M.F.4. Telephone and Dial Replacement Unit

Catalogue No. T155

With the advent of fast and sophisticated exchange switching systems, the use of local M.F. signalling allows the speed advantage to be passed on to the user.  The major advantage, however, of introducing this method is the reduction of investment level per subscriber.  Firstly, because the digit receiving equipment is held for less time and therefore relatively less hardware is needed; secondly, because the advantages of push button dialling can be provided without the need for the expensive electronic circuitry necessary to generate loop-disconnect signals.

In line with this interest in M.F.4 signalling, all future subscriber products which result from the Pye TMC advanced development programme will provide for this facility. The new SPEEDFONE telephone and dial replacement unit are the first of several Pye TMC products which offer multi-frequency dialling as standard.

The selection of L.S.I. Circuit Technology
The high level of dissatisfaction expressed by a number of international P.T.T.s concerning the reliability of existing discreet component designs, was the major influence in Pye TMC's decision to use L.S.I. circuit signalling in the new range of subscribers equipment.  For example, many competitive products currently available are particularly susceptible to keypad contact problems and unstable frequency levels.

The disadvantages of conventional M.F.4 designs
These designs are generally based on an L-C oscillator requiring two large multi-tapped inductors which are both expensive and difficult to adjust to the correct frequency levels.

Individual printed circuit boards linked by a total of fifteen separate contacts are required for the signalling circuit and keyboard control.

A highly complex and expensive keypad needs to be employed with two separate contacts per button plus two or more common contacts.

The advantages of SPEEDFONE
The use of L.S.I. circuit technology in the SPEEDFONE new generation of subscribers equipment provides a number of design and reliability advantages over the current methods.

M.F. frequencies are generated digitally from an ultra-stable reference oscillator which needs no adjustment.

L.S.I. circuit controlled transistor switching removes the need for unreliable common contacts on the keypad.

All of the signalling and keypad control circuitry is contained on a single printed circuit board and is an integral part of the keypad module.

The Pye TMC Mk. 3 single-contact keypad used in this equipment has a proven reliability record based on the use of over two hundred and fifty thousand press-button telephones installed throughout the world.

Mk. 3 single contact keypad
The Pye TMC Mk. 3 single contact keypad is designed to meet the most rigorous standards of reliability in terms of both mechanical longevity and reduced user mis-operation. The low incidence of mis-dialling and minimal maintenance costs inherent in this design have meant that a growing number of telecommunications administrations now express a preference for subscribers equipment incorporating this keypad.

Great care has been taken in the design of the Mk. 3 keypad to provide a unit which minimises the problems caused by subscriber mis-operation. In addition to paying special attention to the ergonomics of keypad layout, an independent collapse action has been incorporated into each key. This action serves to inhibit accidental operation (a key will not operate unless a force exceeding 200 grammes is applied) and also ensures that proper contact is made by guaranteeing that, once deliberately depressed, a key will reach full travel before pressure can be relaxed.

The Mk. 3 keypad has been designed to operate at a minimum performance level of 100,000 depressions per key which in operational terms gives a life expectancy in excess of 7 years (This assumes an average of 10 key operations per call (evenly distributed) with a traffic of 40 outgoing calls per day, 300 days per year). Incorporated into the design are a number of features which serve to maximise the life and reliability of the unit.

For example, by using twinned gold contacts, plated to a minimum thickness of 5 microns, low contact resistance and immunity to corrosion will be maintained over the life of the equipment.

The SPEEDFONE M.F.4 keypad module is supplied either as part of a complete Type 1746 telephone, or, as a dial replacement unit to upgrade existing installations. Pye TMC offers, in addition, an OEM design service which will allow this unit to be incorporated into other manufacturers' equipment.

The module consists of a Pye TMC 12 button Mk. 3 keypad plus associated electronics contained on a single printed circuit board. The telephone speech circuits are switched off during dialling, but a soft click can be heard as each button is depressed to provide an audible indication of key operation. The tone-sender is powered from the exchange line.


Taken from Publicity Leaflet AA/10.77/3M (dated 1977)

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Last revised: February 04, 2019