The strobe should produce only a small blip of light. Repeat this with the lens covered and the strobe should produce a full power flash. TTL failure is frequently caused by low batteries in the camera. This is true for both SLR and Nikonos cameras. Low batteries can even cause the shutter to lock in some SLR cameras.
Batteries are one of the first things to check if a problem occurs.
Our super powerful Substrobe 400 utilizes the same body that originally contained 150 watt-seconds, but everything inside is completely new. Conversion may eventually be offered, but this is not an available option at this time. The receptacles in the bottom of the pack can get pushed up into the pack preventing contact with the pins in the strobe body. The receptacles can then be pushed back into place and secured with a small amount of an epoxy type glue. My strobe does not flash - and I get NO under exposure signal.