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.

Bell-Set

 

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

 

 

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?

Moss under a Microscope

Moss from a path in my garden under a microscope with grains of fine sand. This photo was taken with a USB microscope I bought from Adafruit that I used to inspect circuit boards.

My father has now adopted it as it turns out it is brilliant for looking at postage stamps for the little differences that philatelists need to spot. All his friends at his stamp club have been buying USB microscopes since he showed them. Better to look at a nice large screen than squint through a magnifying glass...

A circuit board using the same microscope at a lower magnification:

DIN to RCA Converter and Switch Box

My father-in-law has a 1968 B&O Beomaster 1400 Stereo receiver which has beautiful sound.

Being from an era when there were no CD players and the myriad of other equipment we now attach to receivers it only has two 5-pin DIN socket inputs, one for phono and the other for a tape deck / aux. For years he swapped a DIN to RCA cable between his CD player and tape deck to cope with the one DIN line-level input.

At about the same time his CRT TV and turntable broke, the new TV adding an extra input (awful internal speakers) and both replacements with RCA connections. He shopped around for a solution but no store had anything helpful.

So I made a box with:

    3 RCA inputs switched to a single DIN output
    A direct RCA to DIN connection for the turntable.

The plastic enclosure is coated on the inside with RF/EMI shielding.

I used a nice selection switch which has a grub screw to hold it in place.

The underside has stick-on rubber feet.

Recycled Door and Mirror

A neighbor replaced a meranti (Genus Shorea) door which had partially decayed and he gave it to me. After leaning against a wall outside for a few years I sawed off the rotten bits and made this mirror from some of it.

The thick base and uprights are from one of the sides of the door and the edges of the mirror from a couple of the slats. The mirror glass was also recycled having previously been stuck directly on a wall.

 

The mirror hinges were surprisingly hard to get. We eventually found them at a huge hardware & timber store, no thanks to any of the store assistants who had never heard of such a thing.

Small Problem

A friend's iPhone cable was working intermittently and he asked me to take a look. The broken wire was hanging on for dear life by a speck of insulation. So tiny.

 

 

It was a bit of a challenge to add the heat-shrink with such little space and not have it shrink while soldering so close. Forceps come in handy as a heat-sink. I'm sure that's why they were invented, or at least that's the excuse I used to buy it. I'm sure two would be better...

I didn't need an excuse to own a shifting spanner and a set-square, but they make nice heat-sinks too in a pinch.

One happy friend.

New Legs

For years a broken chair made from Imbuia sat in my workshop waiting for a use. My wife gave me a mirror whose plastic base had broken to fix and an interesting match was made. 

Spot the Culprit

My wife's sewing machine emitted the magic smoke with a nice bang. Spotting the culprit wasn't exactly hard. Quite impressive, split open underneath too. Fixed it with a salvaged X2 0.1μF part.

Warning: Be aware that X & Y safety rated capacitors operate at mains voltage. These caps are designed to fail open for safety.