Filament source (T398E)

Despite being transistorized, this is a constant current source circuit for 1 V valve filaments such as the 1H4 tube.

Answering Machine (CB18202E)
This 1985 circuit takes advantage of an old cassette recorder to answer and record phone calls,...
Locomotive Sound (CB12323E)
This circuit is from 1990 documentation. It can be implemented with BC548 transistors instead of...
Crystal Test (CB13945E)
We found this 50 kHz 25 MHz crystal tester in a 1978 QST magazine. The FET can be the BF245 and...