Maybe the discoloration seen in the last photo is caused by overheating of the grounding-metal, rather than oxidization? If so, what could be causing this, and what other components should I check to see if have any sort of damage? And how can I fix this, if at all?
I have tried to look at the components on the board where the main output's grounding wire is connected to, but I cannot see anything there that seems out of order.
Everything else on this Polysix seems to be working properly.