It was one of the purple AR2412s that had the problem. I’ve since bought my own, and it is the newer(?) black version with locking XLR inputs. No problems at all with it, although I don’t have a lot of time on it yet. I have not been curious enough to open it up and see if the PSU is the same or similar to the purple one. If it keeps on behaving itself, I probably won’t.
Geoff