Once I had my battery compartment flood on my ys120. Some of them before a certain serial # are suposedly not compatable with nimh batteries. In my case I had just taken them off the charger and they were warm still when I put them in the strobe. Now I let em cool before hand. No problems since.
Great strobes you can pick up on ebay cheap sometimes. Just have some vinager if you get a flood to clean it out good. I put mine back in service and it still works fine.
Good luck. - John