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Celebrating 50 Years in Information Technology. 1970 - 2020

Early NZ Exchange Systems

Unfortunately, documenting our experiences on this equipment, severely dates us ! Yes, we even worked on the Christchurch Rotary Exchange in 1966, known as Christchurch-1. Parts of it were plugged out-of-use by then, as the wiring in the rear parts of the frames had become so brittle, that it was not worth trying to rewire.

It was therefore astonishing to find a section of Rotary switching alive and well at the Ferrymead display. (Staff will even start it all up, and enable you to dial a call through it.) Rotary exchanges had a series of shafts driven by a central motor between the frames. When a switch needed motive power to turn into a selector bank, an electromagnet pulled the drive face against a turning shaft, and used friction to move itself. Thus, our biggest maintenance job was to first wipe clean, then paint on a sticky solution to ensure the friction available was up to the job.
All switch contacts were also open to the atmosphere, so double entrance/exit doors and oil-impregnated 'dust-catch' mats were sited by these doors and changed regularly. Inevitably, dust would still get to the contacts, so the other major maintenance task was to vacuum, and polish the open contacts. Dirt buildup on these was the major cause of call-dropouts and scratchy, hard-to-hear connections.

The Strowger-style exchanges, were more modern and still being installed in the 1960's. Instead of turning shafts though, the Step switches each contained magnets, which would pulse in sympathy with the users rotary telephone dials, driving the mechanisms up on a jagged ladder to the correct selector bank height. Most would then use a second magnet to drive the wipers into the bank, stopping at the first idle outlet it would find, where it would then 'grab' another switch to accept the next number in the sequence. Final selectors would step vertically on the 2nd last digit, then step horizontally into the bank on the last digit, to connect directly to the subscriber line desired. It was all very enormous, very noisey, but worked remarkably well. (Today's exchanges are purely specialized switching computers, are a tiny fraction of the size, and practically maintenance free.)

The systems ran on 50 volt, DC power, which required a number of large capacity rectifiers. Diesel generator sets, and open-topped lead-acid batterys also formed a part of the power system. Part of the junior tech's lot in those days, was to stand on a step ladder, with goggles, gloves and a rubber apron on, to dislodge sediment between the plates at the bottom of these cells, with a long perspex prodder. At the large black power panels, one would have to ensure that one was watching the correct amp-meter when winding down the load, as pulling out the wrong open-copper knife switch was capable of producing a flash big enough to vapourise hands, caused by electrons that suddenly had nowhere to go.(OSH would have a field day, if such conditions existed in the workplace now !).

The diesels were WW2-era Caterpillars, and required an uncooled (and extremely loud) two cylinder petrol motor to be fired up first, so that could be used to swing the Diesel over to an acceptable speed, before we threw the compression lever and switched on the fuel. It was also important to watch that the phase lights were all pulsing at the same speed, before the generator was connected to the power feeds to the exchange. Doing that incorrectly, was liable to break crankshafts and possibly destroy the equipment.

Western Electric 7A Rotary (Ferrymead Post & Telegraph Society Display, Christchurch)
Not quite computers, but the nearest thing back in the 1920-1960 era in New Zealand, Western Electric 7A Rotary and Strowger-style Step exchanges were housed in buildings alongside each other in Hereford Street, Christchurch. Looking after these noisey clacking open-frame systems in the mid-1960's was our introduction to all sorts of more advanced and programmable systems, that came along in the following years.
Strowger Step Switching (Ferrymead Post & Telegraph Society Display, Christchurch)
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