In order to demonstrate the complete principles of an electromechanical exchange, I built a working rig from original components. The unit is based around UAX13 design; an unattended exchange widely used by the Post Office (now BT) for small rural locations. The unit itself is heavy; approx 120Kg when loaded, but all selectors can be unjacked, so it is portable.
Figure 5 is a photo of the finished unit. It was constructed from Dexion shelving parts which can be bolted together conveniently. Normally, in a working exchange, each shelf would contain several identical components, rather than a mixture as seen here. The unit is floor standing and is approximately 3 foot high.
In the photo, all of the selectors have their yellow covers on. These protect the units from dust. The yellow 'straw' colour was normal in most Post Office exchanges, but previously they were grey; BT changed to straw colour in the 50s/60s to make the exchanges brighter and more airy ! Behind the two telephones you can see the '2A' Ringing Generator which provides all tones, ringing and timing pulses. The square badges on each selector cover would normally be printed up with the routing position of that selector - i.e. where it is 'level-wise' in the exchange to enable faults to be traced effectively.
A brief running explanation is as follows :
The unit also includes PG, RA and CSH alarms (indicated by bulbs on the top left).
Figure 7 shows various items which a typical engineer would come across during routine maintenance of a Strowger exchange. You can see a bank (rear left), a uniselector (rear right), meters, relays, line cords, spare wipers etc. There is also a ball of string. This is special waxed string which can be used for bundling wires together neatly. Spare Wipers for selectors would be essential to replace wipers which have become worn or damaged. Damage to wipers typically occurs when someone jacks in or out a selector without first checking that the wipers are clear of the bank - CRUNCH !.
Many thanks to John Macintyre, Ian Hopley and Phil Goodwin for their essential help whilst building the unit.
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