About the Exchange

Catalogue image showing a selection of electromechanical exchange cabinets of various sizes

Youtube Videos

I've spent months getting the exchange ready for EMF, and I made some videos of the process - they're all available in a convenient Youtube Playlist:

https://www.youtube.com/watch?v=m7ct65JLGvc&list=PLccvWE1DGvhKgj3bmgFIgZNmqXGRsoxO8&index=1

I didn't really go into much detail about the exchange in any of the videos, so read on for more history and detail:

What is a Private Automatic Exchange?

Telephone Exchanges sound cool, but what's a Private Automatic Exchange (PAX)? Why have you got one? How does it work?

Prior to 1981, The British Post Office had the monopoly on telephone equipment connected to the UK telephone network. If you wanted multiple phones around a site, or your business spanned multiple offices, you could rent a phone system from Post Office Telephones and they would charge you an arm and a leg!

If you wanted to allow your accounts department to call your warehouse, but didn't need the warehouse to make external calls - using a Post Office Telephone quickly got expensive...

A market emerged for cheaper 3rd party equipment that wasn't connected to the Post Office Network. The companies that provided telephones and exchanges to the P.O were ideally situated to service that market!

Companies such as GEC, British Ericsson, Automatic Telephone & Electric (ATE) etc would all sell you a Private Automatic Exchange (PAX), or you could rent one from companies such as the imaginatively named "Telephone Rentals"

PAXes didn't have to be certified by the Post Office, and didn't have to stick to the same specification as Post Office equipment, so could be manufactured at lower cost, you could have as many phones as you like, and you didn't have to pay for the calls!

If you had some departments that needed to make external calls, you could provide them with a second phone connected to the public network. It was common for execs to have multiple telephones on their desk.

These PAXes could be quite sophisticated, some had features that weren't required by the Post Office, and they could often be linked together in a private network spanning multiple sites that were many miles apart.

Some of these PAX networks (eg for gas/electricity companies, the railways, or large manufacturing companies) became quite large, covering many sites spread all over the country!

When was it built and where?

The example I'm taking to EMF was built in 1956 by "Automatic Telephone & Electric" (AT&E of Liverpool) to a design that was introduced in 1935, and remained largely unchanged until the end of production in the 1970s.

35 years is a pretty decent run for a single design!

There were several sizes of equipment in this range, for installations of between 10 and 200 telephones (I own examples of the 10, 25 and 50 line versions)

As far as I know, my 50 telephone example was originally installed for the Midlands Electricity Board, and formed part of their private telephone network of generating, distributing, and office buildings spread across the Midlands.

It was recovered from MEB in the mid-late 1980s by a volunteer at the Ffestiniog Railway, a narrow gauge heritage steam railway in North Wales.

Several of these PAXes made their way to Ffestiniog, but most were just put in storage, waiting for the right project.

How did I come to own it?

In 2011 the Ffestiniog Railway had a large clear-out of one of their storage sheds. I was volunteering at the Dean Forest Steam Railway at the time, and we went up to collect a load of spare parts to keep our own heritage exchanges going.

I spotted it in a corner and offered to give it a new home in return for a donation to the railway funds... They accepted, so I borrowed a van (and a helpful colleague!) from work and we drove from Bristol to North Wales to collect it, and I installed it in my living room.

In 2019 I saw sense, and built a shed to move it into... by 2024 I'd forgotten how annoying it was to move, and thought bringing it to EMF would be easy!

How does it work?

In simple terms, the PAX is split into 4 sections:

  • Line Circuits: There are 50 of these (one per telephone) and they handle supplying power to each telephone, detecting when it goes off-hook etc.

  • Line Finder: When a phone goes off-hook, the line finder activates and works out which line is ready to make a call and routes it to an idle Final Selector. There are 7 of these fitted, but only 4 are active.

  • Final Selector: There is the capacity for 7 of these, but only 4 are fitted. They react to the signals coming from the telephone dial, and physically move around a 10x10 matrix of contacts to route the call. When they land on the contacts for the phone you wanted to ring, they apply ringing voltage. You need one of these per-call, so as configured this PAX can only handle 4 calls at once.

  • Ringing, Tone and Alarms: This equipment provides all the voltages and tones you hear on the line, and also handles detecting and reporting of faults.

Everything works with electromagnets!

Think "relays, but taken to the extreme" - as well as operating simple switching contacts, magnets are used to activate ratchet mechanisms to either rotate a set of contacts in one dimension (the Line Finders) or in 2 dimensions (the Final Selectors)

Even the tones are generated by electromagnets that are configured to self-interrupt causing them to mechanically oscillate at the required frequencies.

In total, there are over 1600 spring contacts in the equipment, that all have to be adjusted to be the right amount of springy!

Specification

  • Dimensions: 1.4m wide, 0.4m deep, 1.7m heigh
  • Weight: Somewhere between 112Kg and 140Kg
  • Power Supply: 50V DC at 5A
  • Maximum number of Telephone Lines: 50
  • Currently configured for 40 telephones, and 10 "Tie Lines" to connect it to the outside world
  • Numbering Range: 2-digit numbers from 20-69
  • Technology: Electromechanical:

There are no semiconductors in the exchange itself. Everything is done with electromagnets! It's built around the British Post Office 2000-Type selector, which was developed by AT&E

Numbering

The Strowger PAX has lines numbered in a contiguous block from 21 to 60. Not all of these lines are currently working, but I don't need them all for EMF.

I've been through them all and tested them, and made a note of which ones I should avoid using!

1 2 3 4 5 6 7 8 9 0
2x ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ❌ ❌
3x IC IC IC IC ❌ ❌ ✔️ OG OG OG
4x ✔️ ✔️ ✔️ ✔️ ✔️ ❌ ✔️ ✔️ ✔️ ✔️
5x ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ❌ ❌ OG
6x ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ✔️ ✔️
  • "OG" - Outgoing tie line (trunk)
  • "IC" - Incoming tie line (trunk)
  • "✔️" - Tested: OK
  • "❌" - Tested: Faulty (see exchange diary for fault details)

We will only use the Level 3 tie lines for EMF, the 50 tie line is "special" and the related equipment won't be available.

Further Reading

Of course I can only scratch the surface here, but I have collected quite a bit of information about this equipment:

There are other websites that mention this equipment (eg the excellent britishtelephones.com - but the information there is mostly ORC copies of some of the above primary documents.

Optional Extras:

You could buy this equipment with many additional features - most of which are described in the "operating bulletin" linked above. I've got diagrams for some of them: