This circuit was found in a 1967 US documentation. It shows how to build a gain control by digital signals. The circuit can be used as a shield. FET can be a modern equivalent.
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....