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ON EACH MODEL
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The BPO 2 + 6 (introduced circa 1961), 3 + 12 and 4 + 18 type of PMBX switchboard consists of a dark grey key console
mounted in a light grey moulded plastic case. The panel mounted variant, 3 + 12 and
5 + 25,
were installed in a customer provided unit which normally also housed other equipment.
700 type telephones with recall are fitted to extensions and all internal extensions are
connected to the switchboard by 4 wires. External extensions, inter PBX
lines and private circuits are normally connected by 2 wire wiring via auxiliary apparatus
A connection box and power unit, both wall mounted, were provided for each PMBX of this
Made by various manufacturers.
Distributed, installed and maintained by BT.
PMBX's are hardwired "Grandfathered" equipment
Wiring diagrams for PMBXs and associated equipment can be found in the "N"
series, N1003 through to N1887. SA and SAW diagrams that have been identified for PMBX
equipment are also listed below.
Main diagrams for PMBX 2+6 (2/2A)
SAW 71630 & SAW 71631
Main diagrams for 3+12 (2/3A)
SA 7164 & SAW 71640
Main diagrams for 4+18 (2/4A)
N 1104 & N 1128
SA 7165 & SAW 71650
Main diagram for Panel Mounted 3/- PMBX - SAW 7169
The principal TI's relating to installation and maintenance of PMBX's are as follows;
PMBX 2/2A (2+6), 2/3A (3+12), 2/4A (4+18) MAINTENANCE E5 D1351
PMBX 2/2A & 2/2B - INSTALLATION C3 E4001
PMBX 2/3A (3+12) - DESCRIPTION AND INSTALLATION C3 E4002
PMBX 2/4A (4+18) - DESCRIPTION AND INSTALLATION C3 E4003
PMBX 3/5B - FRAMES AND MOUNTING C3 E4035
Maintenance was offered until 30th September 1994.
Customers were sent a letter by post on 17th September 1993 giving them 12 months notice
PMBX 2+6 (Item Code 410217)
PMBX 3+12 (Item Code 395910)
PMBX 4+18 (Item Code 377424)
The Post Office Electrical Engineers Journal
Volume 55, Part 4 (1963)
New Cordless P.M.B.X. SwitchboardsóP.M.B.X.s No. 2/3A and 2/4A
C. M. HALLIDAY, a.m.i.e.e., and E. J. LIDBETTER
The new range of private manual branch exchange switchboards of the cordless
type is completed by the introduction of two further switchboards with maximum
capacities of three exchange lines and 12 extensions and of four exchange lines
and 18 extensions, respectively. These have lamp-signalling and are of
compact design similar to, but providing more facilities than the first
switchboard of the series, which provides for only two exchange
lines and six extensions.
In an earlier article a new type of cordless private manual branch exchange
(P.M.B.X. No. 2/2A) was described. This type of switchboard was the result of an
entirely new approach to the problems of physical design and circuit
arrangement; it has a capacity of two exchange lines and six extensions and was
the first of a new series of cordless switchboards that has been planned to
supersede the existing types. Two larger switchboards similar in general design
to the P.M.B.X. No. 2/2A are now being introduced. The first (P.M.B.X. No. 2/3A)
has a maximum capacity of three exchange
lines and 12 extensions; the second (P.M.B.X. No. 2/4A) is equipped for four
exchange lines and 18 extensions. These two switchboards complete the new range
of cordless P.M.B.X.s.
As a result of research into present-day requirements, the
exchange-line-to-extension ratios of the new switchboards differ from those of
the superseded models, the P.M.B.X. No. 2/2A, which is equipped for two exchange
lines and six extensions (2+6), replaces the old 2+4 type; the new P.M.B.X. No.
2/3A (3+12) will replace the old 3+9 type and the P.M.B.X. No. 2/4A (4+18) will
cater for much of the demand which previously was met by the 5+20 floor-pattern
cord type. The remaining demand for the 5+20 switchboard will in future be met
by a new floor-pattern cord-type board.
