I was an electronics technician back in the late 70's. I had worked for a year or so debugging circuit boards that came off of an assembly line (many parts were soldered on by hand) so I had experience with the simple IC's and CPU's of that time. This was rare skill in those days, and I leveraged that to get a job as the technician at a Computerland store that sold Apple II, Commodore Pet, Atari 400, etc. I didn't know a thing about them at first, but nobody else did either.
We didn't have circuit diagrams for most of the computers so there was little hope for repairing them, but all the IC's in the Apple were plugged in to sockets and were removable. I was able to get a diagram that showed which section of the board was responsible for what subsystem - display, keyboard, memory, I/O, etc. This made it possible to set a working machine on a bench next to a broken one and swap IC's one by one until you reached the defective component. I fixed a lot of Apples that way.
They were hugely expensive. A fully loaded Apple II cost about $2,500 in 1980, which would be about $10,000 in today's money. But people bought them! I think we sold one or two a week.