5. Michael's Strowger Demonstration Unit

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.

Photo of whole unit
Figure 5. The complete unit

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.

Layout
Figure 6. Layout Plan of Michael's Strowger Unit

A brief running explanation is as follows :

  • Either subscriber lifts his handset
  • His SLC marks its level on the linefinder's Veritcal Marking Bank (VMB)
  • The linefinder steps up and stops at the marked level
  • The linefinder hunts rount to find the calling line and then connects it through to the group selector (G/S). If this were a real exchange, there would then be an allotter uniselector to find a free first G/S, but as we only have one first G/S, there is no such choice so the linefinder is hardwired straight to the G/S.
  • The first G/S presents dialtone to the calling subscriber. Dialtone is generated by the Ringing Generator 2a which can be seen in the rear of the photograph.
  • The calling subscriber can now dial the first digit
  • Dialtone is removed and the G/S steps up to the level corresponding to the digit dialled
  • If that level is barred or out of use (detected from the VMB) then NU tone is presented to the caller and the call cannot proceed further. The NU Tone comes from the Ringing Generator 2a.
  • Assuming the level is not barred, then when the G/S has stepped up to the level, it immediately starts stepping round on that level and searches for a free Second G/S on that level.
  • When the first G/S has found a free second G/S, the subscriber can dial the next digit.
  • The same process occurs - the second G/S steps up, then hunts round until it finds a Final Selector.
  • The subscriber can now dial the final two digits, the final selector steps up, then round according to those digits.
  • Once the wipers stop at the selected outlet, the final selector can present NU Tone if the called number is out of use or Busy tone if the line is in use.
  • Assuming that the called number is free, the final selector then applies ringing current to the called line to ring the phone and applies ring signal back down the line so that the caller can hear it.
  • When the called party answers, the final selector sends a metering pulse. This is passed back down the line to the calling subscribers meter but is also detected by the Local Call Timer (LCT) which then starts turning, one pulse every three seconds, and every complete rotation of ten pulses generates another meter pulse. The timing pulses for the LCT also come from the Ringing Generator 2a.

The unit also includes PG, RA and CSH alarms (indicated by bulbs on the top left).

Various Strowger Artifacts
Figure 7. Various Strowger Exchange Artifacts

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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