The introduction of the three new cordless P.M.B.X.s has, therefore, made it
possible to provide a range of equipment in accordance with modern needs.
The adoption of the 4-wire extension principle, hich entails running an extra
wire plus an earth to each extension telephone for supervisory purposes, has
enabled the requisite facilities to be provided in the most economical manner.
An auxiliary unit has been designed for use with external-extension circuits
that are routed in the public-exchange local-line network and so require
conversion from 4-wire to 2-wire working. All the extension circuits on the
switchboards can be used as 4-wire extensions if desired. Alternatively, by
simple rearrangement of cords and straps within the switchboards and the
connexion of the appropriate auxiliary unit, extension circuits 7-12 on the
P.M.B.X. No. 2/3A and extension circuits 10-18 on the P.M.B.X. No. 2/4A can be
used for 2-wire extensions, inter-switchboard extensions or private circuits.
The power supply for these switchboards is derived from an external a.c.
mains-driven power unit 3 supplying 50 volts d.c. and 25 c/s a.c. for ringing
The main features of the two new cordless P.M.B.X. switchboards are as follows:-
(a) Lamp signalling for exchange lines and extensions.
(b) A transmission and signalling limit of 500 ohms for extension circuits and
of 935 ohms for exchange-to-extension connexions when connected to an exchange
equipped for 1,000-ohm lines.
(c) Press-button recall on extension-to-extension calls and
extension-to-exchange calls, under all conditions.
(d) Individual clearing on extension-to-extension calls.
(e) Automatic holding of exchange calls.
( f) Follow-on-call trap on incoming exchange calls.
(g) Connexion of private circuits and inter-switchboard circuits without
modification to the permanent wiring of the switchboard.
(h) Connexion between cabling and the switchboard by a flexible cord and
(i) Arrangements for the provision of a headset in addition to the operatorís
(j) Space for the provision of subscriber-trunk-dialling (S.T.D.) trip meters on
The following additional facilities are provided only on the larger switchboard.
(k) An ďovercallĒ circuit to enable the operator to answer exchange and
extension lines when all connecting links are engaged.
(l) Lamp signals to indicate the next free connecting-link circuit available.
P M B X s No. 2/3A and 2/4A are similar in design. The switchboards are
supplied in two-tone grey with matching Telephones No. 706 as the operating
Both switchboards have a metal chassis and a plastic cover. The P.M.B.X. No.
2/3A is approximately 17in. wide by 8in. high by 13in. deep and weighs 33lb;
the P.M.B.X. No. 2/4A is approximately 22in. wide by 9in. high by 15in. deep
and weighs 56lb.
The thermoplastic French-grey covers are made by injection moulding using a
co-polymer plastic material that is both economical and attractive in
appearance. The leading edge of the cover fits under the front of the face
panel, and adjustable plates on the side of the panel form dust seals.
The face panel is finished in elephant-grey stoved P.V.C. (Organosol) that
provides a durable surface with a leather-grain appearance. The marking required
on the face panel is applied by hot stamping into the layer of stoved p.v.c. and
the characters are white.
If it is desired to use a headset as the operators instrument, the Telephone No.
706 is changed for a Telephone No. 710 and a headset jack is provided on the
switchboard. The change-over from handset to headset is arranged by providing
auxiliary spring-sets in the Telephone No. 710 so that when the handset is on
the rest the receiver and transmitter of the headset are brought into circuit.
When the handset is lifted the transmission path is automatically switched back
to the handset. Under these conditions the normal gravity-switch spring-sets are
short-circuited and the connexion of the operatorís telephone depends entirely
on the operatorís telephone key.
The miniature 1,000-type key 4 has been used instead of the standard lever key.
The key handles are coloured light ivory for the exchange lines and operatorís
keys, and, alternately, light ivory with black inserts and French-grey with
black inserts on the extension positions to assist in identifying the keys
associated with a particular circuit. The alarm on/night service key is light
ivory with a red insert.
