Thursday, 11 February 2016

GPO No. 309 Telephone (Siemens)

My wife has a British GPO No. 309 telephone manufactured by Siemens in the UK that was used in South Africa.

The 300 series telephones were manufactured between 1937 and 1959 for the British GPO although production seems to have continued longer for UK associated markets such as South Africa, Australia, etc...

 It still works fine except for the obsolete pulse dialing. The only repairs I needed to make was to heat shrink over where the fabric cable had frayed back from the handset and the wall socket end. I was interested in putting a pulse to tone dialing converter in it but my wife preferred it to remain unchanged and used for receiving calls and most certainly hearing them!

The phone has three distinct parts from bottom to top: A heavy ballasted base plate, a bell-set and the actual phone.

Phone -- Dial and Handset


The pulse dial mechanism is in the top-center in the photo above. After the dial is turned it is returned to its resting position by a spring at a constant rate regulated by a centrifugal governor device. It is the horizontal device near the top of the picture with a circular brass housing on the right-hand side. It has a worm-gear rod that goes to the left and engages with the gear that is visible there.

The leaf spring contacts produce the pulses by disconnecting the direct current of the line briefly between one and ten times. The voltage on the line is negative in relation to ground to help prevent corrosion. 

The leaf springs parallel and nearest to the terminal block at the bottom are for disconnecting the line when the handset is on-hook. There is a vertical metal spike that is just visible above the contacts which drives down between them when the hook is depressed to disconnect them.

I think the large electronic part on the right-hand side is an inductor.



The bell-set is in the rectangular box and is wired to the phone above it by an external cable. This was required as it separates off the phone into a stand-alone unit with the intention that people might like to have it located somewhere else, like a hallway, so that it could be heard throughout the house. It is very loud so unsurprisingly from what I've read it seems most people just wanted it connected together.

The bell-set has two different sized bells struck by a ball-shaped hammer in-between them, driven by an electromagnet. This gives it a beautiful two-tone ring that sounds much better than the vintage-phone ringtones on smartphones ...  more menacing ... like if you don't answer it will rip your face off, but in a beautiful way.

The white block at the bottom of the bell-set photo is a capacitor. It blocks the direct current and allows through the alternating current of the ringing signal sent by the exchange.

Ballast Plate

The very heavy ballast plate beneath the already heavy bell-set seems overkill so I would guess it might have been attached directly to the lighter phone when the bell-set was elsewhere. On the ballast plate there is a wiring and parts diagram:

The handset has an interesting old moving iron speaker. The cover unscrews to reveal a metal disc which can be slid carefully off horizontally. It is held in place by permanent magnets underneath it and driven by a solenoid. 




Microphone Housing


The microphone's cover also unscrews to reveal a microphone unit that is simply resting on spring contacts and so will just drop out. It also contains the screw terminals for the handsets wiring.

If you ever come across an old telephone like this for sale I would strongly suggest buying it. If nothing else it could be converted into a fantastic alarm clock! Perhaps you could set the time using the dial and turn off the alarm by raising and lowering the handset?

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