There are 2 ways of fault finding:
1. The engineering aproach that means u have the circuit diagram infront of you and you understand how it works then use your electronic skills tracing the schematic diagram with a scope while the UUT(unit under test) is on ( I call it active testing).
This approach is the one I prefer and I can fix anything very quickly.
2. The technician approach, where you dont know much about how the circuit works and or you dont have a service manual or circuit diagram, with power off ( passive testing) you test all the components (resistors, capacitors transistors etc)and if you are lucky you would find one or more components faulty. If not it is probably a dry joint or a broken pcb track. You need to understand all electronic components, how they work and how to test them.
If you have a good device exactly like the broken one then you can compare the two using the component tester.
I used to do repair jobs ages ago but I had enough of the hard work so if I get a repair job I contract someone else to do it and earn easy money.
Whatever you do you should concentrate on safety, all the money is not worth it if you get hurt.
Regards
Mahmood