The keys, lamp jacks and circuit-designation label-strips are mounted on the
panel in such a manner that all screw heads are concealed.
When the lamp cover-strip is removed the lamp-jack screw heads are exposed, by
releasing these screws, the lamp jack may be drawn forward to permit lamps to be
changed without the use of a lamp extractor or the removal of the switchboard
Arrangements have been made for the provision of subscribersí private
trip-meters. These are used on STD. calls and are fitted, as required, in the
key panel of the switchboard in line with the appropriate exchange line keys.
For this reason the exchange-line connecting-circuit keys are more widely spaced
than the extension keys. The removal of the exchange-line designation strip
uncovers a cut-out portion of the panel provided to accommodate the meters; if
meters are provided a narrower type of designation strip is used. The meters can
be provided on the basis of one meter per exchange line
or one meter that can be switched to any of the exchange lines.
When the cover of the switchboard is removed the metal chassis, consisting of
three sections, is exposed. The front and rear panels and the base plate form a
triangular section when the chassis is closed. By releasing the screws at the apex, the hinged panels can be
opened outwards to permit inspection of the wiring and components. The P.M.B.X.
No. 2/4A is similar except that it contains more equipment.
The front panel carries the keys and lamps, and the relays are mounted on the
rear panel under a dust cover; some static components are mounted on the base
together with the connexion blocks. One type of connexion block has soldered
connexions that can be rearranged to provide, for example, barring of the
connexion of private circuits to exchange lines; the other type is equipped with
screw terminals and is used for the connexion of the operatorís telephone and
the line-circuits. Screw terminals are also provided to permit rearrangement of
the straps and cords when 2-wire
extensions, private circuits or inter-switchboard extensions are required.
The switchboard is supplied with a multi-way cord already connected, and with
the free end terminated on small connectors that plug into a wall mounting jack
assembly cabled to the connexion box. The P.M.B.X. No. 2/3A is equipped with a
68-way cord and P.M.B.X. No. 2/4A with a 100-way cord.
For economic reasons a parallel-feed transmission bridge has been adopted for
extension-to-extension calls. The bridge is of the new 50-volt design (used on
P.M.B.X. No. 2/2A) and consists of a 300 + 300-ohm coil with an 0-1uF capacitor
across the output to give improved side-tone balance. This combination is
suitable for all extension-to-extension calls up to a signalling limit of 500
ohms, with either 300-type or 700-type telephones.
Non-removable relay shields fitted to the transmission-bridge relays increase
the crosstalk attenuation between circuits to at least 75 db.
A new lamp (Lamp No. 2, 45 volts) has been developed for use on these
switchboards. It has a reasonably flat lumens/resistance response over the range
0-500 ohms for the line-plus-telephone loop resistance and its use has obviated
the need for a line-signalling relay.
|Note: There are five connecting circuits.
SIMPLIFIED CIRCUIT OF P.M.B.X. No. 2/3A
No hand generator is fitted; the ringing supply is derived from a
frequency-dividing mains-operated unit producing a 25 c/s a.c. output.
The switchboard circuits have been designed to operate from a nominal 50-volt
d.c. supply, but are capable of operating from a supply in the range 45-55
volts, and this has enabled economies to be made in the design of the power
units. Under full-load conditions the current consumption of the P.M.B.X. No.
2/3A is 1 amp and that of the P.M.B.X. No. 2/4A is 2 amp. If auxiliary units are
associated with the switchboard, the power consumption is increased and the
power units are changed to 2-amp and 4-amp types, respectively. Arrangements
have been made to ensure that exchange calls in progress are maintained if a
mains failure occurs. For normal installations it is recognized and accepted
that extension-to-extension and inter-switchboard calls will fail if the mains
supply ceases, but at installations where such a break in service could not be
tolerated the switchboard will be operated from a floated-battery system.
