STC 4119 PAX



THE S.T.& C.  4119 P.A.X.

A Personal Appreciation

By the late Alan G.  Hobbs, G8GOJ

My first introduction to the 4119 P.A.X.  was in 1976, when I joined the Central Electricity Generating Board (CEGB).  At that time, all of the electricity substations had an electro-mechanical exchange, usually equipped for only 25 or perhaps 50 extensions, although some of the larger sites had correspondingly larger exchanges with up to 150 extensions.  Power Stations at that time usually had 200-line exchanges, often of pre-2000 vintage with the rounded covers on the selectors.  The exchange was used to provide general on-site administrative communications and, by means of tie-lines to neighbouring sites, communications over a much larger area.  There were also connections to the separate control telephone system, which was also based on electro-mechanical technology, and was used to pass switching instructions to and from the Grid Control Centres.

The 4119 P.A.X.  operates on 50V DC and is constructed on a substantial angle iron frame, within a wooden cabinet measuring approximately 27 inches wide, by 15 inches deep and 55 inches high.  It can be equipped for a maximum of 25 extensions, and is fitted with four connecting circuits.  The equipment is based on standard Post Office practice, and uses 600 type relays for the line circuits, 3000 type relays for the connecting circuits and common services, and PO number 1 (PO number 2 in later versions) uniselectors for the line finders and connectors.  The connecting circuits, pulse generation and ringing and tone circuits are constructed in 2000 series jack-in groups.  There is provision within the cabinet for one additional jack-in group which could be used for special facilities or a tie-line.  The numbering scheme is two-digit, and is 21-20, 31-30 and 41-45 inclusive.  The extensions are connected consecutively around the banks of the connector uniselector, with the 'home' position being on extension 43.

Ringing and tones are similar to the then PO standard practice.  The ringing current is generated by a vibrating pendulum relay, which vibrates 20 times per second.  This circuit is also used to generate the dial tone and ringing tone, which is a pulsed waveform at 40 pulses per second.  NU tone and busy tone are derived from a thermionic valve oscillator which operates continuously, although early versions used a buzzing relay which was operated as and when these tones were required.  The double beat ringing cadence, and busy tone interruptions, are produced by strapping the appropriate contacts on different banks of the pulse uniselector, which operate the respective switching relays.

An extension wishing to make a call would loop the line and operate the 'L' (line) relay in the line circuit in the usual manner.  This would generate a 'start' signal which is fed to the ringing and tones group, and then to the pulse group, which:-

  1. causes the ringing vibrator to start

  2. causes the pulse circuit to start, with the familiar 'ticker-ticker-ticker-ticker' as the uniselector was stepped at 5 impulses per second, and

  3. causes the line finder uniselector of a randomly selected connecting circuit to search for the calling extension.

The random selection is achieved by using strapping between groups of contacts on another of the banks on the pulse uniselector to distribute the start signal between the four connecting circuits.  The chosen connecting circuit would then hold itself to a common 'lock' lead, which was also generated from the start signal, even though the connecting circuit may by then have been deselected and another connecting circuit selected by the pulse selector.  This system has the advantage that more than one connecting circuit may be searching for the calling extension.  If the originally selected connecting circuit was faulty, a connecting circuit which is subsequently selected should eventually find the calling extension and return dial tone.  Of course, this multiple selection system breaks down when there are already three established calls on the exchange, but routine exchange maintenance should eventually sort out this type of 'no dial tone' problem.

Upon dialling the first digit '2', the connector selector would step two positions and wait patiently for the second digit.  Whereas if the first digit had been '3' or '4', after the initial steps the connector selector would self-drive, during the inter-digit pause, to the start of the 30 group or the 40 group of extensions.  If a first digit other than 2, 3 or 4 is dialled, NU tone will be returned to the caller before the second digit is dialled.  In common with other exchange designs from S.T.  & C., the called side of the connecting circuit transmission bridge could be used to repeat the digits dialled by the caller, which is an ideal situation for tie-line working.  To select this feature, an additional 'OG' relay was fitted in the connector group, which was selected by strapping a point on one of the connector selector banks, which corresponded to the required extension number, to the 'OG' relay.  This ensured that no ringing current would be sent to the called extension, and that the outgoing pair would have an impulsing contact on the 'A' relay inserted into the loop.  Naturally, all four connector selectors would need to have identical strapping, or the results could be quite interesting.

In the patch that I covered, we had a fairly standard numbering scheme at all of the sites.  For instance, the Control room was always 20, and the Telecomms room was always 39.  Because of this, we tried to arrange that tie-lines would be in the 40 group, which was out of the usual extension numbering range.  Apart from any official tie-lines to neighbouring substations, any other tie-lines were the result of the ingenuity of the Telecomms engineers involved, and what pairs on suitable pilot cables could be pressed into service.  My boss, a Telecomms engineer of long standing, and one of his previous colleagues, had been rather keen on providing good communications in 'their' patch, and they had finished up with more tie-lines than would fit into the 41-45 extension group.  However, by examination of the connector circuit diagram, it had been found that the detection circuit which prohibited the use of extensions 46-49 could be defeated by cutting one wire in each connector circuit.  Naturally, there were still only 25 extensions available in the exchange, which meant that extensions 21-24 were no longer available for normal use.  However, this did not upset the standard numbering scheme significantly, and provided many more routes out of this particular exchange.

With this type of network, a good knowledge of the tie-line charts was of paramount importance if calls were to be made to distant sites.  Because if one route was busy, there was usually another way to get to the destination site, even if it did mean dialling lots of digits, and listening for several more dial tones en-route.  Many is the time that I have sat in a Telecomms room, in the certain knowledge that I am the only person on site, to hear the exchange seized and the 'ticker-ticker-ticker-ticker' of the pulse uniselector, followed by two dialled digits and then silence.  Silence which was only broken by incoming digits being repeated to a distant exchange.  Eventually there would be the sound of the call clearing down, and the connector uniselector homing.

The earliest date on the drawings and circuit diagrams for the 4119 is July 1945, with factory modifications still being added to the drawings into the early 1960s.  The exchange that I now own is dated January 1949, although the exchanges of this type that I used to maintain were purchased in the late 1960s and early 1970s during the construction of the 275kV supergrid electricity transmission network.  Sadly, very little of the electromechanical era now remains in active service, although there are isolated exchanges, neglected, and still in situ, because no one is currently prepared to fund the expense of removing (i.e.  scrapping) them.  The vast majority have long since gone into the skip, although a few have managed to find their way into 'new' homes, and some of these are still providing a real service.  However, by the turn of the century, even those that still remain will probably have been scrapped, so that the exchanges now in preservation will be the last of a very long line.



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Last revised: September 28, 2022