This is the circuit for the HM-10ª, an instrument sold by Heathkit. The circuit is from a Radio Electronics from May 1963. The indication is by an analog instrument.
This circuit is from a 1974 publication, but it can still be easily assembled today with BC548...
This driver for audio signals using the TAA435 integrated circuit was found in Philips 1968...
We found this circuit for old tape recorders in the December 1972 Radio Electronics magazine. The...