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By Locha D. Pitts You've followed standard troubleshooting procedures and the "dog" still doesn't work. So, what are you going to do now, friend? Troubleshooting is a difficult art to learn, especially when everyone is too important or too busy to provide apprenticeship training, and employers prefer 25-year-old technicians with 10-years experience in CB-chroma circuits and ECL-logic crystal sets. The electronic technician's world is assembled from intermittent fuses, mismarked diodes, broken pins, leaky capacitors, wires with only one end, corroded switch contacts, sticking meter pointers, missing transformer cores, misrouted wires, hair-line cracks, touchy circuit breakers, and infinite ways for components to go bad. Believe me, every tech needs all of the help he can get. So, let's sit down with cups of coffee, and talk about a few ideas that might help solve some of these electronic problems, which seem to defy all logic. Use Your Test Equipment You shouldn't attempt difficult troubleshooting unless your test equipment is in good shape-and your most important test equipment is your mind and body. The technician who hasn't discovered that he's human (and not a robot) will waste kilo-hours on a difficult problem when he is tired, sleepy, hungry, angry, hung over, or worried. How are you going to fix a "dog" if you are a Cathode Head (charged with negativity)? A lot of exasperating unfixables have become "a piece of cake" after a good night's sleep, a coffee break, or some time spent on a routine task. Ask For Help Once you've decided you don't have to be perfect, you can ask another technician for help. Most employers and customers are concerned only that you fix the hunk of copper, silicon, and carbon. They don't care where the troubleshooting ideas come from, so long as you fix the gear. Ideally, you should talk to a technician who has good judgment, and is experienced with the type of equipment you are troubleshooting. He might tell you to check the plate voltage in the color-killer circuit, or remind you that this particular set obtains a B4--voltage from the cathode of the audio output tube. Or, perhaps while describing the symptoms, you'll remember that you forgot to check the color oscillator, or to replace the modulator transistor. Maybe you'll remember to peak the grid and dip the plate. Sometimes talking to a parts supplier can be helpful. The first color picture tube I ever replaced died after a month. After a counterman at the local distributor told me about the many cold solder joints at the heater pins of the picture tubes, I was able to restore it to new life by applying a few drops of solder. Be Persistent Some troubles require persistence more than intelligence. I recall the case of a kilowatt mobile transmitter which remained inoperative for more than a year because no one had the persistence to remove the power supply from the equipment cabinet, in order to get at the terminals below the chassis. It took three hours to remove the cumbersome beast, but only five minutes to confirm that a power transformer secondary winding had burned up. Read Technical Publications Too many techs fail to read technical magazines and manuals, and thus are unaware of many changes in the field. In the days when a certain resistor in the tuner was supposed to open under any overload, such techs could have prevented much head scratching by reading in the magazines about the problem. Also, if you don't know about scan-rectified power supplies, you probably will waste precious hours testing tie power sources, instead of looking for the defect in the horizontal sweep. If you have a technical manual for the bucket-of bolts you're working on, sit down and read about tie circuits that-from the symptoms-seem suspect. Or, obtain a book or magazine article dealing with similar problems or circuits. Some technicians pride themselves on troubleshooting without a schematic. This attitude was understandable in the days of the five-tube AC/DC radio, but when you get in over your head, you must have a schematic, and you need to study it. It always seems to turn out that pin 7 is supposed to be tied to pin 8. Or, what looks like a multivibrator is really a differential-input stage. Question Your Assumptions Every good technician makes a lot of assumptions when he first opens up a piece of equipment for inspection. These assumptions usually simplify and speed up the work. However, when your frustration has you calling for the men in white coats, you'd better begin questioning all assumptions. Normally, all of us would expect a simple tube layout chart to be correct. But, I once fixed a TV set (after a tinkerer had been playing with it) by deciding intuitively that the tube chart was wrong. I began to swap the two tuner tubes. And, as I brought the RF-amplifier tube near the socket it was not supposed to be in, loud sounds came from the speaker, and the picture appeared. Speaking of assumptions, did you know that the "A" modification of a popular op-amp IC has different pin connections for offset adjustment than the original version did? If you assume the pin connections are the same (who wouldn't) and replace the original with an "A" version, :he circuit won't work. Today, the most dangerous assumption is that the equipment is properly wired with the correct components in the right places. Be especially wary of one-of-a-kind industrial equipment, kits, and low priced consumer gear. I remember the fun of debugging a kit-type amplifier, built by a color-blind technician. He had interchanged only four resistors. But, what output tube works well with 2200-ohm grid resistors, instead of the specified 2.2-megohm value? Then, the VOR Omnirange system had performed very poorly for six months, before I discovered that the complex antenna had been wrongly color-coded at the factory. If you have troubles in one part of the set, make certain the rest of the set is working properly before proceeding. Consider the sync problems you can get from power-supply hum or low B+. Even if the secondary problems aren't causing your primary symptoms, it's a lot easier to work on gear after you're confident everything else is working the way it should. (Sync problems are easier to troubleshoot after you've replacer. that worn-out picture tube.) Years ago, I knew a fellow who fixed TV sets by replacing all tubes. If that didn’t repair it, he replaced ad the capacitors; if that didn't fix it, he replaced all resistors. If you think that's funny, you should have been there when he cut out about 20 capacitors, without marking where they came from! Of course, if you have localized a problem to a specific circuit (such as the focus section of a color TV), it could be cheaper and quicker to substitute three or four dollars worth of parts, rather than spending an hour or two trying to discover which part is defective. What now? The nitty gritty of troubleshooting comes when you seem to have done everything possible, and you've run out of ideas. Inertia at this point can be deadly. Try Something Different Do something! Anything! Make more measurements. Do a few substitutions; try your intuition. Use different test equipment. Make a list of all possible causes of the trouble that you can think of, including those you've already checked out. Compare voltages, resistances, or waveforms with those of a similar and known-good unit. Or, compare with the other channel, if you're working on a stereo amplifier. I suppose you expect me to sum up all this advice, but I'm not going to. Get us some more coffee and I'll tell you about the transceiver power transformer we had to melt out with a blow torch. Also see: |
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