Unless this JC120 is radically different, the preamp channels are one thing, the power amp channels are another. Usually on chorusing amps, the preamp - either channel - is a mono signal, and is fed to both amps for both speakers. Most chorusers use stereo only while chorus is on. Typically they send the dry signal to one amp and the chorused signal to the other. A chorus pedal mixes a chorus effect with the dry and sends it to the amp, but a chorusing amp like this has two speakers with different signals, so it makes a real swirling sound as the wet and dry signals beat against each other.
So in some cases one amp fails and the mono comes out just the other speaker and sounds more or less normal. But turn on the chorus, and we then either get only the dry still or only the effect side, depending which amp is faulty.
In your case, I am not sure what I need the FX board drawing for yet. Power amps would be good to have.
I would see that both power amps work, or does one have a problem. Do the FX returns go right to the power amps? Are there power amp in jacks? Hmm I see there are not. SO do both returns function or not?
I see the send jack is clearly labelled as CH2. Many amps have FX on one channel and none on the other. But the returns are labelled L and R.