We found this precision circuit in a manual from the 80's. The operational amplifier is of a common type at the time, and equivalents can be tested. Transistors are general purpose types.
This touch switch is from a 1978 publication. The circuit should not be powered by a...
The circuit shown uses the 741 operational amplifier, but lower noise and higher gain equivalents...
We found this zener regulator in a 1978 radio amateur documentation. The operational can be...