Reflow ALL the printheaders on the board!
In your picture the first one on the left and the three on the right are all cracked.
While you have the board out, replace the big capacitors.
Do the ground upgrade and reflow the solder on all printheaders.
Do some reading on pinwiki if you want to know more.
https://www.pinwiki.com/wiki/index.php?title=Bally/Stern
Peter