PMBX No. 4
Descriptive Leaflet - 1967
Descriptive Leaflet - Multiple Position - 1973
Descriptive Leaflet - Single Position - 1980
Private Manual Branch Exchange No.
3 CIRCUIT DESIGN FEATURES
5 ASSOCIATED APPARATUS
6 INSTALLATION AND 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.
(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:-
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
(d) Through clearing and dialling
(e) Press-button operator recall from extensions on all types of call
(f) Automatic ringing on calling cords
(g) Automatic hold on exchange calls originated or answered by the operator
(h) 'Click' engaged test at multiple installations
(j) Free line signalling
(k) Transmitter Cut-out
(m) Night service
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
(p) Alarm lamps
(q) Cord and ring test
(r) Position coupling
(t) Inter-PBX circuits
When requests are made for non-standard facilities, reference should be made to TI C3
A0120: "Non-standard facilities at subscribers' stations".
3 CIRCUIT DESIGN FEATURES
(b) Exchange lines
(c) Cord circuits
(d) Operator's and miscellaneous circuit
4 PHYSICAL DESIGN
(a) General Construction
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
(c) Face equipment
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.
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
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.
5 ASSOCIATED APPARATUS
(a) Distribution equipment
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
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
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
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
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.
6 INSTALLATION AND MAINTENANCE
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'.
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.
Last revised: January 18, 2022