Yeah, try removing that memory expansion. I had something similar with a Amiga i rescued from the junkyard. It turned out to be a leaky battery which had destroyed the memory expansion and had also caused some corrosion in the a500 itself. It worked fine after cleaning out the amiga and removing the expansion. The expansion was sadly too badly damaged for me to repair, as i have basicly no skills in that area.
If it is infact a a500+, then it also has a battery on the mobo wich also might have leaked, in that case remove it completly and try to clean of as much as possible. Also make sure every chip is seated properly.
If it is a a500+, then it could be that the previous owner swapped the keyboard or something.
