TELE No. 312

Combined table handset telephone, for use in Automatic areas on Shared Service Separate Metering lines.

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

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

Scope of Instruction

This Instruction describes the "Telephone No. 312" which has been introduced for use in auto. 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" and "Telephone No. 312L" 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:-

(a) 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.

(b) 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.

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)


Chassis - showing components	Chassis - showing switch and rectifier

TELEPHONE No. 312

Last revised: May 07, 2008