The multimeter is our eyes; without it, we would find it difficult to fix various faulty machines. However, blindly trusting measurement results can sometimes lead us into a confusing world, [excluding cases of multimeter malfunction and inaccurate measurements]. Here are a few simple examples: A modified board machine had a faulty abl resistor, causing the tube socket’s focus pole to break down and spark against the board. After replacement, there was a return sweep, the brightness was too high, there were sweep lines, and no image. Adjusting the acceleration pole showed changes, but it still did not meet the requirements. Measuring the three cathode voltages yielded zero; further measurement showed 180V was absent. Upon checking, the rectifier diode and current-limiting resistor were basically normal. Disconnecting the video amplifier tube one by one still showed no voltage. Thus, I replaced it with a 180V rectifier tube, slightly adjusted the acceleration pole, and the TV returned to normal. A Changhong HD TV sometimes starts normally, sometimes does not, but the main voltage remains normal, with the power supply showing 13V sometimes and sometimes not. After disconnecting the rear load, the 13V was normal, leading to suspicion of a load issue. Following the line, it surprisingly passed through a transistor control to the digital board. Suspecting the digital board had a protective issue, I replaced it with a functioning digital board for verification, but it was ineffective. Thus, I considered the power supply itself or protection, and tried replacing the 13V rectifier tube, which allowed the machine to start and stop normally, with no further issues. Later, I swapped back the original digital board, proving it was good. The rectifier tube measured normal resistance in both directions but was no longer usable. For a TCL color TV, horizontal bright lines appeared, identified as an 8177 field block issue. Routine checks showed normal power supply and peripheral components, so I replaced the 8177, yet it was still ineffective. Later, due to a temporary delay of a few minutes, the machine started normally, but this good state didn’t last long, as the fault reappeared after about ten minutes. I suspected that a component was thermally unstable, so after waiting for the machine to cool, I started it again, and it was normal. Using a soldering iron, I quickly heated relevant components in the field section and found that heating the bootstrap diode caused the fault to appear immediately. I promptly replaced it, and the machine returned to normal, no longer showing a bright line after heating. For a Panda TV, it made a clicking sound upon startup. After disconnecting the horizontal load, it operated normally. Initially, I removed the D2553 horizontal tube, which measured normally, suspecting the high-voltage package had an AC short. Testing with a universal package showed the fault as initially suspected, with no signs of damage in all horizontal components after repeated measurements. Later, when starting without the heat sink, I found that the horizontal tube sparked against the heat sink. I moved it away from the heat sink to prevent sparking, fixed it with an insulating piece, and resolved the issue. During a site repair of a three-phase motor that wouldn’t start, I measured the resistance of the three wires, which were all the same, suspecting a winding fault. Upon disassembly, I found the windings bright as new, with no issues. Asking about the motor, which had been purchased recently, I decided to try replacing it with a similar three-core wire, which worked. Typically, if a standard three-core wire cannot handle high current, one or more wires may have a high and unstable resistance, but this was not the case here. After sharing so many examples, I believe many people have encountered similar issues, which can be particularly frustrating for beginners with insufficient experience. Ultimately, I want to emphasize that instruments cannot always be overly trusted.
Original:http://www.jdwx.info/article-791-1.html
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