I;m looking for classic acoustic modem schematic. The type we used in the '80 with our Apple II, MSX, ZX Spectrum and C64.

The Elektor magazine published a circuit in the '80 but I don't seem to find
a copy or reference on the internet.

Can anyone help?

 

Take a look here:
http://www.freepatentsonline.com/4367374.html

You'll need to create an account (free) in order to be able to view it, however.

Keep in mind that such a modem would have extremely limited usefulness nowadays; as they were limited to approximately 110 baud.

 

http://www.discovercircuits.com/M/modem.htm