PLAN NUMBER WORKING
These telephones are for providing economically various extension schemes without the use of separate switches or switchboards.
They are arranged to accommodate key units having from one to nine spring sets of the make-before-break change-over type, i.e., " K " combination.
The spring sets are operated by push buttons, and are illustrated overleaf.
For a simple extension the instrument has a key unit with one set of K springs. For other extension schemes the key units have a number of K spring sets, the standards being for 4 and 9 sets. The push buttons on a telephone with a 4K spring set operate respectively (left to right) 2K-1K-1K spring sets, while with a 9K set, 4K-1K-4K are operated.
Each contact spring is wired to a separate terminal so that the scheme in hand may be readily connected up.
On the 4K and 9K key units there is a mechanism which can be varied to control the non-lock or locking of the two outer keys, the centre key being non-locking. There is also supplied with each of these key units a T piece which is fixed to the cradle lever when the extension plan necessitates the release of a key on replacement of the micro-telephone.
Instruments are also available with battery bells so that plans to suit almost any requirement can be arranged. At each station allowed to call the exchange a separate generator N8486 type is required (shown above).
The telephones are normally produced in black, but if specially ordered, they can be
supplied in Chinese red, ivory or jade green. They can also be obtained without the
instruction tray and directory pad fitment.
In the case of plans requiring only two push buttons, chromium finished dummies can be supplied for the third hole.
The terminal blocks, cords and labels vary according to the plan required, so that full details regarding the scheme to be operated should be supplied when ordering.
Circuit diagrams of various plans can be supplied on request.
The telephones in the table below are complete with instruction tray fitment.
Taken from the Ericsson catalogue, edition No. 49
Last revised: October 13, 2019