TECH GURU's out there, how to clean Circuit Board?

Bombaywalla .. I think you did not read the original post correctly. The amp is stated to be in working condition.

When electronics is working I don't go pouring liquids onto the PCB. Not water. Not alcohol, not even electronics cleaner (as some PCB cleaners will damage some components).

Like I said, if it isn't broken do not "fix" it.

Cleaning the pcb will be beneficial because there can be current leakage between parts, depending on the nature of the spilled matter (type and amount). Also, if you ever want to sell the unit, you would not want to sell someone a component in such condition. 99% pure alcohol can be easily obtained from most electronics stores, along with necessary brushes, swabs, etc.

A electrical parts cleaner CFC free will work wonders. It will be expensive but they have several types that leave no residue. Almost any hardware store like ACE will have this. If you cannot find any go online and search for electical cleaners. There are cleaners that will harm PCBs but most will not. Read the can before you use and most will require PLENTY of OPEN AIR so that you dont become sick from the fumes.

Be careful about alcohol, and any other cleaning agents.
In building inertial guidance systems for ballistic missiles we found that the best fluid for cleaning circuit parts is Freon. We used it for decades. Concern for the Ozone layer forced us to abandon Freon except for a few very critical processes.

After a research project lasting several years it was determined that a Citrus-based cleaner was next best. Since we discovered this many citrus-based cleaners for general home use have appeared on the market. I do not know if these are the same as what we now use for circuitry.

Your best bet is to find out what product is now sold specifically as a Freon substitute for electronic part cleaning.

Seandtaylor99,
my good man, I did read Rapogee's original post very well. Yes, I noted that it "works perfectly but I believe its not a healthy thing".
even if the gear is working perfectly, electronics & "some greasy stuff and cat hair all over the internal board" are 2 things that do not go together! a dirty PCB is a disaster waiting to happen especially in a power amp (a high gain & large signal piece of gear). it should/must be cleaned up otherwise failure over the longer term is almost certain.
It looks like we disagree. OK, so be it!