Author Topic: British GPO 312  (Read 268 times)

Offline countryman

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British GPO 312
« 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.
« Last Edit: February 05, 2020, 03:25:04 AM by countryman »

Offline Jack Ryan

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Re: British GPO 312
« Reply #1 on: February 04, 2020, 05:33:26 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

Offline countryman

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Re: British GPO 312
« Reply #2 on: February 05, 2020, 08:03:27 AM »
Thanks Jack, that helped  :)