TELEPHONE No. 740

Circuit diagram - N840.

Diagram for the Additional Buttons - N849.

Diagram for the Auxiliary Switches - N848.

Drawings - 93022, 93022/0, 90322/1/1, 93022/1/2 and 93022/1/3.

Specifications - S1404 and S1405.

Telephone No. 740 superseded the Telephone No. 710.

There was a Mark 1 and Mark 1A.  The Mark 1 used a case with part number 1/DCO/733 and the Mark 1A used a case part number 2/DCO/733.  The 2/DCO/733 case was the later case with a location slot for the handset.

ENGINEERING INSTRUCTIONS
TELEPHONES
STATIONS
A 1076
Issue 2, Feb 1969

TELEPHONE No's 740 AND 746
Description

General
This Instruction describes Telephones No. 740 and 746, which are counterparts to the Telephones No. 710 and 706 and have been developed from the printed wiring version of the Telephone 706. The principal new features are as follows:-

(a) A new design of telephone cover, which now incorporates a carrying- handle.

(b) Use of microswitches for the gravity spring-sets.

(c) Smaller electrical components, with the regulator permanently connected.

(d) Optional provision of one or two press-buttons on the Telephone No. 746, or four on the No. 740.

(e) A common chassis for both types of telephone.

(f) A plain outer dial ring matching the telephone cover and used in conjunction with a dial having numbers only on an antique silver background. (During the transition to all figure numbering some telephones may be issued with an outer dial ring, having numbers and letters, which matches the handset.)

Many parts of the Telephones No. 740 and 746 are identical with those of the Telephones No. 710 and 706 which are described in A1063 and A1060 respectively. This Instruction describes only those parts particular to the Telephones No. 740 and 746.

The telephones, which as issued are for use in automatic areas only, are suitable for use on lines of up to 1000 ohms T.E.R.

The telephones can be modified for a variety of uses by the fitting of add-on units,' adapters etc. and these will be described in later instructions.

The Telephone No. 746 is available in black, blue, two-tone green, two-tone grey, ivory, red and yellow. The Telephone No. 740 is available in black, two-tone grey and ivory only.

Cover
On both types of telephone the cover is secured to the telephone assembly by a lug on the front skirt which engages in a recess in the leading edge of the base and by a single fixing screw located above the cord entry at the rear of the case. The screw, which is held captive by a bush and a spring, engages in a tapped hole in the cord clamp, a T-shaped metal bracket riveted to the rear of the base.

The telephones, as issued, have the aperture for the press-button(s) closed with a dummy button held in place by a clip. When a press-button unit is fitted the dummy button and clip should be secured within the telephone for subsequent refitting.

The outer dial ring fits round the hole in the sloping face of the cover through which the dial protrudes. The dial ring is secured by dropping the lugs on the ring into the recesses provided on the edge of the hole in the case and turning clockwise until the two recesses provided for the finger-stop line up. To remove or refit the ring the cover must be taken off the telephone assembly.

An 'off-rest' position is provided with the handset resting across the instrument just above the dial.

A carrying-handle, in the form of a recessed finger ledge, is provided between the horns directly below the normal handset position.

Base
Telephones No. 740 and 746 are fitted with a base made of mild-steel with a crackle black-enamel finish. There is a 'knock-out' and three holes in the base for use with a Plan-set N 625 and a cord-entry hole at the rear of the base for a watch receiver, as on the Telephone No. 706. A ring of slots is punched out beneath each of the bell gongs. Two studs, fixed to the base, support the printed wiring board which is secured by a single screw inserted into a tapped hole raised in the base. The bell gongs are supported by two removable moulded pillars; two long screws pass through the metal plate supporting the dial, the gongs and the moulded pillars into tapped holes raised in the base.

The plunger-supporting brackets are riveted to the base.

Printed wiring board
The main features of the printed wiring board are:-

(a) its reduced size.

(b) the regulator is permanently wired in.

(c) the gravity microswitch is permanently wired in.

