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SYSTEM 100 is a simple low-cost switching telephone system that provides access to a single public-exchange or PABX line from up to four telephones. Four variants - 101 to 104 - are offered to suit different applications.

Typically, a system can be provided for a secretary and one or two executives. The secretary can answer all incoming calls and transfer them to the executives as required. The executive can dial all his own calls, although the secretary can dial them for him if preferred. There is priority between telephones: when an executive is using the exchange line, the secretary cannot gain access to it - this ensures that an executive's calls are not overheard.

The system is also an 'intercom' between its telephones, and can be used while an exchange call is being held (exchange and intercom calls are secret from each other).

A single basic telephone type is used for all positions and can be installed without removing the telephone case. A number of versions are available: keypad or rotary dial; table or wall mounted. System operation is by pushbutton switches in front of each telephone's handset: 'EXCHANGE' to connect the telephone to the exchange line; 'HOLD', to hold an exchange call while making an 'intercom' call to one of the other telephones; and '1' and '2', to operate built-in intercom-calling buzzers in the other telephones. A lamp lights on the telephone when the exchange line is in use.

Each system comprises telephones, a power unit (normally AC-mains energized, but batteries can be used) to operate the intercom, lamps, and buzzers, and a common facilities unit (CFU) to interconnect the exchange line, power supply, and inter-telephone wiring. Alternative CFUs provide: visual indication of exchange ringing; visual reminder that an exchange call is being held; and automatic transfer of ringing to the executive's telephone if the secretary does not answer the call within about 15 seconds.


Only one version of the System 100 telephone is required; it is based on the standard GEC746 table telephone, and uses many common parts. Thus, stores keeping and spares are simplified, and additional inventory requirements are kept to a minimum. The detailed differences between the System 100 telephone and the GEC746 are illustrated overleaf. The System 100 telephones are available in a choice of three colours: dual-grey, ivory, and black.

Wall mounting
The telephones can be converted to wall mounting by inverting the dial or pushbutton keypad assembly, fitting a handset-support bracket to the carrying handle aperture on the telephone case, and fitting a wall bracket to the telephone base. If desired, the four control keys can be replaced by keys with inverted legends.

Four main variants are offered, which concern the signalling. The variants are:

  • Rotary-dial signalling at 10 IPS

  • Pushbutton 'dialling' with loop-disconnect signalling to the exchange

  • Pushbutton 'dialling' with multi-frequency output

  • Pushbutton 'dialling' with DC-leg output

The rotary dial variant uses the standard dial assembly with a clear plastic finger plate. The mechanism is a trigger type with a self-compensating action which exhibits no appreciable variation in pulse ratio after 4'/2 million impulses.

The three pushbutton variants are identical to the rotary-dial model, except that the dial and ring are replaced by the appropriate key unit and escutcheon.

The loop-disconnect pushbutton variant is a direct replacement for the rotary dial instrument. The dialled numbers are stored - using large scale integrated circuits - and released to the exchange at the appropriate speed, normally 10 IPS. The dial electronic circuits are powered from the exchange battery.

The multi-frequency variant provides audio-frequency signalling to the exchange (in accordance with CCITT Recommendation Q23): the dialling frequencies transmitted over the telephone line are detected by special receivers at the exchange. An individual combination of two frequencies is generated when a key is pressed.
The fourth variant uses 'DC code C' signalling. It is a low-cost alternative to the other types for P.A.B.X. or PAX with suitable detectors.

Rotary Dial
Press Button Rotary


SYSTEMS 102, 103, AND 104

An outgoing call is made from any telephone in the system by lifting the handset, pressing the EXCHANGE button, and dialling. An incoming call rings the bells at all the system's telephones and is answered at any telephone by lifting the handset and pressing the EXCHANGE button.

Clear down
Replacing the handset clears the call and automatically releases the EXCHANGE button (or the HOLD button, if the call has been held).

