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Descriptive Leaflet
TELECOMMUNICATIONS
INSTRUCTION
C MARKETING
INSTALLATION
3 Internal
E4002
April 1971
Private Manual Branch Exchange No.
2/3
(P.M.B.X. No. 2/3)
Description
GENERAL
The Switchboard, PMBX No. 2/3A is a lamp-signalling cordless
switchboard with five connecting circuits and has a capacity for
3 exchange lines
and 12 extensions. The
switchboard has been designed to stand on a table or similar
support.
Each extension telephone is normally connected to the
switchboard by four wires (A, B, C
and an earth). External extensions may however, where necessary,
be provided on a 2-wire
basis by fitting a Unit, Auxiliary Apparatus, No. 96A at the
PMBX end of each such
extension. Similarly inter-PBX circuits can be connected via a
suitable Unit, Auxiliary
Apparatus, e.g. No. 97, 98 or 99. The switchboard can be used in
CB (manual) and automatic
exchange areas, and also, with Unit, Auxiliary Apparatus, CBS536
fitted at the public
exchange in CBS No. 1, 2 and 3 areas.
Power and ringing supplies are normally obtained directly from
mains operated units but
may be derived from secondary cells when standby power is
required.
PHYSICAL DESIGN
The colour of the switchboard is two-tone grey and a matching
Telephone No. 706 is
normally used as the operator's telephone. The switchboard is 17
in wide by 8 in high by
13 in deep and it weighs 33lb. It has a metal chassis and an acrylonitrile-butadiene-styrene copolymer (ABS) cover.
1. The lamp cover strip is removed by moving the phosphor-bronze
clips at each end inwards
towards the centre of the switchboard and pulling forward. On
removal of the lamp cover
strip the two lamp-jack fixing screws are exposed. By unscrewing
these screws the lamp
jack is drawn forward. The lamps can then be removed without the
use of a lamp extractor
or removal of the switchboard cover. The individual lamp prisms
are a push fit and can be
readily removed by pressure on the rear. A non-metallic
instrument should be used for this
purpose to prevent scratching the prism. A new lamp prism can be
inserted from the front
by the application of slight pressure.
2. There are two designation strips fitted on the face panel,
one for the extensions and
the other for the exchange lines. The extension designation
strip (Strip, Designation, No.
37A) is removed by applying pressure on the lower edge and
sliding the strip upwards
towards the lamp cover strip. This releases the fixing clips,
which slot into the face
panel in each half of the designation strip mounting. The paper
label, which is printed on
both sides, can then be removed for marking as required. With
the extension designation
strip removed access is given to the label for the exchange
lines, which can be removed by
sliding it out of the right-hand side of its mounting.
3. Subscribers' private meters of the cyclometer trip type
(Meters No. 21B, Grey) can be
provided on the face panel in the space normally occupied by the
exchange line designation
strip. When these are required the existing designation strip
(Strip, Designation, No.
36A) is removed by pivoting the locking projection on the under
side, and is replaced by a
narrower strip (Strip, Designation, No. 33) which covers the
upper part of the meter
front. When replacing designation strips care should be taken
not to damage the panel
surface. For details of metering schemes see E5001 and for the
application of these
schemes to the switchboard see Diagrams
N1005 and
N1006. Metering
connections within the
switchboard are made on the auxiliary terminal strip fitted on
the key panel adjacent to
the exchange line keys.
4. The cover is held in position by an angled bracket which
pulls the cover to the rear by
means of the screws in the flange at the rear of the baseplate.
To remove the cover the
screws, which are captive, are unscrewed, and the latching
bracket springs clear of the
projections on the inside of the cover. The leading edge of the
cover fits under the front
of the face panel and adjustable plates which are positioned on
each side of the panel
form dust seals.
5. The chassis consists of three sections, the front and rear
panels hinging on the
baseplate, forming a triangle when the chassis is closed. By
releasing the screws at the
apex the hinged panels can be opened outward to give access to
the wiring and components.
6. The relays in the switchboard are standard PO 600 and 3000
type and are mounted on the
rear panel. A metal cover, retained by a spring slip at each
end, is provided to protect
the relays.
Case removed - keyboard and relay mounting folded back to
expose wiring
7. A terminal block (Block, Terminal, No. 46A) with 122
terminals is mounted on the
switchboard base immediately below the relay mounting plate. The
conductors of a Connector
No. 1068A are terminated on 68 of these terminals. The free end
of the connector is
plug-ended and jacks into a wall-mounted socket to which the
permanent cabling is
terminated. Terminals are also provided for the connection of
the operator's telephone.
The remaining terminals on the terminal block, together with the
tags on a Strip,
Connection, No. 153/10B which is also provided on the base of
the switchboard below the
face panel, permit the termination of 2-wire extensions,
inter-PBX extensions or private
circuits using associated Units, Auxiliary Apparatus, without
modification to the
switchboard permanent wiring.