The printed wiring board is located by mating slots in the front edge of the board and in the sides of the plunger-mounting bracket.

The regulator is an integral part of the telephone circuit and provision is not made to render it inoperative.

The gravity spring-set consists of a microswitch positioned alongside the left-hand support of the plunger-mounting bracket. The switch is operated by a lever pivoted on a bracket at the rear edge of the switch cover, the upper end of the lever being formed into a channel in which a projection on the plunger assembly rides. A coiled spring holds the lever in position. The switch is operated when the handset is on the rest.

When the case is removed the switch can be locked in the off position.  The switch hook is manually lowered and the swinging latch arm, which is fixed towards the top of the switch hook mounting, is lifted so that it locates against the front of the arm.  Once pressure on the switch hook is removed the latch will be held in place by friction.  Pressing the switch Hooks slightly will release the latch.

Dial and dial cord
The telephones are fitted with a Dial, Automatic, No. 21FA (clear). This has a number ring with black figures on an antique silver background and a transparent finger-plate. The dial cord has spade-tags at one end for connection to the dial terminals and ring-tags at the other for connection to the telephone terminals.

Terminal block and line cord
The Telephone 746 is supplied with a four-wire line cord and a four-way terminal block, the Telephone No. 740 is supplied without line cord or terminal block.

Bell
A single-coil bell is under development for the Telephones No. 740 and 746, but early issues will be fitted with a Bell No. 59C-1. The bell is screwed to the base of the telephone.

Telephone circuit
The circuit for Telephones No. 740 and 746 shows one change from that of the Telephone No. 706. The brown wire from dial terminal D3 is taken to telephone terminal T3 instead of gravity spring 5. This short-circuits the transmitter when the dial is taken off-normal and prevents surges in the spark quench circuit passing through the transmitter.

Press-buttons and switches
The press-buttons for use with Telephones No. 740 and 746 are transparent to obviate the stocking of a range of colours for each type of button and legend. The legends available for each type of press-button are shown in Diagram N849.

Press-buttons for the Telephone No. 740 are provided by removing the dummy button and fitting a Part 1/ ... 10/DBU/372 using the pin, Part 1/DPI/203, provided in the telephone.

A single press-button for the Telephone No. 746 is fitted by removing the dummy button and clip and inserting a Part 1/ or 2/DBU/362 which is retained by two pins, Parts 1/DPI/203. When two press-buttons are required, two Parts 1/ ... 6/DBU/363 are fitted, each with one Part 1/DPI/203.

Where a single change-over contact (non-locking) is required in association with a press-button in either telephone a Switch No. 5A-4 together with an operating plunger, Part 1/DPL/1022, is fitted. Other non-locking contact assemblies can be provided on either telephone by fitting the spring-sets described in Diagram N 848 together with a Part 1/DPL/1022.

There a locking single change-over contact is required in association with a, press-button on either telephone a Switch No. 5A-9 is fitted (this switch includes an operating plunger). Other locking contact assemblies can be provided on the Telephone No. 740 by means of the latch-plate as described in A1063 and Diagram N848.

Additional gravity switches can be fitted to either telephone when required as follows:-

Replacement parts
Table 1 lists the replacement parts which are being made available:-

TABLE 1

Part to be requisitioned as:-

References:- A 1060 & A 1063

How to remove a Telephone No. 746 or 740 case

1. Locate the case fixing screw on the rear of the case above the cords.
2. Slacken the screw until it becomes loose - it remains fixed to the case, held by a spring and is not completely removed.
3. Lift the rear of the case upwards and towards the front of the until it is clear of of the dial.
4. Pull front edge of case forward to release from lip on front of base plate.

Re-fitting is the process in reverse, except you may have to slide the casing dial ring to locate it over the dial.

If the casing screw does fall out - fit back as follows:-

1. Place spring on screw.
2. Insert screw and spring into hole in case.
3. Offer up nut with threaded end case side.
4. Screw the screw into the nut, until the threaded section is free.

Additional information

Last revised: August 12, 2024