Thinking more about how to repurpose the Interconnecting Box, Drivers Box, to form a combined Digital Modes Interface and Audio Test/Break-Out Box, I started to consider what might be inside one.
A couple of things were clear. There would be a switch, probably multipole, and a potentiometer. Beyond that all I could make out from the external view photos was the presence of the connectors!
But, thanks to Chris G8MKT, I am now in possession of EMER TEL L802, pt's 1 & 2, the Clansman Harness Technical and Repair manuals. And from pt.2, comes this wonderfully useful diagram -
The second, obvious item is the volume control. That will certainly be used on the receive audio path to control the level into the PC for decoding, as well as remaining a usable volume control for the Clansman audio kit.
More interesting though, is the presence of two transformers, and a relay! The datamode interface requires isolating transformers - could these perhaps be pressed into service there? As for the relay, it may be useful for PTT control.
One thing I do have to decide is how I will connect the completed unit to the radios. If I do so from one of the audio connectors, I require a plug to plug cable. For this it might just be best to use the cable for the loudspeaker, so I need to check how many of the cores are present in this cable to ensure I can make the required connections this way.
Interestingly, PTS Norfolk has the telephone cable terminals available at 75p each, or the sprung wire terminals for £2 a go. Using these would make the resulting project look very authentic!