For understanding and troubleshooting
electrical equipment, there are several bits of paper that could come in handy.
A functional block diagram will give you two important pieces of information: what functions are contained in a circuit, and how they are arranged. Pretty basic. With a functional block diagram in hand and with a basic understanding of electronics, you can often get an idea of where your fault might be located. Some block diagrams also give expected voltage or waveform information, so that you can quickly determine the faulty function using half splitting.
Once you've narrowed down your function... well, it's only inward from there. If your malfunctioning equipment has thousands or resistors or transistors (or dozens of ICs.. or hundreds) it helps to first have a general region before you go trying to signal trace every component.
Then? Break out a good old fashioned schematic diagram and find your fault. Have fun!