Anderson Jacobsen ADC 300 modem

We had (I doubt any are still in use) some modems over here that (IIRC) were made by Plessey, and rented by the GPO (who ran the telephone service, and who owned everything connected to it at the time. You had to rent whatever equipment you wanted from them). Anyway, the modems in question were large -- about 2/3 the size of a PC/AT system box -- metal cases with (mormally) 4 plug-in modules inside. They were the PSU, Modulator, Demodulator, and control. It was all discrete components (well, the 'control' module, which did things like ring detection, hookswitch, etc used a number of relays). The modulator was either a pair of oscillators and a way to swich between them, or a single VCO with a circuit to switch the control voltage.