PMBX No. 4

Descriptive Leaflet - 1967

Descriptive Leaflet - Multiple Position - 1973

Descriptive Leaflet - Single Position - 1980

3 Internal
Issue 1, Aug 1973

Private Manual Branch Exchange No. 4/1A
(P.M.B.X. No. 4/1A)





(a) Calling appearances
(b) Terminations
(c) Lamp indication and audible indication
(d) Through clearing and dialling
(e) Press-button recall
(f) Automatic ringing
(g) Automatic hold
(h) 'Click' engaged test
(j) Free line signalling
(k) Transmitter cut-out
(l) Metering
(m) Night Service
(n) Emergency service
(p) Alarm lamps

(i)-(v) PMBX 4/1A Mark I
(vi)-(xii) PMBX 4/1A Mark II

(q) Cord & ring test
(r) Position coupling
(s) Keysender
(t) Inter-PBX circuits


(a) Extensions
(b) Exchange lines
(c) Cord circuits
(d) Operator's and miscellaneous circuit
(e) Metering


(a) General construction
(b) Dimensions and weight
(c) Face equipment
(d) Keyshelf
(e) Apparatus layout


(a) Distribution equipment
(b) Power equipment

(i) 50 volt negative supply
(ii) 50 volt negative reserve supply
(iii) 50 volt positive supply
(iv) Power distribution and alarm arrangements
(v) Ringing supplies

(c) External exchange line circuits
(d) Signalling Units
(e) SSAC13 equipment


(a) Installation
(b) Maintenance


The switchboard PMBX 4/1A is a cord type switchboard, with lamp signalling, which may be used with associated equipment to provide non-multiple single position installations for up to 160 extensions, 20 exchange lines and 10 inter-PBX lines, or multiple suite installations for up to 200 extensions, 40 exchange lines and 10 inter-PBX lines. The switchboard, as normally supplied, includes face panel equipment for 40 extensions; equipment for 10 exchange lines is provided in the Mark I switchboard, but this is increased to 20 on the Mark II. Face panel equipment for 5 inter-PBX circuits, with wiring for 10, is provided in both cases. On the Mark I switchboard, the wiring for the additional 10 exchange lines is supplied. Pre-wired extension units are also available to increase the face panel equipment to the desired capacity.

Specially designed plug-in relay sets, fitted within the switchboard, and wall-mounted units accommodate circuits to give the facilities listed in paragraph 2. Power for the installation is normally derived from a conventional 50 volt mains-operated power-unit; additionally, secondary cells may be provided if a stand-by supply is required.


Access to the rear of the switchboard showing the relay sets and at the top, the fuse mounting.
Remove of the relaysets is by pulling straight out. 
Removing at an angle as shown is not possible as there is a locating pin on either side which would not allow this.
The picture is obviously posed!

The PMBX 4/1A provides the following standard facilities:-

(a) Calling appearances for up to 160 extensions at single position installations. Calling, duplicate calling and multiple appearances for up to 200 extensions at multiple position installations.

Calling appearances for up to 20 exchange lines at single position installations (in Mark 1 Switchboards, face equipment is initially provided for 10 exchange line appearances only). Calling appearances for up to 40 exchange lines at multiple position installations.

Calling appearances for 10 inter-PBX circuits at single and multiple position installations respectively. Exceptionally, more inter-PBX line appearances may be provided at the expense of exchange line appearances.

(b) Terminations for:-

(i) Exchange lines to any CB type of public exchange. (A modification to the exchange line circuit is necessary for working to TXE exchanges).

(ii) Extensions, internal and external.

(iii) Private circuits.

(iv) Inter-PBX extensions.

(v) Inter-PBX private circuits.

Private circuits and inter-PBX private circuits are barred access to exchange lines, inter-PBX extensions are not allowed outgoing calls to the public exchange and exchange lines may not be connected together. Apart from these restrictions, all five of the above mentioned categories may be inter-connected by cord circuits.

(c) Lamp indication of calling and supervisory conditions
Audible indication of calling conditions and, optionally, of clearing conditions a sub-unit SA7606 is required for the audible clear, but it is operative only to a double clear. A key is provided to disconnect the audible alarm as desired.

