标签: coils

My grandparents' TV went dead. My dad, a radio and TV repair person, took a quick look and thought the damage was caused by a line spike, maybe from lightning. The tuner didn't appear to be damaged. He didn't have the time to take on this repair, although more likely he thought it was a lost cause. He issued me a challenge to fix it. Wait a minute, pop! You want me to rebuild a TV that was possibly hit by lightning? Cool! I knew it wouldn't be pretty, but I accepted. My dad was correct. Only the IF section was toast. Really, fried remains of resistors, coils, and caps burnt to a crisp. It looked like someone had taken a torch to the IF section. I got out the SAMS service manual. I can't remember the exact make. The TV may have been a Philco, RCA, or Magnavox. I started the rebuild with the easy stuff first, replacing the tubes, resistors, and caps. I had to match up the resistor values from the schematic, since the colour codes were different shades of burnt umber... pun intended. The IF coils were my biggest worry. Luckily the coils had only minor smoke damage with a couple of open windings. I carefully cleaned up the coil forms and re-wound the open ones with the same size copper wire. Six weeks had elapsed since I started, and finally the work was done. It was time for the smoke test. It was the most nerve-wracking moment: Would my Frankenstein TV come to life or... just smoke? I carefully flipped on the switch, standing well back, just in case. It was ALIVE! NO smoke! It really worked. The picture and sound were both functional. Actually, very functional indeed considering what I had done to the IF section. On Thanksgiving day my grandparents had their TV back. Note: To protect the innocent the exact details and names have been changed. Plus this happened about 40 years ago, when I was 15 years old and just starting 10th grade! Dave Grindel submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).  

Several readers wanted to know more about some of the simple electronics projects I built in my early pre-high school days, based on instructions I found in a book published 50 years earlier in about 1915 titled "Boy Mechanic: A book that tells how to build things." I do not have any record of the details of how I built the simple radio receiver I wrote about previously, but I did find the instructions for an earlier project that I took back to California from Oklahoma: making high frequency Oudin and Tesla coils with very mundane simple components, including cardboard tubes, magnetic wire, discarded phonograph records, and miscellaneous scraps of brass, wood and cloth fiber. The reason I still have them is that upon entering ninth grade in high school, I used the same plans to build versions for the annual high school Science Fair, part of a trend that became popular in U.S. high schools in the 1960s. Tesla and Oudin coils are both electrical resonant transformer circuits used to produce high-voltage, low-current, high frequency alternating-current electricity. The difference between the two is that the Oudin coil uses a lower current and is thus safer – both for the builder and the "zapee." The first versions built over a summer I spent on a farm in Oklahoma were, as noted earlier, were constructed with a collection of amazingly simple, and common place, components. To make an Oudin coil ( figure 1 below ) the instructions called for the use of a 6 by 11 inch cardboard tube for the secondary, which is first covered with two or three coats of shellac. When the last coat has dried, a single layer of No. 26 magnetic wire is wound on, with the winding started about half an inch from the upper end of the tube, one end fastened down to the tube and the other, about eight inches, left loose for connection to a brass rod.   Figure 1: Plans for an Oudin coil.   When the winding was finished, I gave it another coat of shellac, which was left to dry, while I created some of the other components. Several wooden disks—roughly the same diameter as the cardboard tubing—I made on my uncle's lathe and fit into the ends of the cardboard tube at the center of the secondary coil. The bottom disk was screwed into a wooden base and the top one drilled through the center to accommodate a brass rod leading to a brass ball attached at the top end. A cap for the coil was made from a seven inch vinyl phonograph record disk, with the hole in the center enlarged to make take the brass rod. Small holes were also drilled at opposite points on the disk for small round head wooden screws which were used to fasten it to the wooden disk at one end. I found the brass ball I needed from a metal bedstead in a pile of "stuff" that my uncle had collected in the farm's workshop. The base of the Oudin coil was made of hard wood, supported and insulated from the ground by rubber crutch tips, which fitted over wooden pegs, one at each corner. After completing the secondary, it was necessary to build the coil's primary winding, which consisted of eight turns of one inch copper ribbon attached to the base by four wooden cleats, slotted to separate the individual turns from each other. Flexible leads, with helix clips attached to one end were used to connect to binding posts to complete the coil. Using a half kilowatt transformer and a single unit, oil immersed, high tension condenser, I was able to generate sparks 10 to 16 inch in length from the coil. In the first version, my test subjects were the many hens and roosters in the yard of the nearby chicken coop. What a commotion!! Building a Tesla coil Suprisingly, compared to the Oudin coil, the Tesla coil ( figure 2 ) was even simpler to make and operate and consisted of a secondary winding of a single layer of No. 28 cotton covered magnetic wire over a shellacked 5 by 18 inch cardboard tube. The basic differences involved the number of windings on the primary and secondary and differences in the resistance of the copper winding. It even used the same type of condenser as that in the Oudin coil.   Figure 2: Circa 1915T coil. The actual building of new versions of the coils in preparation for the ninth grade science fair at the high school I was attending near Burbank in Los Angeles, Calif., was not a problem. But obtaining the parts was. What I was able to find so easily on my uncle's farm or nearby ones, was much more difficult in Los Angeles. It required many weekends of searching through hardware stores and other resources I could get to within walking distance. Fortunately we were living near the Burbank airport, which turned out to have a number of aerospace machine shop subcontractors and retail outlets that served their needs. The brass bed posts were a problem, but a machinist I met on my search volunteered to create them when I explained my needs. I finished the project on time for the Science Fair, but I did not win anything. But it was popular with the visitors to my booth and almost everyone got a "charge" out of it, though not quite as dramatic as the effect on the chickens on my Uncle's farm. Also, I was told it would be credited toward qualifying me for inclusion in the special college prep classes the following year in physics and calculus that had just been started at the high school I was attending. Another bonus out of the Science Fair project was that the process of collecting the parts from various machine shops made it easier for me to find a job the following summer. As it turned out, I would need every penny of what I earned. How I learned to think After that summer, the first thing I did on first day of class in the tenth grade was buy the text books for the college prep courses I was eager to sign up for. However, for a variety of reasons, it was decided that I was not college-bound material. I refused to accept that and got the principal to agree to add credits for those courses to my high school transcript if I hired a tutor that was acceptable to him to teach me the same material from the books I had purchased. With his approval, I found a tutor that met the school's requirements, a mechanical engineer I had met at the machine shop I was working at during the previous summer. Unfortunately for me, at the end of the semester the school district refused to allow the principal to add the tutored classes to my transcript. But the one-on-one process of working with my tutor on the physics and calculus problems -, usually in his office at the machine shop on weekends—was the most useful part of my entire high school education. I could not tell you now about anything I learned in my regular high school classes or about any of the teachers. But my sessions with my tutor and the lessons I learned from him are still vivid in my memory. Every chance he could, he would take me out on the shop floor and show me a real world practical examples of the problems I had to work on in the physics and math texts. He also taught me all sorts of tricks for visualizing problems and short cuts that eliminated a lot of unnecessary math. He showed me how to apply those same techniques to all aspects of life, outside of engineering, math and physics. He also taught me strategies for taking written tests and passing them. I firmly believe those lessons in thinking and how to approach problems were a significant factor in getting my Scholastic Aptitude Test (SAT) scores high enough to qualify for entry into college. More importantly, those lessons in thinking and problem-solving are still helping me to this day.