Well, how part of a step by step tele exch worked. The noise from hundreds of these working simultaneously was deafening.

I found it very boring doing maintenance on them. As a trainee tech with the then Government owned Telecoms dept, when we were given the choice, my two were first cable transmission testing on new cables and complex fault finding on existing cables, and installation and commissioning of telecoms radio links, mobile phone bases and some two-way work also. I got my first choice and later that group merged with the radio group. I did about 5 years in the first and twelve in the second before resigning in the late 80s.