My guess is that the capacitor (those are surface mount electrolytic caps, not tantalums, BTW) developed in internal short, got very hot and burned the nearby cable which may have been touching the cap before it burned.
The 47uF caps mostly act as filters between the power supply (5V or 12V) plane and the Ground plane on the circuit board. So, if one develops an internal short, that's a short directly between 5V (or 12V) and ground and will generate a lot of heat.
Regarding your second board, that could be any of a number of things. If you're still using the same power supply, it's possible that the earlier short circuit damaged a component in your power supply which later failed.
However, it's also possible that the "brand new" IIsi board has leaky capacitors which caused a failure after a few days of use. Even though the boards you got were never used, they were still manufactured more than fifteen years ago and that's plenty of time for the electrolytic caps on board to fail.
Because you have two of them, and since there's memory on the IIsi logic board, I would pull the RAM, and all cards, disconnect the SCSI and floppy cables, and just power up the logic board with a monitor connected to the built-in port. See if you get to the flashy question mark floppy image.
If so, then your logic board is probably okay, and the problem is elsewhere.
If not, then try the other "new" logic board that you bought.
That should get you started on the trouble-shooting path.
The 47uF caps mostly act as filters between the power supply (5V or 12V) plane and the Ground plane on the circuit board. So, if one develops an internal short, that's a short directly between 5V (or 12V) and ground and will generate a lot of heat.
Regarding your second board, that could be any of a number of things. If you're still using the same power supply, it's possible that the earlier short circuit damaged a component in your power supply which later failed.
However, it's also possible that the "brand new" IIsi board has leaky capacitors which caused a failure after a few days of use. Even though the boards you got were never used, they were still manufactured more than fifteen years ago and that's plenty of time for the electrolytic caps on board to fail.
Because you have two of them, and since there's memory on the IIsi logic board, I would pull the RAM, and all cards, disconnect the SCSI and floppy cables, and just power up the logic board with a monitor connected to the built-in port. See if you get to the flashy question mark floppy image.
If so, then your logic board is probably okay, and the problem is elsewhere.
If not, then try the other "new" logic board that you bought.
That should get you started on the trouble-shooting path.