Anyone familiar with these? This unit isn't firing at all, with either of my 103 heads. Was working fine, then stopped firing in the middle of a shoot. I unplugged the sync cable, and tried to fire it using the orange button on top. Nothing. Turned the unit off, and it didn't discharge the flash. Tried another head, same symptoms. Pushed the reset button and that didn't help. The modelling light on both heads worked fine though. At this point, I'm thinking it must be the power pack. I don't have a spare to test with, unfortunately. I tried plugging it into a different circuit in my house (don't know what difference that should make though). I'm out of ideas - is this thing toast?