Negative ion detector (CB21001E)

This tube circuit was found in the magazine 73 for radio amateurs in an issue from the 1980s. Note the transformer used to power the circuit.

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Capacitance meter (CB22163E)
Found in an English documentation from 1989, this circuit uses components common today. The...
Simple capacitor test (CB7060E)
The LED frequency flashes depends on the value of the capacitor tested according to the table. We...
Hartley crystal oscillator (CB7541E)
Found in a Popular Electronics from the 90s, this circuit generates a signal that depends on the...