Martin's Personal Telephone Exchange Museum

MNDX 341Page created March 2000 , Updates March 2010.

A quick glimpse at some of the telephone switching equipment I have preserved

I have tried to gather some pictures to give a flavour of the equipment I have saved, but it is not always possible to get ideal shots of equipment which has exceeded the space available for it! I hope this quick tour will nevertheless convey some idea of the work involved and, for those who have never seen inside an exchange, reveal the nature of an electromechanical telephone exchange.

The rural exchange depicted here, is known as a type UAX13. This was my first real big rescue operation back in 1991. These exchanges were designed to be built in Unit contstruction, hence the term Unit Automatic Exchange. Originally designed before the second world war, they served the rural communities of the UK until 1996, when the last ones were closed at Elvanfoot and Crawfordjohn in Scotland. This picture shows my first small hut in which I had preserved only 3 racks, enough to provide pre-STD service to 100 subscribers.

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U13 racks apparatus room

Here is the new apparatus room layout following a major reorganisation in 2010. At last I have all 9 racks of the U13 in one place as they should be. In addition to the first 2 A racks and the C rack on the left, there are 2 F racks, 2 B racks and G and H racks.  F and G racks were added later when Subscriber Trunk Dialling was introduced after 1959. The G rack contains the Coin & Fee Checking relay sets used with Pay On Answer coin collecting boxes.  On the right is a view looking the other way around. In the foreground on the right are the 3 UAX12 racks rescued from Glenprosen in 2009, still undergoing restoration.On the left can be seen the PABX1 and a PABX4 ringer rack. In the far distance are the U13 B, F and G racks. The U13, known as "Lashford", can be accessed from CNET on the STD code 086741.

B Rack

Here are more U13 racks, including a B rack containing some relatively modern apparatus. At the top are two EPG2A electronic pulse generators for local call timing. In the centre is a CAMU electronic device for inserting 5 meter pulses for calls to Directory Enquiries. This was added when charging for such calls was introduced for the first time, and was an example of how electronics became 'bolted on' to Strowger, in order to cope with modern developments which are otherwise trivial to add to digital switches. There are also various other relay sets provided to handle automatic calls from testing routiners. In between the second B rack and the U13 F racks is an audio rack type 62.


PABX3 racks from Crawley Hospital, recovered 1994. The near rack is a Line & Final Rack, and the far rack is a Group selector rack. There is equipment here to form a 100 line PABX. There were 4 L&F racks, two Group racks and 2 RSR racks in the Crawley exchange. Unfortunately my colleague, who also saved some racks, needed the Ringer rack. However, I have enough ringer shelf equipment to construct a ringer shelf on a spare Group rack shelf. There is also one cord board switch section from the 3 originally at Crawley. This exchange has now been passed on to another enthusiast as I no longer have space available.


These are two PABX1 systems. One is 7+35 and the other is 10+49. I feel I don't need two so I parted with the smaller version some years ago. I kept the one that was recovered from a demolition site and needed rack straightening before I could do any other repairs. Before being crushed, it was as new and refurbished with hardly any use. There it was in the rubble, complete with diagrams. The other was recovered in nice condition from Banbury, where a notice next to it declared it to be the last PABX1 ever fitted in the Banbury area.


On the left is a 20+5 system known as PABX5. And beyond that a 50 volt power plant.

On the right is the replacement for the PABX5, the PABX6. Unfortunately this system was scrapped as no one would give it a home, and it had been quite badly vandalised in the derelict building before I saved it.

PABX4 Ringer Rack Power Plant 227

Some more odd racks on the left. There is a routiner docket printer and a ringer rack. On the right is the power plant which charges the 50 volt cells which power all the equipment. Just behind can be seen the 62 type transmission rack equipment.

MNDX 341

This is my mobile Non-Director exchange. Known as MNDX 341, it was made in 1971 and was a batch of 200 ordered by the GPO for use as standby exchanges for emergency use. It is a complete 400 line exchange built to the same design and standards of a main exchange, except that it uses shorter racks and is mounted into a four wheel trailer. Weighing 7 tons, it was only intended to be wheeled short distances into position on site, having been transported on the road by low loader. MNDX 341 was one of about 20 held in the South Eastern region. They were more often used as temporary means to expand exchanges where capacity was running out, as was the case at "West Forest" Mobile exchange at Wokingham. Use in real emergency situations was rare. Some of the interior fittings had unfortunately been removed before I was able to rescue the MNDX, but I have endeavoured to restore it to near original state, as some of the pictures below will show. Access is possible from CNET on STD code 086742.

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RSR1 etc Racks LineFinder Rack etc MDF Maintenance area Power Plant

Clock 46 Clock 36

Larger exchanges used master clocks to produce timing pulses. Depicted here are, on the left, the Clock 36, which produced pulses at 1, 6 and 30 second intervals. Note that no face was ever normally installed in the case of these clocks. I have fitted one here as this particular clock is in my lounge! It also drives all the other pulse clocks around my house and museum. On the right is the smaller Clock 46, which produced fewer pulses for use with manual switchboards.

Tariff Clocks

This is a selection of "clocks" driven from the master clock, which produced further pulses at longer intervals, and also some exchange alarm bells. The box at bottom right is an old howler device, used for making a loud noise on a subscribers line, should he be so careless as to leave his receiver off the hook!

Tester 219A Test Desk Associated with all exchanges are testers built especially for testing a specific function. On the left is a Tester 219A, a trolley mounted device for plugging in to the exchange racks containing the Coin & Fee Checking relay sets used to operate pay on answer coin collecting boxes. This tester was in fact a full routiner, which would automatically step on to each relay set in turn and sequentially test each function for correct working. Faults were printed out on the exchange fault recorder. The tester is fully Strowger in construction, using only standard relays and uniselectors. On the right is a Test Desk. Each main exchange would have one or more of these test positions so that engineers could test lines to diagnose faults which had been reported either by subscribers or by the automatic routine testing.

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K6 Kiosk

And as we come to the end of the first part of our tour, here is my Kiosk, in the snow. This is the famous K6 which used to stand in Jerico in Oxford. The building behind is my UAX13 building. Although it's no A or B type building, the position is quite typical of many kiosks outside small rural exchanges. I have started the long process of restoration. After further removal of loose paint I am ready to start replacing all the glass panels, most of which I have now removed and cleaned out ready for replacement. Many of the existing panels were of the modern acrylic type which went opaque in a very short time. Hopefully I might be able to finish this project, given some decent summer weather!

Postscript. The restoration was finished in 2000. I would add a recent photo but I have to say that after another ten years of the British weather it needs repainting yet again! It does now house a payphone 700 working via the coin and fee relay sets in the MNDX.

Here we pause in the tour. If you would like to see some more interesting telephone equipment, including some of the small PAX equipment in the collection, please click here

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This page is maintained by Martin