(d) Through clearing and dialling
This is normally provided on extension to exchange connections, a double clearance signal being given when the extension clears. Optionally, cord circuits may be modified for non-through clearing with individual supervision. Individual supervision is given on other types of calls.

(e) Press-button operator recall from extensions on all types of call
When recalled, the operator may transfer the call as required.

(f) Automatic ringing on calling cords
Manual ringing is available on answering cords for call reversion purposes.

(g) Automatic hold on exchange calls originated or answered by the operator
This holds the exchange line should the operator restore the cord circuit 'SPEAK' key before extending the call, or before an extension has answered when the call has been extended.

(h) 'Click' engaged test at multiple installations

(j) Free line signalling
This may be provided optionally for exchange lines, inter-PBX lines, or both.

(k) Transmitter Cut-out
A push-button switch may be provided if required to cut out the transmitter circuit of the operator's headset.

(l) Metering
Four types of metering are available, and can be used individually or in combination:-

(i) Metering on cord circuits using resettable meters, to indicate individual call totals.

(ii) Metering on exchange lines, to indicate running totals.

(iii) Centralised extension metering, to indicate running totals using totals or resettable meters.

(iv) Localised extension metering, indicating running and individual call totals.

(m) Night service
Keys provided on each position disconnect the 50 volt supply when operated, allowing cord circuits to be used to extend selected exchange lines direct to selected extensions. (Note: exchange lines using units signalling SA7616 should not be used for this purpose unless it is unavoidable.)

A 'NIGHT BUSY' jack is provided, so that subscriber controlled night busying may be employed when the public exchange is equipped for this service.

(n) 'Mains Fail' service
When the 50 volt supply fails extension to extension calls fail but extension to exchange calls in progress are maintained. The operator is able to receive or initiate exchange calls but cannot extend them. To assist operating under these conditions 'HOLD' jacks are provided to enable exchange calls to be held while the 'SPEAK' key is restored. Exchange lines may be plugged through to selected extensions, as under night service working.

(p) Alarm lamps
Indications are given for the following conditions on the PMBX 4/1A Mark 1 switchboard:-

(i) 'POWER FAIL', indicating a failure of the negative 50 volt supply to the position. On the first position only of Mark I multiple position installations, this alarm lamp is also used to give a mains supply failure alarm for the negative 50 volt rectifier at installations with stand-by power arrangements.

On the second position only of Mark I multiple position installations, this alarm lamp may be used to give a positive 50 volt supply failure alarm.

(ii) 'POWER RESTORED', indicating a restoration of the power supply monitored by the associated 'POWER FAIL' alarm (see above).

(iii) 'FUSE ALARM 1', indicating a failure of a negative 50 volt fuse mounted within the position or in the Fuse Unit No. 2A.

(iv) 'FUSE ALARM 21, indicating a failure of the reserve negative 50 volt battery fuse (fuse 31).

(v) 'FUSE ALARM 3', indicating the failure of the positive 50 volt battery fuse (fuse 32).

For the PMBX 4/1A Mark II switchboard, indications are given for the following conditions:-

(vi) 'POSITION FAIL', indicating a failure of the negative 50 volt supply to the position.

(vii) 'POSITION RESTORED', indicating a restoration of the negative 50 volt supply to the position.

(viii) 'FUSE ALARM 11, indicating the failure of a negative 50 volt fuse within the position.

(ix) 'FUSE ALARM 21, as for the Mark I switchboard.

(x) 'FUSE ALARM 31, indicating the failure of a fuse in the Fuse Unit No. 2A.

(xi) `DIRECT MAINS FAIL', This is used at installations where a floated battery supply is used for either the positive or negative, or both, 50 volt power supplies. It indicates a failure of the rectifier output being used to recharge the batteries, and so gives advanced warning of a battery supply failure.

(xii) '+50V FAIL', indicating the failure of a positive 50V supply. Derived from a power unit.

(q) Cord and ring test
Cord test A, cord test B, and ring test jacks, to test cord circuits for continuity, supervision, noise and ringing.

(r) Position coupling
The operation of the 'COUPLE' key on an un-staffed position disconnects the operators circuit and extends the connections to the adjacent left-hand position.

(s) Keysender
At present, a keysender may be provided in place of the dial only by a non-standard arrangement, but a works specification will be issued in the future to allow a keysender to be provided as a standard optional facility. (Keysender No.1)

(t) Inter-PBX circuits
The facilities available on inter-PBX circuits are dependent on the type of signalling used, and on the type of installation at the distant end. The appropriate diagram notes-for the signalling unit concerned should be consulted for details.

When requests are made for non-standard facilities, reference should be made to TI C3 A0120: "Non-standard facilities at subscribers' stations".



(a) Extensions
Extension telephones are wired to Dgm N 806 as 2-wire PBX Extensions with Recall. Lamp signalling is employed, and the lamp circuit is completed by a loop from the extension via auxiliary break contacts in series on the answer and multiple jacks. The lamp is extinguished when a plug is inserted into any of these jacks. Lamps No. 2/45V are used, which are satisfactory on lines of up to 500 ohms loop resistance. On longer lines Units Signalling SA7611 are employed to repeat the calling signal, and these units are also used for ancillary calling appearances.

(b) Exchange lines
Ringing is detected by a relay in shunt with a rectifier which normally causes the calling lamp(s) to light continuously until the call is answered. Under power failure conditions the calling lamps light with ringing periodicity. When an incoming or outgoing call is extended, two relays are connected in series with the line, one to provide supervision, and the other, differentially connected, to detect an operator recall earth from an extension. In the event of a second exchange call arriving before a previous connection is cleared down, a 'follow-on-call trap' prevents the extension bell ringing and attracts the operator's attention by dimming both cord circuit supervisory lamps at ringing periodicity.

(c) Cord circuits
One key with 'SPEAK' and 'DIAL' positions and two supervisory lamps are associated with each cord circuit. The operation of a 'SPEAK' key and its associated relay SK connect the operator's telephone circuit. The circuit is arranged to allow only one SK relay to be operated at a time thus preventing interconnection between cord circuits should more than one 'SPEAK' key be operated. Ringing at a periodicity of 0.75 sec on and 2.25 see off is automatically applied when a calling cord is inserted into an extension, private circuit, or inter-PBX line jack. Continuous ringing is connected to the answer cord via 'SPEAK' key contacts when the common 'RING ANSWER' key is operated. A capacitor type bridge provides divided battery feed for extension to extension to inter-PBX line connections and gives individual lamp supervision. For extension to exchange connections the bridge is normally switched out of the circuit to give through dialling, through clearing, and double supervision. The circuit may, however. be modified by the insertion of two straps between U points to retain the bridge in circuit, to prevent through dialling and through clearing, and also to give individual supervision.

(d) Operator's and miscellaneous circuit
The Pilot relay, PA or PB operates when any calling lamp is lit on the left or right hand face panels respectively, to  the appropriate panel pilot lamp and operate the audible alarm buzzer. Two relays, MF and MFA, are held operated by the 50v supply to the switchboard and release in the event of a supply failure, to connect a reserve supply to the exchange line calling lamps and to modify the transmitter circuit so that exchange calls may be answered using the public exchange line feed current. A transistorised circuit SA7609 (Sub-unit SA7606) suppresses side-tone and attenuates excessively high level incoming speech by up to 4 dB. This enables the high sensitivity of the receiver in the Headset No. 1 to be fully exploited. A sub-unit SA7606 may be provided optionally to give an audible clearing signal. The sub-unit monitors the voltage developed across the resistors feeding earth to the answer and supervisory lamps in each cord circuit. If both lamps are lit the voltage is sufficient to operate the circuit which connects an earth to sound the alarm buzzer.

(e) Metering
Metering Units No. 3B are used to detect the 50 Hz meter pulses on exchange lines for all types of metering except localised extension metering. The units provide earth pulses to control directly a Meter No. 150C for 'totals' metering on exchange lines, or to control additional relays MA for 'trip' or `centralised extension' metering. The MA relays connect positive battery pulses to the sleeves of the appropriate exchange line jacks and the pulses are used indirectly to control trip meters associated with cord circuits or 'totals' meters associated with extensions. A Meter No. 19 may be connected to an extension to give running and individual call totals, and is operated directly by the 50 Hz pulses from the public exchange.



(a) General Construction
The face equipment, key plate, kicking panel, and plinth are coloured French grey. The top, side, and rear panels, and shelf desk are faced with elephant grey Warerite. All the panels are easily removable to give access for installation and maintenance.

