By Stephen Coan
Years ago in the mid ‘70s I was working at Tektronix in their calibration lab. Tek used a lot of purchased power supplies from name brand manufacturers. Our lab also had a repair shop for non-Tek equipment, so we got to look at all kinds of items. One day I got a small 5-10W power supply in for repair. The output voltage was out of range. After analyzing the circuit, I found that a feedback resistor had changed by close to 20 percent from its nominal value.
I replaced the resistor with the same size and value and the supply worked again. A few months later, I had another of these supplies show up with the same problem. After analyzing the circuit more carefully, I found that the precision 1/4W resistor was passing more like 3/8W of power. I replaced the resistor with a 1/2W part and notified the area that they had a problem and to send in all the power supplies for a modification. Once these were all modified, I never saw one again while working in the lab. It was such a simple circuit, but it was not completely reviewed before it was released to manufacturing. Maybe this company was building-in calculated failures for replacement a few years down the road.