|Private Manual Branch Exchanges
CB 20+100 cord-type (N327)
with automatic or C.B. exchange lines
This switchboard, an indicator signalling type, has capacity for 20 exchange and 100 extension lines with 17 cord circuits. Stock units are available wired and equipped for 10 exchange lines, 50 or 70 extension lines and 15 cord circuits, and can subsequently be extended to full capacity as required.
The switchboard is specially suited for busy establishments where speed of operation is essential. The facility for automatic holding of exchange lines enables the operator to deal with other calls while waiting for a called party to answer. For installations of from 100 to 200 extensions, two units can be bolted together to form a two-position, non-multiple switchboard. When two positions are installed en suite, each is equipped with longer connecting cords and double pulley weights.
Exchange lines are normally for connection to an automatic exchange, but alternatively
can be C.B. Each cord circuit has a speak/ring key and a dial/ring-back key.
The key shelf is arranged with supervisory indicators and switching keys for cord circuits and miscellaneous facilities.
Internal equipment is positioned and mounted for maximum accessibility. Incoming cables enter via covered apertures in the open-constructed plinth, and terminate on soldered terminal fields positioned at the bottom of the unit.
For convenience of handling (e.g. during installation) the hinged key shelf can be raised and its apron lowered to reduce the depth by 8.5in (216mm). The total weight for stock units varies depending upon extension-line capacity
Operating equipment, supplied as standard, includes a lightweight plug-in handset with rocking-armature receiver, a manually operated ringing-current generator, and a dial adjusted to transmit 10pps with 2:1 break/make ratio and with standard numbering, i.e. 1 to 0.
Alternative equipment, supplied in addition to or in lieu of the above includes a lightweight plug in headset, a transistor ringing generator powered from a 24V source, a dial with any desired adjustment and numbering and a ringing-resistance lamp (where an external ringing supply is used).
Internal components, including relays and relay retards, are to British Post Office specification and are afforded tropical finish, i.e. with coils suitably impregnated and metal parts specially finished. Conductors are insulated with PVC, giving protection against moisture, fire, attack by insects and fungoid growth. Connecting cords are additionally protected by PVC sleeving and nylon braiding.
The loop resistance (exchange line via switchboard and extension telephone) is 700 ohms maximum. Exchange lines of lower loop resistance enable higher loop resistance lines to be used to the extension telephones.
The table shows the range of loop resistance values applicable to the PMBX, based on a 50V 200/200ohms public-exchange feed bridge and telephones of type N1900 or 617/1/02161/000 at the extensions.
Note: 100 ohms in PMBX and 130 ohms in extension telephone
Last revised: October 12, 2019