There were no signs of trace corrosion anywhere on the logic board. I had cleaned it thoroughly and inspected with magnifying glass. A little bit of green fur on the negative side tag of one of the caps was the only hint of leakage. I think I caught it before any damage.

Still a puzzle to me how the ROMS would have deteriorated - they should be quite stable. Can only guess that a spike of some sort got into the logic boards. I'd be interested to know if anyone knows what causes ROM failure.

Rick