Seandtaylor99,
perhaps you did not read Rapogee's original post correctly. I cut & pasted here: "Older Rotel amp, there some greasy stuff and cat hair all over the internal board, very dirty and grimy....."
greasy stuff will not be blasted off by compressed air. In fact, if the greasy stuff is at all mobile, compressed air will splatter it farther & wider creating an even bigger issue in other parts of the amp (that perhaps do not have issues so far)!
OTOH, Seandtaylor99, does bring up a good issue that the 90% alcohol does have 10% water in it. I can tell you that I have cleaned PCBs with this 90% alcohol before BUT..........I did use a hair-dryer to dry the PCB + I let the PCB air dry for atleast 24 hrs before I switched on the circuit.
Good catch Seandtaylor99! Otherwise, I would not have mentioned my hair-dryer routine! ;-)
Rapogee: the Chemtronix Flux-Off will not harm the PCB. I've used that chemical routinely to remove sticky/guey flux paste & fluid from PCBs. No harm has ever been done to the PCB.
Like I said above the alcohol will not harm the PCB either. However, make doubly sure that you dry the PCB (make that water evaporate) before you plug the PCB back into the Rotel amp.