This is a combined circuit of tone generator and preamplifier. We found it in a 1975 documentation. Operational amplifiers of that time can be replaced by modern equivalents.
We found this circuit in a 1965 magazine. The circuit can be assembled with current equivalent...
This is a typical circuit of a PROM programmer from the 70s. It appeared in the magazine Radio...
If there is an indispensable household appliance in our kitchen these days it is the microwave...