Norchard Exchange NU Tone Circuit
Normally a U13 provides NU tone on spare group selector levels by appropriately strapping the vertical marking banks. This is somewhat tiresome to do and it was felt simpler to allow the selectors to cut into spare levels and to find a circuit that returns NU tone. The circuit is very simple, consisting only of two relays.
The group selector tests into the battery condition on the P wire and extends the calling loop to operate the A relay. A1 operates the B relay which then returns an earth on the P wire to hold the connection. B is made slow to release so that should the caller continue to dial, then B will continue to hold the P wire and not release during the dial pulses.
Relay B also connects NU tone to the third A relay winding which transmits the tone to the caller via the transformer action within the relay coil.
When the caller clears, A releases which in turn releases B which removes the earth from the P wire thus releasing the connection.