As a newly hired engineer just out of college I was placed on the production line at Tektronix for two weeks to get familiar with production procedures and some of the product line. I was set to calibrating and testing type 547 storage oscilloscopes.
Everything went fine for several days until I ran across one unit that just wouldn't focus a proper spot on the screen. Since the cathode circuitry is sitting at -2000V, it was very difficult to measure anything with the scope power turned on. After getting bit a couple of times by the regulated high voltage, I decided to turn everything off and measure components one by one in the area where I thought the problem was.
I finally discovered a shorted 1N914 diode. Feeling confident, I got another fresh diode right out of stock. The instrument was built with silver-plated ceramic terminal strips, so I had to use special 2 percent silver bearing solder to fix it. I replaced the diode and turned the scope on.
There was no change in operation, so I again started looking for another component that was bad. After half a day of looking I kept coming back to the same diode that I had just replaced, but didn't test it because, well, I had just replaced it.
I finally decided it could be the only component that could cause this problem. Sure enough, it tested bad. I analyzed the circuit and could come up with no way that the diode could be damaged. It had to be bad right out of stock. I replaced it again and the scope worked and calibrated properly.
This entry was submitted by Jack Gilmore and edited by Rob Spiegel.
Jack Gilmore works for Advanced Energy, covering analog and digital design of industrial products using micro controllers including software and hardware design, circuit board layout, and product packaging. He has also designed RF instrumentation, RF matching networks, and a 333kW solar inverter and accessory products to enhance performance.
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