[russell_w_b]

Originally Posted by richrussell

I think I posted on here about four years ago when I last had a go at getting them going, but nobody knew then. There seem to be plenty of four terminal intercom phones, but not many three terminal ones. Unhelpfully the terminals are just labelled 1, 2, 3.

Here's a circuit diagram of a Stirling No:U200. It may be similar to your instrument.