The earliest computers pre-dated the transistor, so instead of "logic boards" we had a standard-sized chassis into which was built some piece of logic that could be accomplished with half-a-dozen (or less) valves/tubes. These small chassis units were called "turrets" for some reason, and were mounted in racks so that they could easily be checked and replaced (thus the large handle on each chassis with which to pull it out if necessary.)

Because of the electrical surge on start-up, the machine could only be started in the morning after the local bakery down the road had switched off its ovens - so we tried not to switch the machine off unless really necessary. But first thing every morning we ran margin tests so that we could identify valves that were getting wobbly and replace them prior to doing production computing.