Something Weirdly This Way Comes - Beacon Trouble Edition
I had some temperature beacon trouble at the remote location. When I tried listening Sunday morning all I heard was a carrier - it turned out to be my beacon was key-down. The beacon has been running 24 / 7 / 365 for years without a problem. Here's a picture: The old Knight CB talkie is OK, the wall wart seems OK. But the beacon in the tin is NG. It is a PIC based beacon which reads and transmits the temperature in degrees Fahrenheit every 10 seconds or so. I also used an op amp to scale the LM-34 sensor voltage and an external voltage reference for the PIC A/D. There is also a piezo at the lower right which clicks in time to the Morse. Diagnosis: Is the keying transistor bad ? It is a 2n7000 MOSFET. I tried replacing it. The beacon worked BRIEFLY but then went key down again. Then it dawned on me: what about the jack ? There is a 3.5 mm stereo jack to the Knight CB which brings ground, +9V and the keyin...