the answer might be one or the other, or both!
that is why you cant replace pieces that look burnt or exploded.
there might be another weak piece somewhere down (or up) the road.
the only way to test it is like this:
1. you take each piece off of the board and test it.
2. you get the specifications and learn what the impedance of each circuit is supposed to be, then just use a multimeter to test the impedance.
if the impedance is way off, then something in the circuit is bad.
real experience would tell the person 'if the impedance is this much different.. then of course these pieces are bad'
but, as circuits get more complex.. it could be a combination of things that amount to the same impedance change.
if there is a more advanced way of doing it, i really dont know it.
and i would expect to learn something like that in college.
i know sometimes you can use an oscilloscope to compare the waveform.. and the waveform difference could easily tell which pieces are bad, based on the change.
usually the experienced electrical person can say 'it is one of these few combinations' and then remove those pieces to test them.
the good ones go back in and the bad ones get replaced.
but that means you have to know how to shop for quality replacement pieces.