This audio oscillator is from a 1957 Raytheon publication. The tone generated depends on C2 and C2 and the handset must be high-impedance magnetic. Equivalent transistors can be tested.
To obtain a signal with a variable mark / space ratio in a ratio of 1 to 20 to 20 to 1, we have...
Other operational amplifiers series, such as the MC34072, MC34074, MC35071, MC35072, MC35074 d...
We found this sophisticated meter circuit in an article in Radio Electronics magazine from June...