This two-transistor circuit is from a December 1964 Radio Electronics. Modern transistors can be tested. The headphone must be high impedance or crystal with a 2k2 resistor in parallel.
This power control circuit was found in an article in the June 1995 Electronis Now magazine. The...
This circuit was obtained in a 80s RCA documentation. The circuit uses components from that time....
This circuit compares the input voltages by providing an output according to the curve next to the...