C 1113
Issue 1, Oct 1950


1. General
'Switchboard, P.A.B.X., SA 8120' is the cordless manual switchboard for use with the P.A.B.X. No. 1 .

The switchboard consists of a wooden case with a sloping front panel, cast in aluminium alloy, and a detachable baseboard.  Equipment is arranged within the switchboard case as described below:-

(a) The baseboard is fitted with two 'Jacks No. 40' to which the cable from the automatic unit is connected, see par. 10.

(b) The cabinet is provided with two 'Plugs No. 3201', which fit into the 'Jacks No. 40', so that it is possible to remove the cabinet without disturbing the permanent wiring.

(c) The face equipment accommodates the keys detailed below:-

(i) Nine lever- type connecting keys for exchange, inter-switchboard manual extension and '0' level lines, and are situated between two rows of lamps; each key serves two circuits.

(ii) Six two position keys designated as follows:-

Dial/Ring: Interrupt/Flash and Cancel: Audible Alarm/Night Service:

Speak Extn./Speak Exchange: Release/Engaged Test: Alarm Cut-off.

2. Use of Position Keys
These are used is as follows:-

(a) Dial. For connecting the dial to the exchange, to automatic inter-switchboard circuits and manual extensions.

(b) Ring. For ringing on '0' level circuits and generator-signalling private circuits.

(c) Interrupt. This enables the P.A.B.X. operator to offer a trunk call to an engaged extension. (A ticking noise is superimposed on the extension circuit, when the operator enters the connexion, to warn the extension that the operator is in circuit.)

(d) Flash and cancel. This enables the operator to call-in the public exchange operator on an established call, or to release the connexion after a dialling error.

(e) Splitting i.e. Speak-extension, Speak-exchange.  This enables the operator to speak on either the extension or public exchange side of the connexion.

(f) Engaged Test. This enables the operator readily to ascertain free lines.  Operation of the key causes a steady glow on the calling/supervisory lamp associated with any exchange and inter-switchboard circuit while the circuit is in use, see par. k.

(g) Release. This enables the operator to release an exchange line which has not been keyed to an extension number.

(h) Alarm Cut-off. This is used to disconnect the alarm signal in the case of a P.C. or other alarm.

(j) Audible Alarm. When this key is in the operated position, an audible alarm is given on every calling signal.

(k.) Night Service. This is operated when the operator vacates the switchboard.  The operation of the key arranges for calls incoming on exchange and inter-switchboard circuits to be answered by selected extensions, who dial '8' to gain access to the calling line.  This key is of a special locking type to prevent accidental restoration when cleaning or dusting the switchboard.

3. Digit Key Strip
This is provided for keying extension numbers only. The digit-key strip is connected to a marker circuit which positions the linefinder of exchange lines, inter-switchboard circuits and the '0' level circuits on to the keyed extension number.  It is not an electrical key-sender and, therefore, cannot be used for calls to the public exchanges.

4. Calling and Supervisory Lamps
One lamp is provided for each exchange line, Inter-switchboard circuit, manual extension and 0-level circuit and these lamps function as calling and supervisory lamps. Thus:-

(a) A flicker signal on a lamp at a frequency of 0.2 sec. on: 0.2 sec. off - indicates a calling circuit.

(b) A steady glow on a lamp (i.e. during the setting-up of a connexion on the switchboard, but see par. 9) indicates a free extension.  This glow ceases, however, when the extension answers.

(c) A lamp flashing at 0.75 sec. on: 0.75 sec. off indicates an extension busy.

(d) The appropriate supervisory lamps glow steadily when the ENGAGED TEST key is operated, to ascertain a free exchange line or inter-switchboard circuit.

5. Alarm Lamps
Three lamps, to indicate various alarms, are provided as follows:-

(a) P.G. Pilot. This is fitted with a white opal and, when glowing, indicates a permanent loop due either to a faulty line, a misplaced handset or a handset not replaced after a call.

(b) Auto Alarm. This is fitted with a red opal and, when glowing, indicates an alarm-type fuse blown or a switch failing to release.

(c) Mains Fail. This is fitted with a red opal and, when glowing, indicates failure of the mains supply.

6. Dial
This is provided to enable the operator to originate exchange calls for automatic extensions which are barred direct-access or for manual extensions; it is also used for dialling over automatic inter-switchboard lines.

7.Operator's Telephone
This is a 'Telephone No, 184 ' with 'Plug 404', and is requisitioned separately and fitted locally.

8. Rear Equipment
This consists of a mounting plate, fitted in the cabinet, on which are mounted the buzzer, retard coils, resistors, capacitors and metal rectifiers.

9. Operation
Detail operating instructions are issued separately on form A1368. Briefly, the operating procedure for the cordless switchboard is as follows:-

An incoming call is indicated by a flicker signal on the appropriate lamp.  To answer, the SPEAK key lever is moved towards the lamp; the lamp will continue to flicker.  Ascertain the extension number required, and key-up on the digit-key strip. If the extension line is free, the flicker signal on the lamp will change to a steady glow indicating the extension bell is being rung.  The SPEAK key may now be restored to normal and when the extension answers, the lamp will cease to glow.

At the conclusion of the call, the connexion will be cleared automatically.  No further action on the part of the operator is, therefore, necessary.

If the extension keyed is busy, the flicker signal on the lamp will change to a busy flash of the calling subscriber is prepared to wait, the SPEAK key may be restored to normal, and the lamp will then continue to flash until the extension becomes free.  The caller is then connected automatically to the extension without further intervention by the operator.

10. Cabling
The cordless switchboard is connected to the automatic equipment by Cable, Switchboard, 61-wire/9oz, Round, which is wired to two 'Jacks No. 40' fitted on the switchboard base.

11. Diagrams 
Dgm. SA 8120 shows the circuit arrangements of the cordless switchboard, which are described in Dgm. Notes SA 8120 and SA 8122

Superseded by Switchboard SA8133.


Switchboard front view


Switchboard with front cover open

Rear view of Switchboard with back cover opened

BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: October 30, 2022