TELEPHONE No. 312


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

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

Most of the Telephone 312's were the older Telephone 310 model that had been converted in the GPO factories.

Mark 2A - has a Receiver, Inset No. 2P in the handset and switch bracket & plungers of an improved design.

The handset is a Telephone No. 164.

Circuit diagram N412.

Click here for the circuit diagram - Web users - CD users

Additional good quality pictures

History, Technical and General Information on 300 type telephones

Click here for Button labels  - Web users - CD users

Keys for use in 300 type Wall and Table Telephones

Lamp Fittings

Labels that fit in the sliding tray

How to wire your Telephone No. 312 to make it work on Plug and Socket

Collectors information

General fault finding on your phone

How to restore Bakelite

Dismantling the Handset

Dismantling your Telephone No. 312


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 C 3140 and maintenance is dealt with in A 5201.

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 "Telephone No. 312F" (dial has figures) and "Telephone No. 312L" (dial has letters and figures) in Black and the standard colours Chinese Red, Ivory, and Jade Green.

Circuit
The circuit for "Telephone No. 312" is shown in Dgm. 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:-

1   Bell No. 59A (unmounted)
1   Bell-gong No. 2 and 1 Bell-gong No. 2A
1   Coil, Induction, No. 27
1   Capacitor, M.C., No. 97
1   Dial, Auto, S.S., No. 10 .... or No. 12
1   Key No. 302B (Complete with Plunger, Part 1/SPL/325)
1   Rectifier-element No. 1/12A
1   Telephone No. 164
1   Thermistor No. 1AN
1   Tray D 62076 .... (or Tray, Dummy, D 62099....)

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:-

1   Label No. 160 .... (see A 3202 and A 3913)
1   Label No. 252Q, Black (see Dgm. 620)

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

Chassis - showing components Chassis - showing switch and rectifier
 
 
BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: October 16, 2012

FM