CIRCUIT DESCRIPTION AND OPERATION
The 4-wire extension principle is used on all internal extensions and can be
applied to external extensions within the curtilage of the subscriberís
premises. For external extensions connected via the local-line network it will
generally be desirable and more economical to use a 4-wire/2-wire conversion
unit. For extensions not requiring a conversion unit the exchange-to-extension
transmission and signalling limit is 935 ohms in exchange
areas equipped for 1,000-ohm line working, 65 ohms being required for the
supervisory relays. On external extensions for which conversion units are
required this limit is reduced to 850 ohms, the difference being due to the
resistance of the signalling relays in the conversion unit.
Two additional facilities are provided by the circuit of
the larger switchboard, the P.M.B.X. No. 2/4A. The first
enables a free connecting link to be selected rapidly by
means of a free-link signalling system. The second
facility, known as overcall, enables the operator to
answer a call when all seven connecting links are
A simplified circuit diagram of the P.M.B.X. No. 2/3A is shown above.
Extension- to-Extension Calls
When a call is originated at an extension by lifting the
telephone handset, earth via contact KX1, the telephone
loop, contact KX2 and the calling lamp, operates the
pilot relay, P, which is connected to -50 volts, and the
extension calling lamp glows. Contact P2 connects the
low-resistance coil of the P relay in parallel with the high-resistance coil. This prevents the voltage drop across the
pilot relay becoming excessive when a number of lamps
are glowing simultaneously. Contact P1 causes an
audible alarm to be given if the alarm on key, KA, is
operated. If more than one extension is calling the
switchboard, overhearing between the extensions is
suppressed by capacitor C3. The call is answered by
operating the operatorís telephone key, KO, associated
with the chosen connecting circuit and the appropriate
connect extension key, KX, and by lifting the handset
of the operatorís telephone. The transmission-bridge
relay, LA, feeds transmitter current to both telephones.
If connexion to another extension is required, the
operator checks that the extension is disengaged by
observation of the keys, and calls the extension by the
operation of the appropriate ringing key. When the called
extension answers, the calling lamp glows and the KX
key associated with the extension is operated to complete
the connexion. The operator then restores key KO and
replaces the handset. Either extension can recall the
switchboard by depressing the recall button on the
extension telephone. An earth (on the fourth wire from
the connexion box) is then extended to the C wire (the
third wire) and the calling lamp glows while the recall
button is pressed. When the call is completed and the
extension handset is replaced, earth potential is extended
via the auxiliary gravity-switch spring-set, SWA, to the
C wire to give a clearing signal on the calling lamp via
key contact KX3.
If the calling extension requests connexion to an
exchange line the connect exchange key, KE, of a free
exchange line is operated. This disconnects the local
50-volt supply from the connecting circuit at contacts
KE1 and KE2 and extends the extension to the exchange line circuit. As relay CT
is normally operated, earth via
contacts CT1, KE1 and KX1, the extension-telephone
loop, contacts KX2 and KE2, relay SA and contact CT2
operates relay SA to -50 volts. Contact SA1 operates
relay SB and contact SB1 releases relay CT. Contacts
CT1 and CT2 extend the circuit through to the exchange line. Capacitor C2 is
connected across relay SA to
provide a low-impedance speech path. The exchange-line connecting keys are
arranged to prevent any
exchange lines being connected together.
Incoming Call on an Exchange Line
Incoming ringing current on an exchange line will
operate relay AC over one coil, via contact CT4 operated.
Contact AC1 lights the calling lamp and contact AC2
completes a holding circuit for relay AC from an earth
via contact CT3 (to provide a locked calling signal on the
exchange line until the operator answers) and also operates
the pilot relay, P. The circuit is arranged to ensure that,
provided the alarm on key, KA, is operated, an audible
alarm is given at the P.M.B.X. even if the exchange-line
calling lamp becomes disconnected. The operation of
relay AC is delayed by the short-circuit maintained
across the hold coil by contact AC2. This avoids false
operation due to line surges when switching takes place.
False operation of relay AC can, however, occur under
certain conditions if the A and B wires of the exchange
line are reversed.
Specific safeguards have also been incorporated to
avoid the possibility of lost calls due to mis-operation of
the switchboard keys. A calling signal is not extinguished
(except when a headset is also provided) until the
operatorís handset has been lifted and both the KE and
KO keys have been operated. When the call is answered
by the operator, relay SA operates. Relay SA then
operates relay SB, which releases relay CT. Contacts
CT1 and CT2 extend the operatorís telephone loop to the
exchange line to trip the ringing if the incoming call is
from an automatic exchange. Contact CT3 disconnects
the holding circuit for relay AC, and contact AC1
extinguishes the calling lamp. The exchange call can then be extended to an
extension, as already described,
or automatic holding conditions can be applied.
Automatic Holding of Exchange-Line Calls
When a call is answered, relay SB is operated by relay
contact SA1; -50 volts, via contacts SB2 and KE3
operated, operates relay H to earth at contact K04.
Relay H locks to earth via contacts H3 and KX4 or to
earth at the auxiliary gravity-switch, SWA, of the extension telephone if the
extension key has been operated.
Contact HI prepares the holding loop across the exchange
line, the loop being completed by contact KOI when the
operator restores the KO key to normal. Contact H2
lights the call held lamp. This lamp remains as a
visual reminder to the operator that supervision of the
switchboard is still required. If the line has been
switched to an extension the holding circuit is released
when the extension answers the call by the auxiliary
gravity-switch contacts, SWA, opening. When the holding
circuit is removed the call held lamp is also extinguished.
The call held lamp is common to all connecting
If the operator wishes to release an exchange call
without connecting it to an extension, the holding
circuit to relay H is released at contact KE3 when the
exchange key is restored. Although relay H is shown in
Fig. 4 as a slow-to-operate relay, advantage is taken of
its slow release, and in so doing further economy and
space saving has been achieved by using 600-type relays
instead of 3,000-type relays. The slow release of relay H
is particularly important when the automatic holding
circuit is used on circuits where line surges are prevalent.
When an exchange-extension call is completed the
clearing signal is given on the extension calling lamp only
when the extension telephone is replaced. Relay SA then
releases and contact SA1 releases relay SB. Contact SB1
restoring to normal permits relay CT, the call-trap relay,
to re-operate. Contact CT4 re-connects relay AC and
capacitor C2 across the A and B wires of the exchange
line, while contacts CT1 and CT2 disconnect the
exchange line from the connecting circuit. Thus a
follow-on incoming call will be trapped on relay AC and
will not ring the extension telephone bell if the call has
not been cleared-down at the switchboard.
A free-link signalling system, similar to the well-known
free-line signal (F.L.S.) used on manual switchboards in
main exchanges, has been provided to increase the speed of answer. This is particularly important with the
P.M.B.X. No. 2/4A as there are 88 circuit keys to scan
before selecting a free connecting link. The free-link
signals are given by small amber-coloured lamps situated
down the left-hand side of the face panel. Two lamps are
provided for each row of keys, one for the upward
movement and one for the downward movement. The
circuit arrangement is shown below. Normally, all
lamps are extinguished in the idle condition. As soon as
an incoming call is received pilot relay P operates.
Contact PI lights lamp LPA and the operator answers
the call using the A connecting link. When the A
connecting link is taken into use the relay contacts LAI
and LA2 of the transmission-bridge relay operate and step the free-link signal
on to lamp LPB to indicate that the B connecting link should be used for the
The sequence is always the same, but if any link is taken
into use out of turn the remaining lamps still glow in the
same order, excluding the circuit taken out of sequence
if it is still in use. When a circuit becomes disengaged the
free-link lamp glows again if it is the first free link in the
sequence. On exchange calls arrangements are made to operate the appropriate
transmission-bridge relay although it is not used to provide the transmission feed.
When all seven connecting links are engaged the eighth
lamp, the overcall lamp (LPH), glows. This indicates
that all connecting circuits are engaged. Should any
exchange or extension call then be received at the
switchboard it can be answered on the overflow or
|Free-Link Signalling Circuit Element
As mentioned above, overcall enables the operator to
answer a call when all seven connecting links are engaged.
