The transistor used in this circuit is uncommon today. AC124 or equivalent such as AC188K and 2N2431 can be used. The circuit is from the 80's and makes use of transformers.
This simple metal detector was found in a 1960 Sylvania transistor manual. The phone must be high...
The figure shows how to obtain an 800 Hz astable circuit controlled by an external signal. The...
We find this circuit in an American 1980s documentation. It can be applied to any operational...