You have done the trouble shooting, now you just need to repair it. Try stretching the compression springs that are on each end of the negative, and replace the batteries. Then turn the transmitter on to check to see if the light is on. IF it isn't, roll the batteries with the palm of you hand with the switch on and see if you get any response on the transmitter light. I have experienced similar things with the older HL radios. On those they used to have plastic inserts between the batteries, that need to be removed. I don't see those on this new radio though

I am guessing you can fix this yourself. Use you multi meter to probe along each individual battery to find out where the connection is lost. Perhaps you need to clean the ends of all the batteries