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 This exchange provided a capacity from 50 to 200 lines and was introduced around 1937. This type of exchange provide the following usual standard facilities:- Coin-box ccts; Automatic metering on local calls; Supervisory signals on junction calls in either direction (controlled by the switch-hooks of the Uax sub); NUT; PBX auxiliary working (on the PBX final-selector units only); Busy tone is transmitted when all free outlets on a group-selector level are busy.

The following modified standard facilities were also provided:- Busy tone of standard pitch, but of 1 second on, and half second off, with no busy flash; Ringing current, generated by a 16 Hz vibrator, having a period of half second on, 1 second off; Ringing tone being supplied from the ringing current, and having the same cadence; No metering on junctions; Forced release applied to PG held group selectors; Busy tone transmitted when all circuits are busy.

The following facilities are omitted:- Dialling tone; Provision for party-lines, omnibus junction working, and earth-return ccts; Interworking to other Uax’s, No ‘Trunk Barred’ indication on calls to the parent exchange; Operator hold; Trunk offering; Uax junctions cannot be terminated on auto equipment at the distance end; No automatic release under CSH conditions.

This type of exchange was assembled out of three types of racks, which were enclosed in the usual sheet steel enclosed cavity racks. The three types of unit were:-

The linefinder unit (A1). Each linefinder unit provides 50 subs line circuits, consisting of three relays per circuit, and the associated meters. There are eight linefinders (of 50 line, uniselector type) associated to a group selector, of pre-2000 type. The linefinders are allotted in sequence by a uniselector allotter. Contacts 1-8 of level two (of the group selectors) are cabled to the eight PBX finals in the PBX final unit. Contacts 1-8 of level three being similar cabled to the eight regular finals in the non-PBX unit. Contact 1-8 of level four are cabled out to the linefinder connection strips, but are not normally used.  The group selector multiple is continuous throughout all the linefinder units, whereas the linefinder multiple is only continuous within each unit. An overflow meter is provided in each linefinder unit, which operates when no free linefinders are available. Busy tone is returned if all outlets on a group selector level are engaged.

The Final selector units are of two types. The ‘unit, final selector A2’ is the PBX final selector unit, which deals with the traffic from the first two-linefinder units, and therefore caters for the first 100 lines of the exchange. A fully equipped unit contained eight 2-10 PBX hunting final selectors, twelve junction relay sets, the ringing and tone sets, and other common equipment. The numbering range of this unit is 200-299, with PBX groups on any level. The PBX groups cannot have 0 as the last digit of the number.

The ‘unit, final selector A3’ is the regular final selector unit, which deals with the traffic from the second 100 lines connected to the exchange. It contains eight regular finals and fourteen junction relay sets. The numbering range of this unit is 300-399, with no PBX working on any line. The multiples of each unit are not extended to any other unit.

As normal the cabling of the various circuits is done to connection strips mounted at the top of the units, with the subs multiple cabled back to the open type MDF.

For junction working, group selector contacts 1-5 on levels 7-9, and all contacts on level 0, are connected to the linefinder unit junction connector tags.  The group selectors are designed in such a way that only one junction route can be accommodated on each level, with the parent exchange on level 0. The junctions can be arranged for bothways working, or incoming only. Each junction is associated with a junction relay set, and is connected to the linefinder banks via a subs line cct (therefore each junction also needs a subs line cct to allow incoming calls, and for the purpose of guarding the junction during the interval which occurs between the replacement of the receiver and the removal of the plug at the manual exchange). The total number of junctions outlets available is 25, but if the exchange has only the PBX final unit the number is restricted to 12 (the amount of relay sets that can be accommodated in one final unit is 12), 14 becoming available on the addition of the regular final unit. (The ‘spare’ relay set being used when more than the normal amount of junctions is required)

Coin box lines use prepayment boxes, and local calls are completed automatically. When a junction call is made a discriminatory tone signal is received on the depression of a key associated with the junction at the manual exchange.

As normal a fault test number was provided, and if there are no faults inverted ringing tone is returned. If NU tone is heard, it indicates that either a fuse has blown, a selector has failed to restore to normal’ or the charging plant has failed. No tones are heard if the ringing equipment has failed.

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