Found in a 1980s telecommunications documentation, the circuit presented acts as a product detector. The modulator or mixer in the square may be that of the previous circuit.
The purpose of this circuit is to keep the other phones on the same line muted when one is used....
Measuring capacitances and resistance this simple instrument was found in Radio Electronics...
The circuit shown is from the 1977 GE semiconductor manual. More modern equivalent SCRs can be...