There may be a short somewhere inside between the lead going to the pin and the one going to the camera body, such that when the capacitor has a full enough charge, the flash itself is firing (thinking that it has been triggered by the camera, just without the camera). Unscrew the foot - two small Philips screws, and check to see if any of the wires have damaged or melted insulation, and ensure they're not crossed. The ones to watch for primarily are the RED lead and the black lead. Repair any damaged insulation with electrical tape, and carefully reassemble and test.