I’d probably first check you have continuity from the various areas of the board which relate, corrosion can eat away at traces, vias and resistors.
Here’s a continuity made semi complete for one side
and this is the otherside done by @jkyoho
In both cases are not a complete map but should give you a basic idea.
Provided continuity all good then you’d start looking at components which could cause the fault. Check these threads for related info.
Search the forum too, plenty more threads with this issue 
