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Keyphone 75

Push Button Telephone

The GEC keyphone 75 pushbutton telephone is a direct replacement for rotary-dial telephones. It is suitable for step-by-step (Strowger), crossbar, and reed-relay exchanges, operating from normal loop-disconnect dial pulses.

No additional exchange equipment is needed: replace the rotary-dial telephone by the keyphone 75 and conversion to pushbutton working is complete.

With the growth of subscriber dialling for both national and international calls, codes inevitably contain an increasing number of digits - eighteen digits is the international maximum. In consequence, dialling becomes more laborious and more liable to error. With the keyphone 75, the subscriber can 'key in' the required number at the speed of thought and the likelihood of dialling errors is minimized.

The telephone is a GEC746 instrument in which a keyset and electronic store replaces the rotary dial. It has the same transmission performance as the GEC746 and, apart from the pushbutton circuits, uses the same parts. In addition, four switches can be fitted to give the operational features offered by the GEC740 switching telephone.

The number 'keyed in' via the keyset is held in the telephone within a storage unit which uses metal oxide semiconductor (MOS) integrated circuits. These circuits combine high reliability and low power consumption with small size.

The maximum of 18 digits can be stored. The store is sampled and a train of loop-disconnect pulses is transmitted to the exchange in accordance with the number 'keyed in'.

Electronic impulsing circuits and dry-reed relays permit very close control of impulse speed and ratio with negligible change in performance during life.

The telephone is line powered, from the exchange battery, with an additional small nickel-cadmium cell within the telephone to power the impulsing circuits during sending: this cell is recharged during conversation.

The keyset is a 10-button unit with button layout in accordance with CCITT recommendations. The buttons have a 'fall through' action to ensure positive operation; the contacts are gold plated and are enclosed in the keyset housing to provide protection against dust.

The GEC keyphone 75 pushbutton telephone provides an Administration with a quick and economic method of providing an up-to-the-minute pushbutton system without incurring high capital investment or adding specialized interface equipment between exchange and subscriber.

The GEC keyphone 75 pushbutton telephone is a GEC746 table telephone in which a 10-button keyset and an associated store/convertor unit replaces the rotary dial. It is suitable as a direct replacement for a rotary-dial telephone. The store/convertor accepts and stores up to 18 digits keyed in from the keyset and transmits them with appropriate inter-digit pauses as loop-disconnect (decadic) pulses to the exchange.

Abridged Specification

Catalogue number

Case and handset moulded in high-impact ABS plastic. Seven colours available: ivory, yellow, dual grey, dual green, red, blue, and black (in dual-colour telephones, the handset and cords are the darker tone). A recess is provided on the escutcheon to accept a number card (protected by a plastic cover) and is accessible without removing the telephone case. The case can be completely removed for maintenance.

PVC-covered cadmium-tinsel-conductor cords. Held in position by grommets which relieve the conductors of any stresses.
Line cord - straight: 1830 mm (72 in)
Handset cord - coiled: 250 mm (10 in) unextended; 1700 mm (68 in) extended.

Cradle switch
Sealed microswitch.

Keyset -10-button keyset with black numerals on a white background; the buttons are arranged in the 3 + 3 + 3 + 1 format to CCITT recommendations. Each button has a 'fall through' action (resistance to applied pressure decreases as the button is displaced) to ensure positive operation. Button travel is 4 mm (0.156 in). Button contacts are enclosed in the keyset housing to provide protection against dust (the keyset assembly is show to the right).

Store - accepts 18 digits keyed in as fast as the user wishes; metal-oxide semiconductor (MOS) integrated circuits used.
Convertor -  the information keyed into the store is converted to a series of loop-disconnect pulses for transmission over the normal telephone line.
Impulse relay - sealed-reed unit.
Impulse speed - 10 IPS 3%, can be strapped to 20 IPS.
Impulse ratio - break-to-make ratio 2:1 (66%) 3%; can be strapped to 1 5:1 (60%) or 1 :1 (50%) ; no appreciable variation in the ratio after 10 million impulses (for example, 1 million 'dial 0' operations).
Interdigit pause - 800 ms at 10 IPS, can be strapped to give 400 ms; 400 ms at 20 IPS, can be strapped to give 200 ms.
Power supply - loop-disconnect impulsing relay energized by a built-in rechargeable nickel-cadmium cell. The cell is recharged during conversation and no current is taken from the exchange in the on-hook condition. The battery capacity at half charge is equivalent to 700 consecutive digits without recharging (for example, 100 7-digit numbers).

Receiver inset - rocking armature, replaceable capsule. Sensitivity +46 dB relative to 1 dyne/cm2/root mW available power.
Transmitter inset - carbon granule, replaceable capsule. Sensitivity +30 dB relative to 1 mV/dyne/cm2 at 20 dynes/cm2 input.

An automatic volume regulator is fitted to limit speech-signal amplitudes on lines shorter than 400 ohms; introduces virtually no attenuation on longer lines.

Double-coil, d.c. resistance 1000 ohms; impedance 2000 ohms at 25 Hz, 20k ohms at 1000 Hz. Accepts ringing currents of between 16 and 30 Hz. Bells, double gong, harmonizing tones; 3-position volume control with optional 'bell off' position. Extension bell (BE1100 or BE1200) can be provided for noisy surroundings.

Suitable for use in tropical climates. Protected against entry of insects.

Dimensions and weight
Complete telephone
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).
Handset weight - 230 gm (8 oz).

Take from The GEC-AEI Catalogue Leaflet DLT30A (Dated 1970)

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Last revised: January 27, 2011