Use a commercial board cleaner available from an electronics supply. It is usually provided in an aerosol can. THe white stuff is probably oxidation. If you use a acid core solder and do not clean the board thoroughly after repairs this will happen every time. Battery leakage can have the same result. If you have a schematic I would check the affected traces with a ohm meter to verify continuity of the circuit. To start with though I would verify voltages at different points on the board, and check to be sure the oscillators are working and then verify data, address, etc line are active. This will pinpoint what area you should be attending to.
