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

If it aint broke ......

I would take the lid off, go outside and use a can of compressed air to blast off the excess dust and hair. The only reason I would do this is to ensure good air circulation inside the amp and prevent fires. IF you're very careful you could use a vacuum cleaner.

I would not clean the PCB, certainly not with 90% alcohol (the other 10% is water). If the PCB is working then it doesn't need cleaning.

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.

No need to resort to patronizing language. Non conductive gunk on a PCB is almost certainly NOT a failure waiting to happen. If the gunk were likely to cause a failure it would probably have done so already. Using any cleaning agent risks doing more damage than the "gunk" that you're trying to remove. If you are willing to risk damaging components on the PCB to make it clean go right ahead and clean it. I would leave it alone.

Excessive dust and hair impedes airflow and is best removed to prevent any chance of overheating (though even this, I feel, is very unlikely).

Maybe better to use q-tips with cleaner, so that you don't have to spray directly onto the board, and risk stuff disolving and being carried to even less accessible areas. I have done this to clean excessive flux off parts I was repairing in the past.

This stuff would probably do it if not used too liberally.

http://www.radioshack.com/product/index.jsp?productId=2102649&cp=&kw=cleaner&parentPage=search