Electrokinetica The Electro-mechanical Museum

GEC SE50 100-line private automatic exchange

Introduction

Commercial premises were sometimes equipped with internal telephone systems having no connection with the public telephone network. Such systems could be operated manually by a telephonist, or automatically using dial telephones. In the latter case, the exchange was termed a PAX (private automatic exchange), by contrast with a PABX (private automatic branch exchange) where ‘branch’ signifies a connection to the public exchange. This unit is a 100-line PAX, built by GEC in 1961. The components and circuits employed are generally similar to those standardised and widely used by the Post Office for the public telephone network. The switching mechanisms are two-motion selectors of ‘4000 type’, designated ‘SE50’ by GEC.

How it works

Selector mechanisms

Selector mechanism

Selector mechanism

In the Strowger system of automatic telephony, switching equipment is assigned to a call on as ‘as-needed’ basis and remains associated with the call for its entire duration. The switching mechanisms which carry out the routing of the call from one subscriber to another are termed selectors, and in the 100-line PAX a very simple scheme is adopted in which only one selector is required to complete each call. Each selector responds to a two-digit number and selects one destination out of one hundred. The operation of each selector is controlled by a set of relays and associated components built into the selector chassis. The number of selectors required in the system is governed by the number of calls likely to take place at one time. Since the selectors and their associated circuits are complex and expensive, they are usually few in number by comparison to the number of lines. In this particular PAX, there are five selectors for each group of 50 lines, i.e. one tenth of the telephones can simultaneously make outgoing calls. For each telephone making a call, another must be receiving it, therefore one fifth of the connected lines can be in use at once, provided that the originating telephones are evenly distributed between the two groups of 50 lines.

Line circuits and linefinders

In order that a line can requisition a selector and make a call, there must be a circuit associated with the line to detect that the telephone handset has been lifted ready for use, and a circuit associated with the selector which finds that line and connects the selector to it. These are termed the line circuit and the linefinder respectively, and their work is carried out in the fraction of a second between lifting the handset and dialling the first digit of the number. The line circuit must also register within the exchange that the line is busy, to prevent other lines trying to call it unsuccessfully.

Common equipment

In addition to the line circuits, linefinders and selectors, there is a group of miscellaneous equipment which provides general housekeeping functions for the exchange as a whole. Included in this category are the tone, pulse and ringing generator circuits which are switched on when calls are in progress, the allotter which assigns a free switching train to each new call, the alarm circuits which light indicator lamps in case of an equipment fault, and the power supplies. Associated with the alarms are timing circuits are provided to deal with the undesirable condition of a selector or called extension being held out of action on an unsuccessful call or in the event of a line fault. In this case, if the calling line does not hang up within a certain time, the selector is released and a fault condition registered which prevents the offending line from getting hold of another selector until the call is cleared or the fault repaired.

2-motion selectors

2-motion selectors

Wiring side

Wiring side

Initial cleaning

Initial cleaning

Sequence of operation in detail

Acquiring the 100-line PAX from St. Pancras Chambers

Covers off the PAX

Covers off the PAX

The PAX was identified as deserving of conservation during our initial survey on the site. At that time, it appeared to be in near perfect condition apart from a few scrapes to the exterior paintwork. Before the recovery could be carried out some damage occurred when someone apparently tried to remove one of the side covers using a crowbar! The cover was ripped off and destroyed but more alarmingly the end of the crowbar had hit and grazed a selector contact bank. These are extremely difficult to repair or replace as no less than 800 small wires are soldered to the back, none of which have any slack. Fortunately the damaged bank is not required with the PAX configured as found; it was for one of three extra selector positions provided to allow for increased capacity in the future. The recovery was straightforward as the unit is small and self-contained with the exception of the power supply. A mains power unit was recovered with it that did not operate in conjunction with a battery; this is unusual for any exchange but the smallest. The PAX was taken to the workshop, cleaned out, and the unwanted stubs of outgoing cables removed. The selectors were removed to allow the banks to be cleaned, after which all the mechanisms were lubricated.

A few basic electrical tests were made, then the power was connected and the line circuits and linefinders checked. Then the selectors were inserted and tested one by one. A few faults showed up immediately. One selector had a faulty contact on its B relay and did not signal the busy condition properly. One selector cradle made poor contact at first, while another failed to line up with the bank and required mechanical adjustment. Apart from selectors, two line relays were damaged, four alarm lamps were blown, one wiper was bent and the common services relays needed attention, particularly to the dial tone vibrator. All of these items were attended to within a few hours and the PAX has since worked flawlessly.

-----

All content © copyright Electrokinetica 2007-2017 except where otherwise stated • Valid XHTML 1.0Valid CSS