Second generation, table, battery keyphone with provision for four press buttons.

Superseded by the Telephone No. 758.

Introduced in around 1977.

Circuit diagram - N868.

Specification - S975.

3 Internal
Issue 1, June 78


This instruction describes and gives information on 10 pps self-contained keyphone based on the Telephone No. 740.

Telephone 768 differs from Telephone 740 in that a push-button unit (PBU) and escutcheon plate replace the dial and outer number ring. A recess is provided below the keypad to take the customer's number label.

The PBU occupies the same mounting as the dial it replaces. The instrument accepts the standard range of add-on parts with limitations.

The electronic circuits in Telephone 768 are powered by a re-chargeable nickel-cadmium battery.  Battery Secondary No. 22 (red) is used for Telephone 768.

With the addition of auxiliary apparatus as required Telephone 768 is suitable for use on PBX extensions (with or without recall) and those extension plan installations which could otherwise be met by the provision of Telephone 740 except for the following:-

  • Extension Plan 2A.
  • In conjunction with Lamps Signalling Units Nos. 1 and 2.
  • In conjunction with Head Set No. 1 associated with PMBX 2/2A and 2/2B switch boards (N1102).

Charging over exchange lines or 50v PABX extension lines may be employed for one telephone on Extension Plans such as a Plan 2 where the instrument is terminated direct to the exchange line.

50v PABX - Same applies as above excluding PABX No. 1 extensions arranged for direct extension night service.

Charging From Power Unit
A Power Unit will have to be provided for this purpose except for those that have existing power feeding then, all other applications will require the provision of a power unit. In addition, a power unit must be provided in lieu of the line charging when switching facilities are provided to divert the line from the telephone to other equipment. Power Unit 53A should be used and, if required up to 80 keyphone batteries may be charged from one power unit. However it is recommended that no more than 10 batteries be charged from a single fused outlet. An instrument cord with extra conductors and a larger block terminal than that shown in the appropriate N diagrams may be required.

In some cases it could be advantageous to utilise the existing power unit on Key and Lamp units and on PMBX 2A switchboards.

Connection of SC Keyphones to PABX Types ARD 561/7 and 562/7
Connection of more than one SC keyphone within each 'Tens' group of the ARD PABX causes a loss of power that can result in a permanent calling condition to the SLMT marker.
Separate power supplies must be provided for the SC telephones.

Telephones 768 will accept the same auxiliary units as Telephone 740 (See Diagram N 868) with the following exceptions:-

  • A maximum of 2 auxiliary gravity switches may be provided.
  • A maximum of 12 additional terminals are provided.
  • Lamp Signalling Units Nos. 1 and 2 cannot be provided.

To make a call, lift the handset and, on the receipt of dial tone key in the number, one digit at a time. It is not necessary to pause between key operations while a digit is pulsed out, unless an access digit is keyed, in which case secondary dial tone must be obtained before proceeding.

For information see Diagram N 868.

When Power Unit 53A is required it should be installed in accordance with C3 Q0020.

Ensure that the handset is replaced on the rest to prevent excessive battery drain.

The pulsing of Telephone 768 is identical to that of a normal dial telephone and can be tested for speed and ratio in the normal manner. Tests should be carried out when commissioning keyphone to ensure the correct pulsing out of all digit buttons.

Recovered instruments should be returned to Section Stock. Batteries Secondary No. 22 should be removed from the instruments and handed to stores for recharging in dispenser No. 4A.

To change between the different methods of battery charging, straps within the telephone must be moved.  The telephone is supplied with the Battery strapping set to Line charging.  The straps are set T26 - T27.

For 50v local power feed - run the power to the telephone, remove strap from T26 - T27 and strap across T27 - T28.
For 10v local power feed - run the power to the telephone, remove strap from T26 - T27 and strap across T28 - T29.

Line charging

To check the charging circuit connect a milliameter in place of the strap T26 (-ve) and T27 (+ve).  With the handset on the rest a current of 2 - 3mA should flow after 30-100 seconds.  Replace strap after testing.

Charging by local PSU
To check the charging current connect a milliameter in place of the strap T27 (+ve) and T28 (-ve) for 10v PSU's or T29 (+ve) and T28 (-ve) for 50 v PSU's.  With the handset on the rest the current flow should be 2 - 3mA.  Replace strap after testing.

Additional information

ModelMarkGreyIvoryIntroduced Remarks
Tele 768Mk 1yy5/77  


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Last revised: February 25, 2021