Inspiron 620, motherboard issue, replaced 3 times

Long Story. I have an Inspiron 620, 8GB ram, Core i5, and 1TB HDD. I used it as a work computer until a broken USB device caused it to crash due to a power surge when it was unplugged (I think). The computer then shut down and refused to power on after this, and the amber light on the motherboard and PSU started flashing/pulsing. I tested the PSU, which worked just fine. I replaced the motherboard with an Optiplex 390 board (as it is the same as the original just with a different config) and it worked fine. The USB device caused it to crash yet again after a silly accident, and the same thing happened. I replaced the motherboard with another Optiplex 390 board and it was up and running again. Out of curiosity, I wondered if I could fix the old motherboards to put them to use. So far, nothing. I tried resetting the CMOS multiple times by replacing the battery or using the jumper. The amber light on both of them still flashes. Any answers, or are these boards shot? (BTW, I ditched the broken USB device and I'm never using it again.)