Within the metal framework of the switchboard, which includes a 3" deep plinth,-hinged mountings at the top accommodate connection strips for wiring and cabling of extension answering and multiple jacks. Stile bars are provided at the front so that jack and lamp strips may be secured by jack fasteners. Below the face panels an apron is welded to the framework to house the key and supervisory lamp mounting assembly. The shelf desk is secured to the apron and may be hinged downwards to allow the switchboard to be taken through narrow doorways. A cord rail assembly, terminal blocks for the battery and earth feeds to the lamps, and miscellaneous circuit components are located behind the apron. Towards the rear of the position a relay frame provided with jacks may accommodate up to 13 plug-in relay-sets for the operator's circuit, 15 cord circuits, and 12 exchange line circuits. A fuse panel is mounted at the top of the relay frame and connection strips for exchange lines, private wires, position coupling, and miscellaneous circuits are mounted at the bottom.

Details of the switchboard assembly are shown on Drawings 92495/0 and 92495/1 for the Mark I and Mark II switchboards respectively.

(b) Dimensions and weight
The switchboard is 1220mm high, 730mm wide, and 850mm deep. The depth is reduced when the shelf desk is hinged down. The unequipped switchboard, as normally supplied, weighs 216 lbs, but when fully equipped with relay-sets, etc, is approximately 470 lbs.

(c) Face equipment
Two panels each provide mounting space for up to 24 half inch jack or lamp strips. The top four mounting spaces on each panel can only be used if the multiple cable support brackets are removed and are therefore only available at single position installations. Each switchboard PMBX 4/1A is supplied already equipped with 4 pairs of lamp and jack strips catering for 40 extension lines, one pair for 5 private or inter-switchboard lines, and one pair for miscellaneous alarms etc. The Mark I switchboards have 1 pair of lamp and jack strips, catering for 10 exchange lines; the Mark II switchboards have 2 pairs, catering for 20 exchange lines.

The unequipped space is fitted in with French grey melamine faced spacing strips which may be removed to fit additional lamp and jack strips as required. Drawing ES 6283 gives details of how lamp and jack strips may be arranged to suit installation requirements.

Individual jacks have moulded plastic bodies designed to fit into light metal frames, to form strips of up to 20 jacks. The bush end of each jack clips into the front of the frame and the tag end is secured when the top end of a T shaped lug which passes through a slot between jacks is turned clockwise through 900. The jacks are easily removed for replacement or maintenance by turning the lug 900 anti-clockwise using an Adjuster Detent No. 3.

Jack strips with 20 jacks are used for the extension multiple; strips with 10 jacks are used for extension answering appearances and exchange lines; strips with 5 or 10 jacks are used for private wire or inter-PBX circuits, and strips with 7 or 9 jacks are used for the miscellaneous circuits.

Lamp jack strips with 20 individual lamp jacks are provided above exchange line jacks and strips with 10 lamp jacks are used above extension answering jacks. The lamp jacks positioned directly above the jacks are used for calling lamps and positions 450 to the left of the jacks are used for free line signalling lamps if required. Lamp strips initially provided above the inter-switchboard jacks have lamp jacks only directly above the jacks but additional lamp jacks may be added when free line signalling is required. Lamp shrouds for 10 or 20 lamps are clipped into the front of each lamp jack strip. Individual lamp jacks may be inserted or withdrawn from strips by removing the lamp shroud, springing open the top and bottom edges of the frame, and sliding the lamp jack in or out from the front. Opal lamp caps which are hot-stamped with the circuit designation fit into the shroud over calling lamps. Light French grey dummy caps are fitted in unused positions, and caps with an arrow pointing diagonally downwards are fitted over free line signalling lamps.

PMBX No. 4 Extension Units Nos. 1 and 2 are used to add extension answering appearances. Each Unit consists of a jack strip and lamp jack strip for 10 circuits, wired to a connection strip. A lamp shroud is provided with each unit, but lamps and lamp caps have to be ordered separately. Units No. 1 are fitted in the left hand face panel (as viewed from the front), and Units No. 2 in the right hand face panel.

By using Extension Units Nos. 1 and 2, the maximum of 160 extension answering appearances may be provided at a single position installation. This maximum is reduced to 80 appearances per position at multiple position installations. (Refer to Drawing ES 6283).

