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By Carl Babcoke, editor Theory versus practice Some electronic technicians think of general service information as being "theory," while specific information is called "practical." After they graduate from tech school, many believe no more theory ever will be needed. That is wrong! A technician who uses correct theory in practical ways is certain to find defective components or wrong conditions much faster than if he only tapped, prodded, twisted and replaced parts instead of making accurate tests with proper equipment. I proved that in my own case. I was a competent tech although I had little formal training, but I diagnosed faster and more accurately after I learned correct theory. In addition, one engineer in the Bureau of Standards is quoted as saying that nearly 50% of the "facts" in electronic books either are wrong or incomplete. I believe that statement is true. That's why most of our articles that describe circuit behavior use original material and not textbook explanations. Some of our descriptions of tube-powered and SCR sweep circuits have become classics of corrected theory. Fortunately, the choice is not "theoretical or practical" but it can be "theoretical plus practical." You should have the best of both. Effects of solid-state Unfortunately, many of the quick-and-dirty techniques we used successfully with tube circuits are useless when applied to solid-state circuits. One reason is that solid-state circuits are not standardized as much as tube circuits were. A helpful shortcut for one brand and model of solid-state product probably won't work right if applied to another model and brand. Another reason is the inability of solid-state devices to tolerate overloads (a transistor will blow before its protective fuse can open). I am not foolhardy enough to attempt some unproven shortcut that might ruin a whole string of transistors. Nor will I recommend such methods to you. Plug-in modules have reduced the amount of troubleshooting by allowing a choice of total replacement or diagnosis plus repair. Too much information One of the answers for successfully servicing these new-technology machines is for technicians (who already know how the circuits work) to use better test equipment while making many more tests. However, a common discouragement for techs who try this kind of upgrading is when their tests give them information they can't handle. As one tech said, "My new scope tells ma more than I want to know." He meant that he could not interpret the confusing readings. If you keep trying and learning, eventually you'll advance to where these sophisticated tests are clearly understood. They then will be valuable and indispensible to you. We believe Electronic Servicing can help you reach this desirable goal. Space-age information In early 1976, we began a continuing series about the circuits and servicing of modular color TVs. Our coverage of seven different models has been quite detailed, with original explanations of all major circuits, including actual voltage readings and special waveforms (some never printed before). Many discussions went beyond the immediate circuit, so you could adapt the information to other circuits and models. Two unusual training devices we have emphasized are current waveforms and dc waveforms. These dc waveforms included a line marking the average voltage or zero-voltage point. One reason tube theory can't be used also with transistors is that silicon transistors ignore any forward bias of less than 0.5 volt. These low voltages do not cause C/E current. DC waveforms also are essential to our clear understanding of digital circuits. Digital signals are pulses of dc voltage. Therefore, proper measurements must include the positions of zero and supply voltages in relation to the pulse waveform. In short, we at Electronic Servicing have been attempting to prepare you for successful and profitable servicing of whatever circuits the future may bring. Our efforts have not always been appreciated. One technician, whose address was obliterated, commented that even the engineers who designed these circuits could not understand our explanations. He wanted "practical" information he could use at his bench each day. Of course, we believe a correct knowledge of circuit operation is the first (and probably the most valuable) step in any kind of troubleshooting. What kind of articles would you like to see in Electronic Servicing? Write to the editor at: P.O. Box 12901, Overland Park, Kansas 66212, USA. (adapted from: Electronic Servicing magazine, Dec. 1978) |
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