My HP8640 died!
My HP8640 died! This is me catching up on past projects. I did this back in 2022, figured someone would benefit from this information, since the 8640 is still around in great numbers at hams all over the globe. I bought a nice looking HP8640 at our local hamfest back in 2022. It was working when I got it home, but a day or two after I got it home, it started acting erratic, with RF ouput dropping out. eventually, there was no output whatsoever. Did some tests, and got to the conclusion that the cavity oscillator was not oscillating. Following various clues online, I tracked down the issue to the A3 unit (cavity), and a series resistor, which had drifted in value. By measuring the series resistance between the + feed and the A3Q1 socket and the - feed and the A3Q1 socket, I could see that the + lead seemed reasonable at 370ohms, whilst the - lead had drifted to 19Kohms, where I was expecting around 900ohms. The A3Q1 (HP21 pictured below) transistor sits under the small hex cap om t...
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