Cordage enters the switchboard via the two entry slots in the
flange at the rear of the
base. When viewed from the rear the right-hand slot is for the
operator's telephone cord
and the second slot is for the main 68-way cord. A dummy grommet
is also provided on the
right-hand side of the rear base flange. This is removed and a
Jack No. 84C fitted when a
Headset No. 1 is required in addition to the handset. Wiring is
provided in the
switchboard for the connection of this jack. When a Headset No.
1 is fitted the Telephone
No. 706... is replaced by a Telephone No. 710, for details see
Diagram N1103.
The picture above shows an Autodialler
associated with the Switchboard.
This is not a standard item and would have been installed at
extra cost.
FACILITIES
Circuit connections
Four-wire extension circuits may be connected on all extension
positions. Private
circuits, inter-switchboard circuits, inter-switchboard
extensions, and 2-wire extensions
must, however, be connected on extension positions 7-12.
A Unit, Auxiliary Apparatus, No. 96A (or 96) should be provided
on all 2-wire extensions
and terminated in accordance with Diagram
N1127.
Private circuits etc require the provision of a Unit, Auxiliary
Apparatus, No. 97, 98 or
99. Details of the particular N diagrams applicable to any
signalling group are given in
E0402.
Lamp calling on exchange lines and extensions
A Lamp No. 2-45V which has a reasonably flat lumen/resistance
response is used to
eliminate the need for a line-calling relay on extensions. The
lamp is connected in series
with the battery feed via the pilot relay and glows when the
extension loop is completed
by removal of the telephone handset at the extension. The pilot
relay is shunted by an
electrolytic capacitor to prevent overhearing between calling
extensions. The exchange
lines use the same type of lamp and a locked signal is given
when relay AC operates to an
incoming ring. The pilot relay has a low-resistance winding
which is connected in parallel
with the operating coil when the relay operates in order to
reduce the amount of series
resistance added by the relay. If the calling lamp of an
exchange line becomes
disconnected an audible alarm is given (provided the ON key is
operated) as the holding
circuit for relay AC is via the pilot relay.
Clearing and recall
The use of the 4-wire extension principle separates the
functions of transmission and
supervision. As a result it has been possible to arrange that
individual clearing and
recall signals are given by each extension signalling lamp.
Clearing
A clear is given when the extension handset is replaced on both
extension-to-extension and
extension-to-exchange calls. On the 4-wire extensions control is
by an additional gravity
switch spring-set in the extension telephone. In the case of
2-wire extensions the control
is by a supervisory relay in the Unit, Auxiliary Apparatus No.
96A (or 96) (see Diagram N1103
explanatory figures).
Press-button recall
This is a standard facility and a switch is provided in
accordance with Diagram N806 on each
extension telephone. Recall is provided on 4-wire extensions by
connecting the make
contact of the switch in parallel with the additional gravity
switch spring-set provided
in the extension telephone. On 2-wire extensions where a Unit,
Auxiliary Apparatus, No.
96A or 96) is provided the press-button is arranged to earth the
telephone loop and
operate a differentially connected relay in the unit; the relay
contacts operating
complete the extension signalling lamp circuit. The facility is
effective on both
extension-to-extension and exchange-to-extension calls.
Holding of exchange calls
Automatic holding of exchange calls is provided on all
connecting circuits. If on an
exchange call the operator restores the operator's key with no
extension connected to the
circuit, a previously operated hold-relay connects a 910ohm
resistor across the exchange
line to hold the exchange equipment. The hold condition is
removed by the
re-operation of
the operator's key or the restoration of the exchange line key.
If, however, the exchange
line has been extended to an extension circuit, the holding
circuit of the relay is
dependent upon the earth extended over the C-wire from the
extension telephone auxiliary
gravity switch, and the hold condition is maintained on the
exchange line while the
extension handset is on its rest. The hold condition is removed
when the extension
answers. If the extension does not answer the hold condition is
removed either by the
re-operation of the operator's key or the restoration of the
exchange line key. Visual
indication of a call being held is given by the glowing of the
CALL HELD lamp which is
common to all connecting circuits.
Exchange and extension line limits
For transmission and signalling resistance limits see E0110.
Follow-on-call trap
When an exchange-to-extension connection is cleared down by
replacing the extension
handset relay CT is re-operated in the switchboard. Contacts of
this relay disconnect the
extension circuit from the exchange line circuit and reconnect
the calling relay AC to the
exchange line. Therefore if a follow-on call is received on an
exchange line which has not
had its exchange line key restored after a previous call, it
will be trapped on relay AC;
the exchange line calling lamp will glow but the extension
telephone bell will not ring.
Night service
The NIGHT SERVICE key should be operated and selected extensions
connected to the exchange
lines whenever the switchboard is left unattended overnight or
for long periods. Under
night service conditions the pilot relay is disconnected from
the extension calling lamps
to prevent these from glowing. The exchange line calling relays
are also disconnected,
i.e. relay CT is released to prevent the exchange line calling
lamps glowing. A 200k ohm
resistor in series with a capacitor is connected across each
exchange line to ensure that
the lines do not test as disconnected should the NIGHT SERVICE
key be operated without the
exchange lines being extended to extensions.
