Thanks to all who posted. I read or heard about someone having built one of the
first computer modems in the 1970s and that it was very unstable and had to be
tweaked every X minutes because it would run hot and heat would affect it. I
have no idea what it was. Perhaps the solution indicated by Jinx involving a 555
baud rate generator would fit the description. I have built a simple simplex
modems using FM with CD4047+CD4046 and they worked very well. A very similar
circuit was used for muzak over powerline, and for power line intercoms, with
4046 demodulators in the speaker amplifiers.

Peter


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