News:

"The phone is a remarkably complex, simple device,
and very rarely ever needs repairs, once you fix them." - Dan/Panther

Main Menu

British GPO 312

Started by countryman, February 04, 2020, 04:13:36 PM

Previous topic - Next topic

countryman

I could use some education what the rectifier and the thermistor are for? I understand this phone was a special design for a type of party line (?) but not how that worked.

Jack Ryan

Quote from: countryman on February 04, 2020, 04:13:36 PM
I could use some education what the rectifier and the thermistor are for? I understand this phone was a special design for a type of party line (?) but not how that worked.

The Tele 312 is for two party shared service with separate metering.

Roughly:

A subscriber lifts the handset and listens for signs that the line is in use. If it is not in use, he presses and releases the "Call Exchange" button which which seizes the line and assigns the correct charging circuits at the exchange. He then dials the number as usual.

If the button were pressed during a call, the loop would be disconnected and eventually the call would be dropped. The diode prevents that from happening.

As this is a shared service the phones are effectively connected in parallel. When one subscriber dials, the other phone would ding. The thermistor acts as a small delay or filter so that single pulses won't ding the bell but "constant" ringing will.

Jack

countryman

Thanks Jack, that helped  :)

FABphones

#3
https://www.britishtelephones.com/t312.htm

'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'.
A collector of  'Monochrome Phones with Sepia Tones'   ...and a Duck!
***********
Vintage Phones - 10% man made, 90% Tribble
*************

tubaman

From @FABphones link:

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.