hi matt
check carefully the lead as it goes into the handset itself ,check the position of red and black wires especially ,On earlier ko sets the red (positive wire) was n not in the "middle" of the plug and i blew a set up by plugging in wrong way round by accident
normally a futaba receiver type plug is fitted to battery and will fit it sounds a bit like you already have an adaptor lead between handset and battery
can you show us another picture
regards dave