PRIVATE MANUAL BRANCH EXCHANGES
Desk top, Cord-type Switchboard Type N 171
PMBX 7 + 40
This private manual branch exchange brings a new dimension to the field of PMBX design. Sturdily constructed, yet superbly styled to complement the most luxurious surroundings in offices and other business premises, it employs a new Plessey component development - the cord-storage cassette - to present desk operation with cord-switching simplicity. Matching this unique feature is the switchboard's capability to accommodate a line capacity outside the
practical or economic limits of conventional desk designs. Seven exchange lines are catered for, together with 10 cord circuits, and a basic 40 extensions, which may be increased to 50 if needed. Lamp signalling is employed throughout.
The cassette is provided on the basis of two per cord circuit and consists essentially of a cord carrying polycarbonate spring-loaded drum rotating within a nylon housing. The cord occupies 31 turns of the drum in the resting position, giving a total extended length of cord outside the plug shelf of 2ft. Tension is the minimum for quick, reliable retraction, and cord life is high due to a reduction of both friction and cord flexure (each portion is flexed only twice per call, as against four times with pulley weights). Cassettes are retained by a single screw, allowing quick installation of a new unit when a cord becomes faulty or worn.
The compactness of the cord storage has permitted the same freedom in styling as would be possible in a key-type design. The switchboard occupies small space, and is contained in an attractive walnut-veneered cabinet constructed in resin-bonded plywood. The plug-shelf, inclined at the optimum angle for ease of operation, is of sheet steel, finished in a durable dark blue stove enamel.
Lamp strips, mounted immediately above the relevant jacks, are fitted with new type square lenses giving high-level uniform illumination.
Ancillary jacks for ringing-supply, test and cord-test are provided; a neon lamp is
associated with the former. The cord-test facility utilises the inserted plug's
supervisory lamp as a fault indicator. A fuse alarm lamp is fitted adjacent to the
ringing-supply neon, and the common-calling alarm pilot lamp is positioned centrally
beneath the jack field.
The jack strips used throughout are of a demountable design, enabling individual worn jacks to be removed without disturbing
others in the strip.
"Call" and "answer" supervisory lamps, and two similarly associated keys "speak/ring call" and "dial/ring back" are provided on each cord circuit.
Keys grouped at the left of the shelf provide for night service, buzzer cut-off, and changeover from the built-in transistor ringing
generator to an external source when available.
The raising of the plugshelf on to its strut support gives ready access to wiring and components beneath, and does not require the breaking down of existing connections. Further calls can, in fact, be handled.
Relays are Type 3000, mounted in detachable vertical strips. Access is via the rear door, which also serves to mount a fuse panel incorporating moulded alarm-type fuses.
An elephant-grey operator's handset, supported on a fixed horizontal cradle, is provided at the side of the switchboard, and connects into the circuit via the jack below the front edge of the plugshelf. In addition, a matching lightweight headset can
As is conventional, exchange calling lamps glow continuously on incoming calls until the answer plug is inserted, and extension lamps remain under control of the calling - instrument cradle - switch until answer.
Cord circuits apply an automatic hold to calling exchange lines when the "speak" key is released; the hold is maintained until the called extension answers. At termination, both supervisory lamps glow when the extension replaces his handset-an action that immediately releases the exchange equipment. The effectively metallic plug-to-plug connection, which provides for this feature, also permits extensions to dial their own calls if desired. Direct-clearing on calls between extensions, the associated supervisory lamp glows as each extension clears.
Weight of Switchboard: 1081b (48.9kg) net.
Extension loop resistances of up to 500 ohms are permissible.
Extension lines are 2-wire
Operating voltage is 50V d.c..
Plessey Publication (1970)
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