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 source was found in a 1982 American publication, but can be easily assembled using 2N2995 (PNP...
September 2016 - It can be purchased at Mouser Electronics the new REF6000 precision voltage...
This circuit uses the well-known 8038 to generate 3 waveform signals. Frequency bands are selected by...