Depends on what is broken, and what your electronics skills are. If it's something simple like a bad solder connection, you might get lucky and find it. If it's burned out components, you really need to know what you're doing to isolate the problem. Sometimes manufacturers will sell you a schematic diagram of their amps if you inquire.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's