I've been running a Line 6 Relay G90 for a long time, always loved it but run into the same issues as other users have described, intermittent drop-out. The original transmitter used to do it a bit and when I checked it out the whole unit was turning off and then on again, like the batteries were not making proper contact. However, the battery door which appeared to be pretty flimsy was making full contact. I tried replacing the lead but that didn't do it, then it got worse and worse until one day it refused to turn on.
Being busy and impatient, I just bought another at a cost of about 120 Squigglies and got on with rocking. But with a tiny amount of free time last night I decided to re-visit the knackered transmitter and see if I couldn't figure out the problem. Cue an hour of swearing at the tiny Posi-Drive screws they use to hold the whole thing together, resulting in me taking a drill to the one remaining screw which was refusing to reveal the hidden treasure inside.
After getting the unit open, everything looked to be intact apart from dis-colouration on one of the battery terminals. I grabbed some vinegar and a cotton bud and proceeded to clean it up, but still nothing. In one last attempt before tossing it in the bin, I tried making contact with the batteries using the door from my functioning unit and, hey presto, it comes to life. Cue some scrubbing of the door contacts with vinegar (there was no visible dis-colouration) and there we have it, another functioning transmitter.
Now, if I can just find the lead that I thought was the original problem, I can be a flash prick with a pack for my guitar and one for my back up.
I've had some issues with the G30 transmitter recently. The battery door is a terrible design, pops open and basically needs to be taped down tight.