Many decades ago, all HP measurement instruments were delivered with wonderful service manuals, which explained in great details the schematics and the mode of operation.

I have learned much more from those manuals (and also from the manuals of many other companies which have disappeared or from which only empty skeletons survive, even in the case when they still have big revenues, e.g. Tektronix, Motorola, National Semiconductor, Fairchild, General Electric, Philips, Siemens, Thomson and many others) than from most university textbooks.