Mac's Service Shop: Technician or Consulting Engineer?
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After reading the first paragraph of this "Mac's Service Shop" technodrama™ entitled, "Technician of Consulting Engineer?," I expected to be told a story about the seemingly excited customer exiting the shop as he returned from lunch. Had the man made an unreasonable demand on Mac McGregor, the proprietor, and was rebuffed appropriately? It never materialized. In the second paragraph underling technician Barney mentions having seen two men exiting the shop on his way in, not just one. At that point I'm wondering what sort of melee had just occurred. As it turned out, my interpretation of the event was totally incorrect. While explaining the happenings to Barney, he imparts, as he is want to do, a couple clever bits of technical information in the process. This appeared in the October 1960 issue of Electronics World magazine. Check out the third problem in "What's Your EQ?" from the June 1964 issue of Radio-Electronics magazine. Mac's Service Shop: Technician or Consulting Engineer?
Barney, returning to the service shop after his lunch hour, stepped aside just in time to keep from being run over by a vigorous middle-aged man dashing out the front door. Cautiously the young technician opened the door and stepped inside. As he entered the service department, his eyes swept the receiving bench quickly; then he turned to Mac, his employer, already busy at the bench. "What did those last two jokers want?" he demanded. "I saw two guys come out of here while I was parking the truck. Neither one was carrying anything, and apparently neither one brought anything, either. That last one was a competitor. What was he doing? Spying?" "Not exactly," Mac said with a chuckle; "so you can let your hackles down. I should warn you, though, you may have to start calling me 'mister.' I'm getting up in the world. Those two sought my services as a consulting engineer." "OK, Mr. McGregor, don't stop there," Barney insisted. "The first man who came out was Mr. Harkle, an engineer with the spring factory on the north side of town. Right after you left on that last service call before lunch, he telephoned to say he wanted to talk to me about a problem. I agreed, and he came right away with a bunch of blueprints under his arm. "First he showed me a print of a large metal disc about seven feet in diameter with holes bored through it all around the rim. At the factory springs are loaded in these holes, and the rim of the big disc rotates slowly between two high-speed emery wheels that are properly spaced to grind both ends of each spring simultaneously. The rotating mechanism of the big disc is indexed so that at regular intervals it stops while finished springs are removed from a section or while unfinished springs are loaded into empty holes. "The problem is that now and then a shaft breaks or a key shears on the shaft that rotates the big disc, and then the latter starts free-wheeling under the friction drive from the emery wheels. You can imagine the damage to the spring loading and unloading mechanism and the real danger to persons nearby which results when that big disc tries to turn as fast as those emery wheels: What was wanted was a device that would shut off the emery wheels the instant the big disc accelerated beyond its normal slow rotation. To be effective, the device had to detect acceleration that took place over a very small portion of a complete revolution." "That rules out anything based on centrifugal force," Barney observed. "Right. Mr. Harkle had a hazy notion the problem might be solved somehow by electronics, and I agreed it could. For example, a light could be arranged to shine through the holes in the centers of the springs onto a photocell. This cell could work into an amplifier designed to cut off at low frequencies such as those produced by the slow lighting and darkening of the cell as the disc rotated at normal speed. When the disc started to spin, this 'interruption' frequency would go up, pass through the amplifier, be rectified, and operate a relay to cut power off the grinding wheels. Another possible solution would be to mount little PM magnets around the rim of the disc so they passed close to the soft iron core of a coil. Voltage induced in this coil by the passage of each magnet would depend, among other things, on the speed of the magnet past the coil. Voltage thus produced could be amplified and used to operate a cut-off relay when it exceeded a certain value-such as that resulting when the disc started to rotate more rapidly. "But I kept thinking of all the things that could go wrong with such arrangements: dirt could cut down the light received by the photocell; the exciting lamp could burn out; amplifier tubes could go bad; the magnets could weaken; the pickup coil could be jarred out of position; and so on. Suddenly I remembered the 'velocity trip' arrangement used on record changers that permits the tone arm to move in toward the spindle the diameter of one groove per revolution without tripping; but when the needle strikes the tripping groove and the tone arm swings in rapidly, the change mechanism is set in motion. In an instant I had what seemed to me a simple and nearly fool-proof solution. "I suggested a small synchronous motor and cam arrangement be used to press the end of a rubber-tipped rod firmly against the rim of the wheel once every second. The rod would be mounted so its tip could be carried sideways a small amount by the normal motion of the disc during the fraction of a second the two were in contact; but when the rod pulled back each time, it would return to its original position. When the wheel started to spin, however, the end of the rod would be carried sideways much farther during the contact period. This increased lateral motion could easily be arranged to operate a micro-switch that would remove power from the grinding wheels. I pointed out to Mr. Harker that the arrangement would be very quick acting and positive; it used only a few parts; and it could be visually inspected at any time for proper operation. He gathered up his blueprints and took off in a happy frame of mind. "Just before Mr. Harker left, Tony Klinck dropped in to report a solution I had worked out for one of his problems was a great success. Tony is a good TV service technician, but he got into the repair game since the advent of TV and lacks the broad background of general electronic experience that comes from growing up with the business since the early days of radio. He never hesitates to yell for help when he needs it, however; and a couple of weeks back he came