TELEPHONE No. 312

Introduced in 1949 this combined, Bakelite cased, table handset telephone, was used by the BPO in Automatic exchange areas on Shared Service Separate Metering lines.

The telephone was supplied complete with a tray for the subscribers personal directory.

The press button and button label were fitted as standard and the phone incorporated a Thermistor (to stop bell tinkle).  The introduction of the Thermistor made the Bellset No. 41A and Telephone No. 310 redundant.

Produced in the colours; Black, Chinese Red, Ivory and Jade Green.

Most of the Telephone No. 312's had been converted in the GPO factories from the older Telephone No. 310 model.

Mark 1 - has a Receiver, Inset No. 1L in the handset and switch bracket with plungers of the early type.
Mark 1A - has a Receiver, Inset No. 1L in the handset and switch bracket with plungers of an improved type.
Mark 2 - has a Receiver, Inset No. 2P in the handset and switch bracket with plungers of the early type.
Mark 2A - has a Receiver, Inset No. 2P in the handset and switch bracket with plungers of an improved design.
Mark 3A - unknown changes

Early models had the back contact of the press button brought out to T7 and were fitted with a Block terminal No. 20/8 and a Cord, Instrument No. 5/14B.

A Rectifier Element No. 1/12A would have been normally fitted, but later supplies had a Valve Electronic CV8790 or CV7040 fitted directly on tags 2 and 3 of the Key No. 302B.

Telephone includes (1956):-
1 x Bell No. 59A, Unmounted.
1 x Bell-gongs, No. 2.
1 x Bell-gongs, No. 2A, nickelled.
1 x Block, Terminal No. 20/4, Colour ....
1 x Capacitor, M.C. No. 97.
1 x Coil, Induction No. 27.
1 x Cord, Instrument No. 3/62B, Colour .... 54".
1 x Cord, Instrument No. 5/28E, 5".
1 x Dial, Automatic No. 12Fa or 12LA.
1 x Key No. 302B.
1 x Label No. 252Q, Black.
1 x Part 1/DBR/220.
1 x Part 2/SCA/20, Colour ...
1 x Rectifier-Element No. 1/12A.
1 x Telephone No. 164 (Mark 2), Colour ....
1 x Thermistor No. 1AN.
1 x Tray, D 62076, Colour ....

The picture above shows the Thermistor No. 1AN mounted on the chassis under the gravity switch plate, the press button switch in the middle and to the right of the switch is the rectifier. 
This is the improved, Mark 2, chassis.

GEC made some of the Mark 3A models - their model ST3551 or TEL/3P/ATS with Circuit Diagram KS10659.

The handset is a Telephone No. 164.

Circuit diagram - N412.

Drawings - 90319 and 63250.

Diagram for Button labels - N620.

Drawing No. 63022 - Telephone components parts.
Drawing No. 63023 - Chassis components.
Drawing No. 63250 - Chassis components (improved switch hook buttons).
Drawing No. 90319 - Telephone parts (Mark 3A).
Drawing No. 90373 - Chassis components (improved switch hook buttons).
Drawing No. SBA12 - 300 type base plate.
Drawing No. SCA17 - 300 type complete case.
Drwaing No. SCA20 - 300 type case.
Drawing No. SCA47 - 300 type complete case (improved switch hook buttons).

Additional good quality pictures of the Telephone No. 312

How to wire your Telephone No. 312 to make it work in the UK

300 Type telephone circuit description

Dismantling the Telephone No. 312

Dismantling the Handset

History, Technical and General Information on 300 type telephones

Collectors information

Adjustment of Magneto bells

Keys for use in 300 type wall and table telephones

Lamp Fittings

Labels that fit in the sliding tray

General fault finding on your phone

How to restore Bakelite

Comparison chart of GPO 300 type telephones
 

Typical 300 type chassis showing dial cord wiring arrangement

P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
TELEPHONES, STATIONS
A 1312
Issue 1, 11/52

TELEPHONES No. 312

Scope of Instruction
This Instruction describes the "Telephone No. 312" which has been introduced for use in automatic areas on shared service (separate metering) lines.

Installation details for this telephone are contained in C3140 and maintenance is dealt with in A5201.

General
The "Telephone No. 312" is a combined set (one-piece), see A 1010, and is similar in appearance to "Telephone No. 330....."; it is fitted with one key-plunger and two external screws for securing a "Label No. 252Q, Black".

It is available as the "Telephone No. 312F" (dial has figures) and the "Telephone No. 312L" (dial has letters and figures) in Black, Chinese Red, Ivory, and Jade Green.

Circuit
The circuit for "Telephone No. 312" is shown in Diagram N 412. The chief points of interest are as follows:-

• The provision of a "Key No. 302B" and a "Rectifier-element No. 1/12A" to extend an earth over one line wire to establish calling conditions and to obtain meter discrimination. The rectifier prevents the earth being extended over the other wire and also maintains the holding loop during the release of the key.
   

• The inclusion of a Thermistor in series with the bell to prevent bell-tinkling.  A Thermistor is a non-linear resistor with a high negative temperature coefficient; this particular Thermistor is enclosed in a small glass tube and has a nominal resistance of 200,000 ohms when no current is flowing.  The resistance decreases as the current rises and is less than 500 ohms when there is a steady current of 14 ma.  The duration's of surge voltages from dialling or other signalling conditions are too short to lower the resistance of the Thermistor sufficiently to permit enough current to pass to tinkle the ball.  During ringing, the voltage is applied long enough to change the temperature of the Thermistor; the current under this condition rises sufficiently rapidly for the bell to ring, with only a slight initial clamping of the ringing.

Components
The telephone includes the following components:-

The "Thermistor No. 1AN" consists of a bead-type glass-tube Thermistor, mounted on a piece of  'SRBP' and is fixed to the chassis (by means of a bracket) under the plate of the cradle-switch.

The "Rectifier-element No. 1/12A" is mounted on a bracket fixed between the plunger-key and the dial-cord terminal-strip.

The current version of the telephone is Mark 3A which is fitted with redesigned plungers, a "Receiver-inset, No. 2P", a "Block, Terminal, No. 20/811 and a "Cord, Instrument, No. 5/146, 54 in". Later versions will be fitted with a "Block, Terminal, No. 20/4" and a "Cord, Instrument, No. 3/62B....... in".

Items to be requisitioned separately
The following items should be requisitioned separately and fitted locally:-

References:- A 1010, A 3202, A 3913, 5201, C 3140

Mark 2 Chassis
Switch hook mechanism has rollers

Chassis - showing switch and rectifier

Chassis - showing components

Last revised: March 10, 2023