Steel-Framed C.B. P.B.X. Switchboards
July 1950

Shortages of materials during and since the war have given impetus to investigations into the use of alternatives, and to means of effecting economies in such materials, a case in point being the substitution of steel for much of the seasoned timber normally used for telephone switchboard carcases.
The idea of, so-called, all-metal switchboards is by no means new, such boards were used during the war, being specially constructed to suit conditions which would not be met in ordinary circumstances, and for that reason they were not perpetuated.

Switchboards of large type are almost invariably built on an angle-iron or similar strong metal framework, to ensure rigidity, but in the case of smaller P.B.X. boards of the type described here, the extensive use of metal is rather unusual.

This lamp signalling C.B. P.B.X. switchboard is of a type manufactured by the Company in considerable quantities for the Australian Posts and Telegraphs Department and was supplied in the first instance in 1947.

The design effects a considerable saving in timber and also permits an advantageous re-arrangement of the interior equipment in comparison with boards previously made, partly due to the fact that the front, rear, and side panels are all removable, a circumstance which greatly facilitates assembly, inspection and maintenance.

The framework comprises three main assemblies, viz., right and left-hand sides and key and plug shelf frame. The sides are rectangular welded structures made of channel-section pressed steel on which are transverse metal bars for supporting relay set shelves, and vertical bars for carrying fuse panel and terminal blocks. Top and bottom rails are welded to the two sides to form a rigid chassis to which is bolted the key and plug shelf framework (also a welded pressed steel assembly) and the pressed steel top.

The wooden portions of the switchboard are mainly resin-bonded plywood, hardwood being used only where necessary, thus, the removable side, front and rear panels, and the apron bottom of the key shelf, are all of plywood, while the lock rail, key and plug shelf, cord connecting blocks and reinforcing battens for the sides, are hardwood.

Fig. 1 N341A Type Lamp Signalling C.B. P.B.X. Switchboard

The front and rear panels have hand recesses, the former being fitted with an 0.125 inch thick cork linoleum kicking panel on the outside, and leather-covered felt on the inside to act as a cushion against the swing of the cord weights. A similar cushion is provided on a partition separating the weights from the rear equipment. The front and rear panels are placed in position in the same way as the ordinary lift-out door.

Wood screws are used to fix the wood parts of the key and plug shelf assembly which is of orthodox design. The key shelf is hinged and is provided with a locking stay. A covering of black fibre protects the top surface as may be seen in Fig. 1 which is a front view of the switchboard.

There is space in each of the two front equipment panels for fifteen half-inch lamp or jack strips, giving capacity for the following circuits mounted in sequence from the bottom upwards:-

(a) 20 night switching circuits (10 per panel).

(b) 15 auto exchange lines, 3 C.B. or magneto tie lines and 2 cord test jacks.

(c) 100 (nominal) 80 (actual) extension lines.

The board is wired for the full complement of circuits, irrespective of the amount of equipment fitted, the capacity for item (c) being regarded as 80 for this purpose, although the extra 20 circuits can be included when necessary. Extension of the equipment is greatly facilitated by this arrangement of pre-wiring. It will be observed that only 30 extension lines are equipped on the switchboard in the illustration.

At the top of the panels are fitted pilot and fuse alarm lamps, and plunger keys for night alarm and power generator connections, while above these, on the metal framework, are a pressed steel notice frame for an instruction card, and a battery cut-off switch.

Drillings for sixteen cords are provided in the plug shelf, although only twelve cord circuits are shown equipped in Fig. 1, the unequipped positions being fitted with dummies, as are also the vacant supervisory lamp positions on the key shelf. Two double-throw keys on a common mounting plate are associated with each cord circuit and are designated respectively, Ring Front - Speak, Ring Rear - Dial and Hold, from front to back; in addition, a coupling key is fitted in front of the automatic dial at the extreme right, while the lock rail has an 'alnico' generator mounted at one end, and twin, 4-point operators' jacks at the other, this completing the equipment of the key and plug shelf. For the size of the board, the interior capacity in terms of relay equipment is remarkably large, due to the fact that the apparatus is mounted on jack-in units, the utilization of which is of advantage both in the factory during the initial assembly and wiring of the board, and also in connection with maintenance repairs and extensions. The units are mounted on shelves supported from the transverse bars at the side and are arranged as shown in Fig. 2, the single relay set in the top position accommodating telephone circuit and miscellaneous apparatus, and the ten sets below, all having bases of 1 6-relay capacity, being allotted to cord circuits (4 per base) and tie line circuits (3 per base) in the upper row, and exchange line circuits (3 per base) at the bottom.

At the top left in Fig. 2 are terminal blocks for line and miscellaneous circuit terminations, thus, as the cable entry is directly below at the bottom of the board, and the side panel is removable, connection of the external cables is much simplified. A second hole is provided on the opposite side for interconnections between adjacent boards, or as an alternative entry, and is fitted with a cover when not in use.

The fuse panel, which is located at the top right, opposite the terminal blocks, can be swung round into alignment with the relay sets when it is necessary to replace a fuse.

The board is designed for 40-50 volt working with 6-volt lamps which require the introduction of resistors in the extension line circuits. These resistors are mounted on the lamp jack strips, in the manner shown in Fig. 3, an arrangement which effects economy in both wire and mounting space. The resistors are British Post Office type No. 112, each consisting of two windings connected in series and terminated so that one can be strapped out when necessary, to adjust the current flowing through the lamp on high-resistance lines.

The finished switchboard presents a neat and attractive appearance, as the metal work is painted with light brown enamel and wood parts are stained to match.

The interior is enamelled white, thereby lightening the work of the maintenance officer in every sense of the word.

The overall dimensions are much smaller than is usual with boards of this capacity, as may be noted from Fig. 4, and this will have a strong appeal.

The board is coded N341A type and has been supplied with equipment varying from 15 exchange lines, 8Oextensions and 16 cord circuits, down to 6 exchange lines, 20 extensions and 8 cord circuits.

The Company has developed a switchboard of the same pattern and size, but with a resin-bonded plywood frame covered with an ornamental plastic known as Warerite. One of these boards is illustrated in Fig. 5, the finish in this case being imitation birdseye maple, while others in imitation walnut are in course of manufacture. It will be observed that the top and sides are not removable on this type of switchboard.

Last revised: October 14, 2019