to me with quite an interesting little problem. "A drive-in eating place over on the west side has installed a whole new intercom system with speakers on posts in the parking area to relay orders to the kitchen. When a button on a parking lot speaker is pushed, this turns on a light above a switch in the kitchen. The order-taker throws this switch down, steps on a foot switch that operates a relay performing the push-to-talk function of the intercom system, and says, 'May I take your order?' Releasing the foot switch transfers the kitchen speaker transformer back to the output of the intercom amplifier and the post speaker transformer to the input of the amplifier so the customer can place his order. "An added feature is the playing of music through post speakers not being used for talking. A tuner or record player working into a forty-watt amplifier accomplishes this. When all the switches in the kitchen are in the 'up' position, all post speaker transformer primaries are connected in parallel across the output of the music amplifier. Throwing anyone switch down removes the attendant speaker transformer from the music bus and connects it to the intercom circuit. The rub was that music was heard through post and kitchen speakers, even when they were switched to the intercom circuit, interfering with orders. "A couple of things complicated the situation: first, I could not make my own tests. It was Tony's job; so it simply would not do for me to go nosing around. I had to depend on him to do the testing. Second, the system had been struck by lightning shortly after being installed, and the owner could not be sure if the trouble had been present before then or not. "Tony had checked over all the wiring carefully to make sure it was correct and that there were no short-circuits feeding the music into the intercom circuits. I suggested several tests aimed at isolating the part of the circuit where this transfer was taking place, and he soon reported that it was happening somewhere in the banks of switches that transferred each post speaker from the music bus to the intercom circuit, and vice versa. Remembering the lightning, I thought possibly an arc had carbonized a path across the insulation of one or more switches; so I suggested the switch banks be disconnected and individually checked for leakage with an ohmmeter. This test revealed no degradation of switch insulation, and that left just one possibility: the music was being transferred across open switch contacts through the capacity between them. Capacity between contacts on one switch would be quite small; but when dozens of switches were wired in parallel, the total would be high . "I could think of no practicable way of reducing this capacity appreciably; so I decided to try to 'buck out' the music in the same way a transmitting tube is neutralized; that is, supply a bucking signal equal in amplitude but 180° out-of-phase with the signal being transferred through the switch capacities. I worked out a circuit to do this and tested it here on the bench. I let the audio generator represent the music amplifier. One half the primary of a small output transformer was connected across the output of the generator, the center-tap being grounded. The output of the generator was fed through a 20-μμf. capacitor to the a.c. v.t.v.m.; and the output of the generator, operating at 1000 cycles, was adjusted for a 10 millivolt reading. A 35-μμf. variable capacitor was connected between the loose end of the transformer primary winding and the hot terminal of the v.t.v.m. The 20-μμf. capacitor represented the capacity between switch contacts through which the unwanted signal - 10 millivolts in this case - was feeding. I knew any signal appearing at the hot end of the portion of the transformer winding across the signal generator would produce another signal equal in amplitude but 180° out-of-phase at the other end of the winding. This phase-shifted signal would be transferred through the variable capacitor to the v.t.v.m., representing the intercom bus in our test setup. When the variable capacitor was adjusted to 20 μμf., the two signals should cancel - and they did! With the capacitor adjusted for maximum attenuation, the reading could be dropped 35 or 40 db; and since the two signal paths were identical, this attenuation would hold all the way from 50 to 5000 cycles. "All Tony had to do was connect one end of a center-tapped output transformer winding to the music bus, ground the center-tap, and connect the other end of the winding through a small variable capacitor to the intercom bus. He stopped by to tell me that when he set the variable capacitor equal to the capacity between switch contacts, the music dropped out of the intercom as if by magic, and stayed out." "How about pay, Mr. Consulting Engineer?" Barney inquired. "In Tony's case, when he asked for my bill I suggested he could pay me by doing some antenna tower work out at the house. He is a crackerjack at this and well equipped. As for the spring company, I'll send them a bill for consultation and advice. The point I want to make, though, is that the radio and TV service technician has acquired a wealth of knowledge, not necessarily restricted to repairing ailing radio and TV sets, that others are willing and eager to buy - if he is alert to this fact. "On the other hand, the technician must be careful not to get in over his head. For example, yesterday I had a long distance call from the director of the hospital over in Carlston. He wanted to install a two-way radio system at the hospital and in ambulances so that the emergency ward and doctors could be alerted and ready to handle accident patients when they arrived. He had received some quotations from people manufacturing and selling this type of equipment, but he did not like the prices. He had heard of the fabulous 'bargains' to be had in surplus electronic equipment, and he wanted me to go with him to a war surplus depot, pick out suitable equipment, convert it, and install it in the hospital at a big fat saving to the hospital. I politely declined after pointing out several of the many pitfalls involved in converting war surplus, getting the equipment licensed, maintaining it, etc." "I don't blame, you," Barney remarked. "Dependability would be very important in such a setup: He had better wait until he can afford commercial equipment. That's a case where you probably gave away advice worth several hundred dollars to the hospital director." "Oh well, it's a good cause," Mac said with a grin.
Posted May 31, 2023 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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