Fwiw, I’ve fried a couple megas myself, it comes down to remembering not to move the steppers while they are plugged into the board. the motors backfeed voltage through the drivers and burn out both the ramps and mega. I also used an old pc psu when I first got started building mine. turns out the older ones may not be able to handle the em interference of the steppers. my first atx psu would whine along with the steppers. gpt a newer model atx psu and the problem went away.