Introduced in 1949 this combined, Bakelite cased, table handset
telephone, was used by the BPO in Automatic areas on Shared
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
Labels that fit in the sliding tray
How to wire your Telephone No. 312
to make it work on Plug and Socket
General fault finding on your phone
How to restore Bakelite
Dismantling the Handset
Dismantling your Telephone No. 312
P.O. ENGINEERING DEPT.
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)
Installation details for
this telephone are contained in C 3140 and maintenance is dealt
with in A 5201.
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
key-plunger and two external screws for securing a "Label No.
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
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
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.
The telephone includes the following components:-
||Bell No. 59A (unmounted)
||Bell-gong No. 2 and 1 Bell-gong No. 2A
||Coil, Induction, No. 27
||Capacitor, M.C., No. 97
||Dial, Auto, S.S., No. 10 .... or No. 12
||Key No. 302B (Complete with Plunger, Part
||Rectifier-element No. 1/12A
||Telephone No. 164
||Thermistor No. 1AN
||Tray D 62076 .... (or Tray, Dummy, D
The "Thermistor No. 1AN" consists of a bead-type glass-tube
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
plunger-key and the dial-cord terminal-strip.
The current version of the telephone is Mark 3A which is fitted
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.
Items to be requisitioned separately
The following items should be requisitioned separately and
||Label No. 160 .... (see A 3202 and A 3913)
||Label No. 252Q, Black (see Dgm. 620)
References:- A 1010, A 3202, A 3913, 5201, C 3140
|Chassis - showing
||Chassis - showing
switch and rectifier