this makes me thinking. I think (and can be very wrong) that the powerpack only supply high voltage to charge the capacitor and low voltage DC to run the control electronic in the head. I think the capacitor is actually in the head and not in the powerpack. So it seems once the flash is charged, and the actual firing doesn't involve the powerpack but your experience with swaping indicate it's the power pack problem. So I really don't know and it bothers me.