Repairing teletype and some crypto equipment was my job in the Air Force from 1966-70. Really enjoyed them but they did require a lot of preventative maintenance to keep them running well. Main challenge was getting through the tech school, if you failed two weekly tests in a row you flunked out and became either a cook or a air policemen. 
They would have us disassemble major sections of the machine and they would put them into a paper bag, shack them up and then have us put it all back together and demonstrate the machine operating OK. Written tests was memorizing all the individual part names in every section, and a spring wasn't named a spring, but rather for example a "code bar latch lever spring'.
Lefty