Found in an April 1977 Radio Electronics issue, this circuit uses the well-known XR2206. The C1 values for the various frequency bands are given next to the diagram.
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....