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3-28-1974 Repairing an NCR Class 400 electronic accounting machine at Knox Service Center a Goodyear dealer in Radcliff Kentucky.
Most of this machine was built with discrete components. The components were soldered to wire wrap modules with interconnecting wires wrapped to the module pins. en.wikipedia.org/wiki/Wire_wrap The faulty transistor, diode, capacitor etc. had to be found and replaced. Because of this one image, I can remember more about this incident than I would have otherwise. The problem was a thermal intermittent, there is a can of circuit freeze next to the scope. The problem was on one of those few circuit boards, the solid state Z line.
The disk memory can be seen in the cabinet directly below the oscilloscope. The disk was about 3/8" thick with a magnetic coating on the top. Fixed read/write heads had a clearance of .0005" . The capacity was 200, 13 digit words.
The back of the electronics unit had 4 larger panels of wire wrap modules, populated with discrete components. This photo of a Class 395 presents a view of that electronics from the back.
NCR Class 400 Operating Instructions:
NCR Class 400 school for service personnel: