Well, there's always something new. Got two cubes in today for repair. One had a burned plug (fixed, I need to order another box of plugs) with all four dies now working. The second one was weird: The user said it would start mining then shut down the whole contoller within a new minutes. So I hooked it up, put a good heat sink on it, and took a look.
Interesting one. Very, very interesting. Yes, it does come up on four dies (one supply is bad on die 1) and yes it does crash the controller and in one case shut down my power supply. Never seen this one.
Did some work on it. Die 1 is bad. It works, but it's still bad, what happened is that it overheated and damaged both its power supply and the rest of the board to some extent. In this case, when it comes up it goes into thermal runaway as it heats up, ultimately shorting the supplies and the board. You can see the burning on the back of the board, it got very hot.
Turning it off clears that but the other three dies also run very hot. After experimenting for a bit and doing a reflow I found a solution: Run the remaining three dies at a much lower voltage. Running dies 2,3,4 at -.0806 at 275mhz gives you a cube that runs at 57mh while running at 42c in a box with the original style fan. That's a pretty good running cube and about what you should expect from a cube normally. So that's fixed as well.
Never know what you will run into in this business....