The normal wiring arrangement prohibits more than one telephone at a time gaining access to the line. 'Line in use' When the exchange line is in use, the lamp on each telephone is lit, and automatically switched off on clear down. Other lamp indications that can be provided are described under CFU.

Ringer loudness
All telephones have a three-position volume control which protrudes from the telephone base-plate. The lever can be modified so that it also switches the bell off: one of the telephones in the system (usually the secretary's) is not modified. Thus all incoming calls are answered by the secretary, without disturbing the executive; but if the secretary is absent, the executive telephone's bell can be switched on. An optional delayed-ring feature is described under CFU.

An intercom call is made by lifting the handset and momentarily pressing the appropriate intercom-calling button ('1' or '2'). A call is answered by lifting the handset, without operating a button.

To make an intercom (enquiry) call to another telephone during an exchange-call conversation, press the HOLD button (which releases the EXCHANGE button): this connects the handset to the intercom circuit without releasing the exchange call. An intercom call can then be set up (which is secret from the exchange call). At the end of the enquiry, either telephone can be connected to the waiting exchange call by pressing the EXCHANGE button on the appropriate telephone; the handset is simply replaced at the other telephone.

To provide PBX operator recall (or to make use of a PBX enquiry-and-transfer feature) button '2' is used: with the EXCHANGE button pressed, '2' applies an unbalance to the exchange line and does not operate an intercom buzzer; with the EXCHANGE button released (or the HOLD button operated), '2' is used for intercom calling.

A local 10-12 V DC local power supply is used to operate lamps, buzzers, intercom, and common facilities unit. Normally a wall-mounted AC-mains power unit is used, although some systems can be operated from batteries: details are given overleaf.
In the event of a local power supply failure, the system can still be used to make and receive exchange calls, but
the other features are inoperative.

The exchange line, inter-telephone wiring, and local power supply are interconnected via a CFU. Different versions of the CFU can provide a number of additional operating features: their application is tabulated overleaf.

Visual ringing
Each telephone lamp flashes in synchronism with exchange ringing to provide a visual indication at all telephones, regardless of whether the telephone bells are switched off.

Flicker hold
When an exchange call is held, each telephone lamp flickers as a reminder.

Delayed ring
This feature provides an alternative to the standard 'bell off' arrangement. A time delay is introduced between the onset of ringing and the operation of one bell in the system. Thus, a secretary can answer calls without the executive telephone's bell ringing: however, if she is absent, the executive telephone's bell rings after a delay adjustable
between five and fifteen seconds.

System 101 differs from the others in that one telephone terminates two exchange lines. One line is allocated primarily for outgoing calls (it replaces the intercom circuit), and the other for incoming calls. A call on the 'incoming' line is answered, or made, after pressing the EXCHANGE button. To deal with a call on the 'outgoing' line, it is not necessary to first Dress the EXCHANGE button. This system does not use lamp. buzzer, local Dower unit or CFU.




The SYSTEM 100 switching telephone is based on the standard GEC746 instrument, to which have been added the necessary pushbutton switches, lamp, and DC buzzer to provide the operating features: the same telephone instrument type is used at all positions in a system. Each telephone is provided with four switch mechanisms. Many of the telephone components are standard GEC746 items, minimizing stores holdings.


Available in black, ivory, and grey; table or wall mounted; rotary dial or pushbutton signalling.
Loop-disconnect signalling, using rotary dial, normally 10 IPS - type 99480/colour/2
Loop-disconnect signalling, using pushbutton keyset, normally 10 IPS - type 99480/colour/7
Multifrequency signalling, to CCITT Recommendations - type 99480/colour/3

Type 98569 - wall mounted, 104 mm x 68 mm; incorporates terminal field, inductor (for common intercom feed), and capacitor (for exchange ringing circuit).
Type 99502 - wall mounted, 134 mm x 84 mm; incorporates terminal field, inductor, capacitor, and multivibrator to provide a lamp-flicker indication when an exchange call is being held.
Type 99581 - wall mounted, 165 mm x 120 mm; incorporates terminal field, inductor, capacitor, multivibrator, and a high-impedance ringing detector/relay to provide lamp-flash indication in synchronism with ringing current. Also incorporates a time out which removes a short-circuit from one or two bells in a system after a delay adjustable from 5 to 15 seconds (at least one bell in such a system always receives undelayed ringing).

