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The whole MDF with GPO tester (left) used for testing the extension line wiring.
The strips at the top are where the PABX 3 extension and PABX 3 exchange line wiring terminates.
The strips at the bottom are where the site distribution wiring terminates.
GPO/BT incoming exchange line terminations are on the far right. |
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Site distribution wiring termination - fuse side.
The red and grey fuses are dummies. The glass fuses are real
fuses.
Red fuses are for special circuits.
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Wiring side of the same block. Here you see the jumper
wire (blue and yellow), which is routed to the PABX 3 extension
terminating strips - pictured below. |
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Close up of the PABX 3 extension termination blocks with the exchange line termination strips nearest (not the grey
boxes to the right!). These termination strips are the old type with lightning
protectors fitted. |
Opposite the MDF are the Extension Line Units. These cater for up to 100
extensions per rack.
The equipment from the floor upwards are:-
Two rows of 1st Group Selectors. These deal with the
first digit dialled by an extension and are connected to a free 2nd
selector. A digit 9 will connect to a free exchange line relayset
and 0 to the manual switchboard.
Two rows of Linefinders (50 extensions per row) - consisting of line relays (under the covers) and
Uniselectors (the picture below
shows a Uniselector with the mechanism removed, exposing the contact bank).
When an extension goes off hook a linefinder selects a free 1st group selector
and then dial tone is returned.
Two top rows, of which the lower is used in this instance, of Final Selectors.
These deal with the last two digits of an extension number and then ring
the phone. The two Selectors on the
right hand side of the very top row are Enquiry
finders, used when transferring calls.
The picture below shows a Uniselector with the mechanism
removed.
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This is a typical installation drawing of the line units, showing the installers how many selectors are to be fitted.
The emboldened lines show population. You will notice that not all the shelves are fitted with selectors. A traffic study would have taken place before
installation and this would have given an indication on how much equipment was needed.
Additional selectors could be fitted later. LF = Line finder
G/S = Group Selector
F/S = Final Selector |
Walk around the Line Units and there is the Group Selector rack. This houses all the 2nd
Selectors and these deal with the second digit dialled and connect the
call to a free final selector.
These are shared around the Line Units.
The Final selector tests the line dialled and will either:-
- Ring the phone, giving ringing tone to the caller
- Give busy tone if the line is in use
- Give Number Unobtainable tone if the line is not used.
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Behind the Group Selector rack you find the Relay Set Racks (RSR). In general these are equipped with the ringing
machines, Exchange Line relay sets and Enquiry Circuits. This is a
real life example of an RSR rack with the positions and uses of the
relay sets clearly marked. |
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If ringers are fitted to the RSR then they are installed at the bottom of the rack. In this instance we have two
ringers, one is in reserve in case the other fails. The relaysets
above the ringers are associated with the ringers.
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The Exchange Line Relaysets are fitted at the top of the RSR rack. One
line per Relayset.
One large sites the Exchange Line relay sets will have their own
rack.
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Close up of a ringing machine, actually called a Dynamotor. They
are quite weighty. |
And finally - behind the RSR are the batteries and rectifier unit. The batteries are
provided to smooth out
the power supply and ensure that there is no noise or hum. Their other secondary function was to provide power in the
event of a mains failure.
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The switchboards are, of course, in another room, as the noise from the PABX equipment would have obliterated any
conversation.
Later switchboards were covered in grey Formica and were of a similar appearance to the
PMBX 4. |
Thanks to Malcolm Saffin for the use of the PABX 3 pictures. |
At this point the GPO engineer asks you to leave as
the operators have just made him a cup of rosy lea. Tour finishes |