[Whatever]

I have a theory on this....

When you use a 9V battery, there isn't a ground path in the beacon. When you use a 9V supply, you introduce a ground path in the beacon. The fields themselves do not have a ground path built into them, so using a 9V supply means all static built up on the field and in the robots eventually gets discharged through the beacons. That's my best guess for what is causing the issues.

I wanted add - I served as queuer and did general field repairs at an earlier event. The tournament started with the 9V being hardwired and 2 beacons went bad during field testing (pretty much as described). At that point all the beacons were switched to 9V batteries and we ran the next two days without any beacons going bad (which was good because we only started with two back ups). I know the event Nick is referring to used a supply.