250-250v AC 50-60hz input, 10v DC output - type 97206
110/220-240v AC 50-60hz input, 10v DC output - type 99768


Case and handset moulded in ABS high-impact plastic.

PVC-covered cadmium-tinsel conductors. Held by stress-relieving grommets.
Line cord: straight; 2 m (78 in)
Handset cord: coiled; 200 mm (8 in) unextended, 1.7 m (68 in) extended.

Choice of rotary dial or pushbutton keyset.

Dial: characters printed black on antique-silver background beneath a clear-plastic finger plate. Trigger type with self-compensating action. Projects through self-centring ring on the case. No appreciable variation in pulse ratio after 4% million operations. Loop-disconnect signalling - impulse speed 10 IPS; break-to-make ratio 2:1 (66%); inter-digit pause 240ms. Other speeds, ratios and inter-digit pauses can be supplied.

Pushbutton keyset: 10 or 12 buttons, with black numerals on a white background, arranged in the CCITT recommended 3 + 3 + 3 + 1 format with 'fall through' action to ensure positive operation.
Choice of:

  • Loop-disconnect, Multifrequency, or DC-leg signalling. MF and DC-leg signalling use 1 2-button keysets.

  • Loop-disconnect signalling: impulse speed 10 IPS, break-to-make ratio 2:1 (66%); inter-digit pause 800 ms; other speeds, ratios, and inter-digit pauses can be supplied. 18-digit store.

  • Multifrequency signalling: 2-out-of-7 frequency calling, to CCITT recommended frequencies.
    DC-leg signalling: ground (earth), high resistance, or low resistance applied to each of three wires between telephone and suitably equipped PABX.

Receiver inset: rocking armature; replaceable capsule: sensitivity + 56dB relative to 1 Pa/root w
Transmitter inset: carbon granule, replaceable capsule; sensitivity -10dB relative to 1 V/Pa at 2 Pa input.

A built-in automatic volume regulator limits speech-signal amplitudes on lines shorter than 400f1 ; virtually no attenuation on longer lines.

Exchange: double-coil ringer; DC resistance 1000 ohms; accepts ringing currents between 16 and 30 Hz; bells, double gong, harmonizing tones; 3-position volume control with optional 'bell off' position.
Intercommunication: DC single-coil buzzer, moving armature type; DC resistance 55J1., A signal lamp is fitted to each telephone to indicate when the exchange line is in use.

Power supply
10 to 12 V DC to operate buzzers, lamps, and intercommunication circuits. This can be provided either by a GEC power unit or 12 V DC dry batteries. In the event of a mains supply failure, the exchange line remains in service, but lamp and buzzer indications and the intercom circuits are inoperative.

Suitable for all climates; protected against the entry of insects.

Dimensions and weight
Height: 119 mm (4.75 in) Width: 250 mm (10 in) over handset; 140 mm (5.5 in) over case.
Depth: 216 mm (8.5 in) over case. Weight: 1.7 kg (3.5 lb) complete telephone; 230 g (8 oz) handset.


Number of telephones

CFU   type CFU optional features Power supply SYSTEM
1 Not used   Not used 101
2 98569
Flicker hold
Flicker hold ' Delayed ring ' Visual ring
Mains or battery
Mains or battery
3 98569
Flicker hold
Flicker hold ' Delayed ring ' Visual ring
Mains or battery
Mains or battery
4 98569
Flicker hold
Flicker hold ' Delayed ring ' Visual ring

Taken from the GEC Catalogue Leaflet 1850/1077/278 (1978)

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Last revised: January 27, 2011