Operation of the NIGHT SERVICE key connects an earth to the NS
terminal which can be used
as a switching condition to provide non-standard night service
arrangements. Contacts of
the NS relay (which is released) provide night service switching
arrangements for any
Units, Auxiliary Apparatus which may be connected to extensions
7 to 12. The operator's bell
circuit is also disconnected from the first exchange line when
the NIGHT SERVICE key is
operated.
Prohibition of exchange service on private circuits
These circuits terminate on extensions 7-12 and a prohibition
relay is connected to the P
terminal of an extension on which a Unit, Auxiliary Apparatus is
fitted and strapped to
provide exchange prohibition. The contacts of this relay are
arranged to disconnect the
private circuit from an exchange line when both the exchange and
extension keys are
operated.
Audible alarm
This is switched on when the ALARM ON key is operated. An
additional bell or buzzer can be
provided if required by making use of the NA terminal. Any type
of bell or buzzer suitable
for 50V working may be used. Lightweight headsets.
A Headset No. 1 may be provided in
addition to the operator's telephone. been this
facility is required the operator's telephone is changed from a
Telephone No. 706 to a
Telephone No. 710 When fitted, the headset utilises the
regulator and induction coil
contained in the telephone. Additional spring-sets in the
telephone disconnect the headset
when the handset is removed from the rest. The Telephone No. 710
is used because its
gravity switch has the greater mechanical advantage needed to
operate the additional
change-over springs required.
Operation under mains failure
If the power supply to the switchboard fails at installations
supplied by a mains-operated
power unit, exchange connections in progress are maintained but
extension to extension
calls fail. An audible indication of an incoming call on the
first exchange line is given
by ringing the bell in the operator's telephone. Under power
failure conditions the first
exchange line can be used by the operator for both incoming and
outgoing calls. The
remaining exchange lines are connected through to selected
extensions to give normal
exchange access.
POWER AND RINGING SUPPLIES
The switchboard has been designed to operate from a nominal 50V
dc supply but is capable
of working in the range 45-55V. This supply is normally obtained
from a mains-operated
power-unit but a battery float system is available for use at
installations where the
limited facilities available under mains-failure conditions are
insufficient.
A 25 Hz ringing supply is provided by a mains driven ringing
converter at installations
using power units and by a dc operated ringing converter at
installations provided with
stand-by batteries.
Details of power and ringing equipment for mains operated power
unit installations may be
obtained from Diagram N1103 and for battery float system
installations from Diagram N2332.
CIRCUIT DESCRIPTION AND OPERATION
Diagram SA7164 is the schematic diagram for the switchboard and the
detailed circuit
description is given in Diagram Notes SA7164. Diagram N1103 gives
details of the operator's
telephone circuit (with and without headset), the apparatus and
the circuit elements
together with installation wiring and cabling instructions.
On all internal extensions three wires plus an earth wire are
provided to each extension
telephone; four wires should also be provided for external
extensions subject to the
restrictions as detailed for extension Plans 1A in C0010. For
all 2-wire extensions a
Unit, Auxiliary Apparatus No. 96A (or 96) should be provided and
connected as shown on Diagram
N1127.
The switchboard has a parallel feed transmission bridge for
extension-to-extension calls
using a 300 + 300ohm feeding coil with a 0.1 PF capacitor
across the output to improve
the sidetone balance. Non-removable relay shields are fitted to
the transmission bridge
feed relays; these reduce the crosstalk level between connecting
circuits to below 75dB.
For extension to exchange calls the transmission feed is
obtained from the exchange.
INSTALLATION
The switchboard should be installed in accordance with Diagram
N1103. The 80-wire cable to the
Box, Connection No. 5B should be terminated on the Connector
No. 101/1A in accordance with
Diagram N1103 and in the manner shown on Drawing
SD113.
The cabling between the switchboard and the Box, Connection No.
5B must not exceed 100ft
to avoid an excessive voltage drop under full-load conditions.
The extension instruments should be terminated in accordance
with Diagram N806.
STD meters, if required, should be provided in accordance with
E5001 and Diagrams N1005 and
N1006.
For a mains-operated installation the power-unit should be
installed in accordance with
Q0020.
For a battery float installation the rectifier and battery
should be installed in
accordance with Q0060 or Q0071.
The above picture shows from top to bottom, a
Unit, Auxiliary Apparatus, No. 96A (one each external
extension or inter-switchboard circuit), a Power Unit and a Box,
Connection, No. 5B
User Guide
Drawings - 91687/1 and
91687/2.
Specification - S669.
Label No. 415 - Exchange lines.
Label No. 416 - Extensions.
How to wire the switchboard cable
connector plug
Click here for metering
schemes and apparatus required - Telephones, PBX, B3104.
POEEJ
article - 1963
Made by Ericsson
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from
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