This circuit was found in an American medical documentation from 1970. The circuit rejects the 60 Hz noise pickup and generates 2.5 Hz signals. Equivalent components can be tested.
The transistors are 2N918, 2N3464 and other RF equivalents of the time for Q1. For Q2, 2N3399, 2N2894...
This circuit, which can be used as a shield, was found in a 90s Electronic Design. The circuit...
This interesting intercom circuit was obtained from an Argentine publication from the 1970s....