Control equipment was based around the 'Line Replaceable Unit' (LRU) ethos and consisted of 'trays' of circuit boards and equipment arranged in two cabinets. No.1 housed just the usual circuit breakers, relays, reversers and so forth. No.2 also housed standard equipment but included the complex electronic control circuits that ultimately were the un-doing of the class. Each tray could be slid in and out of the cabinet (with the locomotive switched off) with ease and each had a test connector. Specific testers were made that simply compared the electrical signals and stimuli on the test socket with the design values. If any were not as expected, the whole tray was replaced and the locomotive was good-to-go... at least that was the theory. The use of testers plugged into the test socket removed the need for engineering staff to be competent electronic engineers (as opposed to electrical engineers which they undoubtedly were), but often faults on one tray would only show themselves in combination with faults on other trays. This led to confusing diagnostics and locomotives were often failed for long periods pending in-depth exploration of a fault. Faulty trays were sent for repair. This early attempt was bold and fundamentally sound. Electronic circuit arrangements in modern locomotives use this approach almost unchanged. Unfortunately, powerful control and computational electronics were in their infancy in 1968. Had the resources of just ten years later been available, it is arguable that Class 74 would have been a success.