PAX 25/4

TYPE 25/4

This PAX is suitable for organisations requiring between 10 and 25 telephones.

The exchange has a capacity for 25 extension lines and permits a maximum of 4 simultaneous conversations, but it can be partially equipped for 10, 15 or 20 extension lines and 1, 2 or 3 speech circuits.

Conversations are secret.

Priority service can be given to any telephone by the installation of additional equipment.

By fitting additional equipment, various special services can be made available, namely, tie lines, code calling, watchman’s service and fire alarm.

The working principle of this exchange, employing only relays and uniselectors, is the simplest possible and therefore its maintenance is extremely easy.

This PAX works with automatic desk or wall telephones, a description of which is given in the respective catalogue.

This FAX has a maximum capacity for 25 extension lines and permits 4 simultaneous conversations. It may, however, be supplied partially equipped for 10, 15 or 20 lines and with one, two or three speech circuits,

It operates from 50 volts D.C. which is provided by a built-in battery eliminator. The FAX can be supplied without the eliminator if it is to work from a floating battery and charger.

In this case the following power equipment is recommended:-
a) An alkaline battery comprising 40 cells and having a capacity of 15 AH or a lead acid 24 cell 15 AH battery.
b) Charger - Input: 110/200V A.C., 50 c/s single phase. Output: 50V D.C. 1.5A
c) Ringing current generator.


  1. Automatic working and secret conversations.
  2. Up to 4 simultaneous conversations.
  3. Numbering scheme: 21 to 26, 80 to 89, 91 to 99.
  4. Automatic telephone instruments, standard type.
  5. Priority facility for any extension.
  6. Two wire extension lines.
  7. Maximum loop resistance of extension lines is 1000 ohms.
  8. Dialling, ringing and busy tones are provided.
  9. The mains input of 220 volts AC is protected with fuses.
  10. The 50 volt DC and 70 volts AC ringing outputs are protected with fuses.
  11. Special services, such as tie lines, priority, code calling, watchman’s service, fire alarm, etc., can be supplied, on request, by the installation of additional equipment.
  12. Tropically finished.

A fully equipped exchange comprises 25 line relays, 8 relays per speech circuit and 8 relays for common
services. An electronic signal generator is also included.

The exchange is housed in a metal cabinet painted hammered grey.

The relay sets are mounted on hinged shelves to permit access to the rear of the equipment.

The dimensions are 29.5” x 23.6” x 7.6” (75cm x 60cm x 19.5cm).

The weight is approximately 147 pounds (67Kg) when completely equipped.

The battery eliminator is housed at the top of the rack and is suitably separated from the relays.

The battery eliminator is of the full wave type, fitted with a suitable filter, and supplies 50 volts D.C., 1.5A for the relays and 70 volts AC for the ringing current.

The ringing current, on passing through a low capacity condenser and a rectifier, provides ringing tone.

The mains input can be 110 or 220 volts AC ± 8% 50 cycles.

Taken from sales literature 201/1 by Plessey Automatica Electrica Portuguesa (SARL)

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Last revised: October 10, 2019