Likewise, I found a studio tape recorder in a shop once (silver metal, so probably 80's or 90's era), it came with the manual which was mostly just electrical s

Likewise, I found a studio tape recorder in a shop once (silver metal, so probably 80's or 90's era), it came with the manual which was mostly just electrical schematics so that you could both figure out exactly how it works, and figure out how to repair or replace components of it.