PMBX No. 4 Extension Units Nos. 3 and 4 are used to provide the extension multiple appearances in the left hand and right hand face panels respectively. Each unit consists of a jack strip for 20 circuits wired to a connection strip.

A maximum of 200 extension multiple appearances per position is possible, Extension Unit No. 3 catering for Nos. 0-99, and Unit No. 4 for Nos. 100-199. The extension multiple jacks are fitted above the extension answering appearances, in accordance with Drawing ES 6283.

Exchange line circuits may be added by ordering and fitting 1 jack No. 710BR, 1 jack lamp No. 61/20A, and 1 cover lamp No. 8/20A, per 10 circuits. Wiring is provided for the first additional 10 circuits in the PMBX 4/1A Mark I switchboard, the Mark II being initially provided with face equipment for 20 exchange lines. For subsequent circuits, another connection strip has to be fitted and wired to the jacks and lamps.

Five additional inter-PBX circuits may be added by fitting individual jacks (part 2/DJA/18) and lamp jacks (part 1/DJA/13) to the inter-PBX jack and lamp jack strips, and connecting them to the wiring provided in the switchboard. Exceptionally, inter-PBX circuits in excess of 10 may be added, using space normally reserved for exchange line jacks.

A multiple of up to 40 exchange lines and 10 inter-PBX lines can be provided. The miscellaneous jack and lamp strips are located at the bottom of the right-hand face panel of the switchboard. Provision is made for jacks and lamp, for night busying, operation under power fail conditions, alarms and cord circuit, testing, as described in sections 2(m), 2(n), 2(p) and 2 (q) respectively. The remaining 9 jack and lamp positions (7 for a Mark II switchboard) can be used for further alarms, or for the provision of non-standard conference facilities.

One ticket recess and one pilot lamp are fitted per panel.

(d) Keyshelf
The Keyshelf is initially equipped with a row of ten cord circuit keys, each having two associated supervisory lamps and a pair of cords. The keys are designated 'SPEAK' in the forward position and 'DIAL' in the opposite. The pairs of cords are coloured yellow, black, green and blue, in rotation.

Provision is made for the addition of a further five sets of cord circuit equipment; the keys for cord circuits 11-15 are initially mounted below the key- shelf, the resulting holes being fitted with dummy key blanks. When extra cord circuits are required, the appropriate keys are released from below the keyshelf and refitted in their working positions. Extra cords, plugs, pulley weights and supervisory lamps have to be provided.

The single common key on the left of the keyshelf is the combined 'RING ANS' and 'COUPLE' key. Space for an additional key is also provided. Of the two common keys on the right, the one nearer the face panel is the 'NIGHT SERVICE' key, and the other is the 'ALARM CUT-OFF' key.

The operator's instrument jacks are provided in the front left hand face of the keyshelf. Access to the rear of the panel carrying the operator's jacks is obtained by the small panel under the keyshelf. A transmitter cut-out (switch No. 5A) may be fitted between the jacks when authorised.

A removable metal cover (part 1/DFA/15) with a textured PVC finish is provided on the working part of the keyshelf. The markings on the cover are obtained by hot-stamping into this PVC finish. The cord circuit keys are screwed to the keyshelf framework, the handles passing through clearance holes in the cover. The cord circuit supervisory lamp caps are a push fit into the cover.

The keyshelf cover is retained by two ball catches fitted to the key frame locating into the front edge of the plate. It can be released by applying pressure to the bar positioned centrally under the keyshelf.

A readily removable dial and mounting (Mounting Dial No. 22A) is fitted on the right of the keyshelf. For installations in manual exchange areas, with no inter-PBX circuits to PABXS, the dial and mounting can be removed, and a mounting plate No. 5A substituted for the No. 5B originally fitted.

A small notice frame (Frames, Notice No. 38A) is fitted on the left of the keyshelf, to display any relevant local information. This may be replaced by a visible index file (Frames, Notice No. 25 grey) if required.

Where cord circuit metering is required, the keyshelf cover (Part 1/DFA/15) is replaced by a cover (Part 2/DFA/15) which has apertures provided to permit the reading and resetting of trip type meters. These meters (Meters No. 21B) are fixed to the keyshelf framework, below the cover, and in front of the relevant cord circuit key. (NOTE: metering may not be required on all cord circuits.)

