Anybody having trouble with mk 1 collars, having a bugger of a job with mine at the moment both new ones and older ones, i think it is down to batterys, can put in brand new ones straight from the pack and they wont work, put them in another they will, swap them again to the origional they might work. all terminals are bright and clean, sometimes how tight the caps are done up seems to make a difference. Hand units have new batteries and work well. Usually buy all the batteries from battery force because i get through so many, but have recently tried the ones deben reccomend still the same. Th