I do not profess to be a EE. But my Service manager was one of the smartest people BPT had. He was of the opinion that it was the transistors turning OFF that created a short. Hence the SMS card. If you were in rapid and hit a limit or pressed the E stop, you would blow a transistor. If the T2 voltage exceeded 63VAC, you needed to invest in a tray of transistors. I could tell when a customer put in a non standard transistor as the static voltage would be a lot higher to have the same current. As I said the drives are strange. Static they are 9.5 VDC and 8 amps. At rapid they are 60VDC and 2.2 amps. If you do not have a SMS board and have a current clamp on the DC side and pree Estop, the current drops to zero like a stone. With a SMS card it drop to about half an amp and then decays. If one of the power resistors on the saturable core reactors went open (common in old machines, check for cracking on the borttom of the resistor), you would burn the SMS card and melt and burn your ACC card. The fan at the bottom of the logic card rack is critical. If it fails, your boards will fail due to heat. I have seen all of this more than once. I had a machine in Kearny NJ where the fan in the back quit. The insulation on all the wires became brittle and fell off. The ERS board has the BOSS software buned onto a chip at the bottom of the board. It has a tendency to collect dirt from the fan beneath it. This device runs hot to begin with and will fail if not kept clean. Preventative maintenance goes a long way with these machines. Many have been brought back to life by cleaning the boards and cleaning the edge connector with a pink eraser. The screw in fuse holders, when we came across them, were immediately replaced. They caused nothing but grief.

George