Meters, Dummy No. 8 are fitted to cover the apertures of non-metered cord circuits. Transparent plastic windows (Part No. 1/DWI/87) and, if required, Ticket Clips No. 18A are fitted for those cord circuits with metering facilities.

(e) Apparatus layout
Access to equipment mounted within the switchboard is achieved by removing the top and rear panels from the framework. Figure 3 shows the rear view of the switchboard with the rear cover and relay-sets removed showing cords and pulley weights for 15 cord circuits.

Connection strips are fitted in the bottom of the switchboard for exchange line, inter-PBX line and operator's and miscellaneous circuit terminations. On the PMBX 4/1A Mark I switchboard, a connection strip is also provided for keysender attachments but this is unlikely to be used. Cabling access to all terminal blocks is made via entry holes in the base of the switchboard, one at each side and one at the rear.

The two nickel-cadmium batteries fitted per position at power-unit operated installations are mounted in a small bracket in the bottom of the switchboard, on the right hand side as viewed from the rear. A single battery jack is provided adjacent to this bracket.

The strip-mounted plug-in relay sets for cord circuits, exchange line circuits and operator's and miscellaneous circuits are mounted one above the other in the rear of the switchboard. The relay sets are located on the framework by two guide pins, one on each vertical frame member, passing through a hole in each end of the relay-set plate. They are secured by a barrel nut at each end of the relay set screwing on to threaded studs fitted to the frame. Figure 4 shows the rear view of a partially equipped switchboard with the rear cover removed.

The allocation of relay set positions is shown on the left hand vertical frame member. This is such that the relay sets most frequently required (i.e. the operator's and miscellaneous circuit, the first six exchange line circuits and the first ten cord circuits) occupy the lower positions on the frame. This arrangement minimises the disturbance that is caused to a working switchboard when access to the rear of the face panel is necessary.

A maximum of 15 cord circuits and 12 exchange line circuits, together with the operator's and miscellaneous circuit, can be accommodated internally at a single position installation. Further exchange line circuits have to be mounted externally, as Units Signalling SA7616.

At multiple position installations, the relay sets for exchange line circuit numbers 10, 11 and 12 ate not fitted to avoid damage to the face equipment wiring.

The buzzer, smaller components associated with the miscellaneous jack strip, and terminal blocks for the battery and earth connections for the face panel jacks are all mounted on the rear of the pilot lamp rails.

A fuse panel is mounted at the top of the switchboard, towards the rear, and provides the distribution for the power supplies required at PMBX 4/1A installations. This panel can accommodate 30 fuses for negative 50 volt circuits, together with two individual fuses, one of which is used for the reserve battery supply.

By removing the top panel of the switchboard, access can be gained to the extension connection strips, which are mounted in the top of the switchboard. Those for the extension multiple terminations are towards the front of the switchboard, those for extension answering terminations are at the rear. Both sets of these connection strips can be individually hinged upwards to facilitate installation and maintenance work.

At multiple position installations, the extension multiple cabling is supported below the extension multiple connection strip, between the face panel and the fuse panel. Access to this cabling is achieved by hinging the connection strips as previously described.

All auxiliary equipment associated with PMBX 4/1A switchboards is designed to be mounted on or against a wall, with the exception of batteries.

(a) Distribution equipment
Mounting D92221 provides fixings for test jacks and connection strips. All extension, exchange and inter-PBX lines are taken through a test jack for maintenance testing purposes.

The mountings are positioned above a plinth (Drawing D 92370) and may be stacked vertically or arranged side by side.

Typical arrangements of plinths and mountings are shown in Drawing SD 136, and further details are given in Specifications S 1155 and S 1156.

(b) Power equipment

(i) 50 volt negative supply.
The PMBX 4/1A is operated from a nominal 50V negative supply. It is designed, however, to function within the limits of 45V to 55V. The power is derived from a mains operated unit, the type used depending on the current required for the particular installation (refer to Specifications S 1155 and S 1156).

It should be noted that a maximum current of 12 amps can be supplied from these power units; any installations requiring a greater current must be provided with stand-by batteries and rectifiers, as indicated in Specifications S 1155 and S 1156.

Any installation requiring a current of less than 12 amps at which the subscriber is unwilling to accept the limited facilities available under mains failure may also be provided with stand-by batteries.

