We found this circuit in a 1965 magazine. The circuit can be assembled with current equivalent transistors and the precision in generating the frequencies depends on the tolerance of the capacitors.
The circuit shown is only for analog telephone lines. I found it in Popular Electronics (1991). It...
In this circuit from the 60s, the power supply is made with 22.5 V, which is the voltage of the...
This circuit is from 1967 General Electric documentation. It uses SUS which are not very common...