first-aid cabinet at once and apply an antiseptic. The quicker you do this the less likelihood will there be that germs can enter the bloodstream through the cut. Burns should be treated just as promptly. Remember, too, that the baking soda in the cabinet is for neutralizing battery acid that may happen to splash on the skin. "Finally, you know how strongly I feel about every technician being an expert at administering artificial respiration. Probably this is the best weapon we have against unnecessary death from electrical shock, and I think it is near-criminal for anyone who works with electricity to be ignorant of this life-saving process. To work with electricity and not be able to administer artificial respiration is like a professional snake hunter plying his trade without a vial of snake-bite serum in his pocket." "That would definitely be un-good," Barney agreed. "Perhaps I'd better do a little brushing up on my first aid techniques!" 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 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 August 4, 2015 |
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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 ... All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. My Hobby Website: AirplanesAndRockets.com
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