WB900 Pair Gain Equipment

WB900 has nothing to do with the Cold War, it was a system for connecting two customers to one pair of wires to overcome shortages in the underground cable network. The second customers line was connected via a carrier system on the first customers line. A 64KHz carrier carried the speech and ringing signals to the second customer. A 40KHz carrier from the second customer carried the speech, off hook and dialling signals towards the exchange. The battery in the remote carrier unit in the second customers house was trickle charged over the line.

Everything about this system was wrong. It was not a Wire Broadcast (WB) system, as it carried signals in both direction, by definition broadcasting is uni-directional. The engineers who had to work with it hated it. The customer was charged full line rental unlike the highly unpopular 'Shared Service' which was in abundance throughout the 1960-70's. The engineers were not supposed to tell a customer with WB900 there was someone else sharing the line. Shared Service offered no privacy between the two customers sharing the same line, at least WB900 offered full independance. The service was unreliable due to faults and flat batteries.

Here is a picture of the internal printed wiring board and inside cover of a "Subscribers Unit WB900, Adapter No.1A" a small white box approximately 80mm square.

[courtesy of Andrew Blofield]