The circuit is shown below. To answer an extension
call under these conditions the speak on overcall key,
KSO, is operated and relay OC feeds transmission
current to the operatorís telephone via contacts KH1 and
KS02, and contacts KSOl and KH2. Relay OC operates,
and contacts OC1 and OC2 changing over disconnect the
ringing supply and extend the operatorís telephone to
the extension ring key, KR. The operator can now
speak to the calling extension by holding operated the
appropriate KR key. The KSO key must be restored
before answering a normal call.
An exchange call is answered by operating the KSO
key and the appropriate exchange overcall key, KH,
which locks in the operated position. Contacts KH1
and KH2 disconnect relay OC and connect the exchange
line to the operatorís circuit. The transmission feed for
the operatorís telephone is now supplied by the public
exchange and contact KH3 connects bridging coil IL to
provide a holding condition. This permits the operator
to leave the circuit and offer the call to an engaged
extension and then, if required, breakdown the connexion
and extend the exchange call to the extension. When the
call has been dealt with, keys KH and KSO are restored.
Overcall Circuit Element
As the switchboard has been designed to work from a
mains-operated 50-volt d.c. power-supply unit, the
circuits have been arranged so that the exchange-line
service is maintained under mains-failure conditions. If
a mains failure occurs, relay CT (Fig. 4) releases and
contact CT4 connects the bell of the operatorís telephone
to the first exchange line so as to give an audible
indication of an incoming call. The operator can
answer the exchange call under mains-failure conditions
by operation of the appropriate KE and KO connecting
keys and by removing the handset from the operatorís
telephone. Outgoing exchange calls can also be
originated by the operator in the normal manner. The
operating instructions for these switchboards will advise
the subscriber to connect the remaining exchange lines to
selected extensions (as for night-service working) if a
mains failure should occur, and the selected extensions
will then have direct access to these exchange lines.
Each extension telephone is provided with a press-button key (a make contact in
parallel with the auxiliary
gravity-switch spring-set) for recall purposes. Operation
of the button connects earth potential (on the fourth
wire) to the C wire (the third wire). Recall is available
on all extension and exchange calls.
Night-service arrangements are provided by the
night service key, KNS, and relief relays that disconnect the pilot relay and
the extension calling-lamps.
Key contact KNS3 releases relay CT, which disconnects
the AC relay of each exchange line thus rendering the
call-trap circuit ineffective. Selected extensions can be
connected to the exchange lines by operating the
appropriate KE and KX keys as required.
Two-Wire Extensions , Inter-switchboard Extensions and Private Circuits
Additional terminations have been provided on
extensions 7-12 of the P.M.B.X. No. 2/3A and on
extensions 10-18 of the P.M.B.X. No. 2/4A so that by
suitable inter-connexions these extensions can be used
for 2-wire extensions, inter-switchboard extensions or
private circuits, without alteration to the permanent
wiring of the switchboard.
When any of these facilities are required the
connexions are made so that the switchboard controls
the switching relays in the various auxiliary apparatus
units to provide for automatic hold on 2-wire extensions
and inter-switchboard extensions, or prohibition of
exchange-line connexion to private circuits.
Provision is also made to control the relays in the
4-wire/2-wire conversion unit in order to provide
through-dialling facilities on exchange calls. Should the
operator enter the circuit on this type of connexion,
spring-set SWB on the operatorís telephone short-circuits resistor R on the control wire and operates a
relay to provide transmission feed from the conversion
unit to the extension. On extension-to-extension calls
the divided transmission-feed arrangements are applied
by earth from contact KE4 normal short-circuiting
The P.M.B.X.s No. 2/3A and 2/4A incorporate new
circuit design, provide better facilities, and should give
greater reliability and ease of maintenance. These
advantages, coupled with their improved appearance,
should make them welcome substitutes for the switchboards they supersede.
The authors wish to thank Ericsson Telephones, Ltd., who, as the liaison
manufacturers, worked in close cooperation with the Post Office on these