Fixing electronic audio gear can be difficult without some good testing equipment. It is important to first consider what it is you wish to repair and which equipment is specifically needed to accomplish those tasks. On a basic level, you first want to start with a multi meter especially one that can measure capacitance, transistors and diodes. Then an oscilloscope can measure the shape of your signal. In this setup example I have a Frequency Generator, Audio Oscillator, Audio Power amplifier, Jitter Meter, Resistor Decade Box, Distortion Meter, Frequency Counter, Flutter Meter, Audio Oscilloscope, AC Variac, Variable DC Power Supply & AC Outlet connected to the AC Variac Output. 

Here are some of the manuals of the equipment in this setup.