I’ve seen this a number of times on several QMX rigs that I’ve worked on. ?In my experience it has always been a short inside an IC. ?Most often it has been the cpu. ?I’ve also seen a Capacitor on the VDD rail blow up and short. ?My belief is that it happens when some voltage that is too high (5v or 12v) gets put out on one of the pins of the cpu or onto the VDD rail itself. ?Why this happens is less clear. ?My guesses are: a microscopic wire or solder bridge creates a short, a mechanical event (like pressing a button) causes a short, a meter or scope probe accidentally bridges 2 pins. ?I’ve seen evidence of all these on different rigs.
I usually find the short with a milliohm meter, and by lifting power pins from ICs or by removing caps to confirm. ?It’s rather tedious but often leads to the dead IC. ?
73
Jeff
W1NC