(ii) 50 volt negative reserve Supply
At power unit operated installations, 2 batteries secondary No. 16 are provided per position as a reserve 50V negative supply. These are trickle-charged from the main negative 50V supply to the position, and are accommodated in the bottom of the switchboard itself.

(iii) 50 volt positive supply
When metering other than `totals' and/or 'localised extension' is required, a positive 50 volt supply is required. This is derived from a mains operated power unit, the size of unit used being determined by reference to Diagram N 2250.

(iv) Power distribution and alarm arrangements
The negative 50 volts and positive (earth) leads are taken from the power source to a Fuse Unit No. 2A. From there, individual fused outputs are provided to the switchboard position(s)', ringing converters), signalling unit(s), etc. The output to each position is terminated on an alarm type fuse mounting, within the switchboard itself, serving exchange line relay sets, cord circuit relay sets, etc. An associated fuse alarm is provided at each position. The alarm circuit from the Fuse Unit No. 2A being connected to an alarm lamp on one position. Normally, position 1 is used as the control position at a multiple position installation.

The reserve and positive battery supplies are connected via individual alarm fuses with visual alarms on the switchboard positions.

(v) Ringing supplies
The interrupted ringing and continuous ringing supplies are obtained from Converters, Ringing Nos. 11A and 12A.

Converter, Ringing No. 11A provides for both interrupted (0.75 sec on, 2.25 see off) and continuous ringing supplies, and is suitable for installations of up to two positions. Three or four position installations require the addition of a Converter, Ringing No. 12A. This provides a continuous ringing supply, and makes use of additional contacts provided on the No. 11A to interrupt the current.

Both items are wall mounted and are provided on the standard 12 inch wide base, with a 7.5 inch projection from the wall.

(c) External exchange line circuits
Units, Signalling SA7616 are wall mounted units accommodating one exchange line circuit. These are provided when more exchange line circuits are required than the number which can be mounted within the switchboard positions themselves.

For single position installations, a maximum of 12 exchange line circuits can be mounted internally; at multiple position installations, this figure is reduced to 9 circuits per position. Further information on the provision of these units is given in Specifications S 1155 and S 1156.

(d) Signalling units
A range of signalling units, numbers SA7612 - SA7615, has been developed for the connection of inter-PBX circuits to the PMBX 4/1A switchboard. These units are all provided on the standard 12" wide wall mounted unit, with a 7.25" projection from the wall.

All inter-PBX signalling systems are catered for by these units, with the exception of SSAC13 (see below).

Further details, and references to N diagrams, are given in Specifications S 1155 and S 1156.

A signalling unit SA7611 is also available, providing long extension circuits for lines over 500 ohms resistance, and also duplicate answering circuits.

(e) SSAC13 equipment
The terminating equipment at PMBX 4/1A installations for the 1vf SSAC13 system is Equipment, Signalling No. 25/2A. This comprises open type floor-mounted racks, catering for two inter-PBX SSAC13 circuits each. These are supplied complete with shelves, shelf-wiring and jacks, on which the appropriate relay sets are mounted.

If required, Covers D92417 can be supplied, finished in elephant grey, to encase Equipment, Signalling No. 25/2A. With these covers, the overall dimensions of the SSAC13 equipment are 5' 4" high, 1' 9" wide and 1' 7" deep.

Further information on the SSAC13 equipment is included in Diagram N764, as well as in Specifications S 1155 and S 1156.


(a) Installation
All PMBX 4/1A installation information is given in Specifications S1155 and S1156 for single position and multiple position installations respectively. When extending installations installed prior to October 1971 reference should be made to Diagrams N 2231 and N 2232.

(b) Maintenance
Maintenance information for the PMBX 4/1A switchboard is given in TI E5 D1401: "Switchboard PMBX No. 4/1A, Maintenance". A full list of diagrams relevant to the switchboard and its ancillary apparatus is given, together with maintenance instructions.

User Guide

Introduced in 1967-68.

The PMBX No. 4 can be fitted with a Keysender No. 1 which was available mid to late 80's as an `extra'.

Additional Pictures

The above switchboard was probably installed in a Hotel.

It has all the extensions on one position and non-standard keys at the top right for connection to a paging system.

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Last revised: January 18, 2022