My Fox II 286 mainboard just blew a capacitor. I haven't yet powered it on, so maybe the cap was bad. How can I diagnose this problem, and eliminate other bad caps?
The wear of solid electrolyte tantal caps as these are is neglectable. They are however extremely sensitive to overvoltage. As this is a cap right near the power connector, I suspect the power supply is broken.
This seems to be the +12V or -12V line. Check these on the power supply before reconnecting. Maybe you are lucky and nothing else got fried.
(Chances you accidentally mixed up the two halves of the power connector? What you have there at the back of the photo also doesn't seem to belong to an AT power supply.)