Single biggest problem with the relay system. The transmitters invariably fail. Line 6 knows this full well. Why do you think the transmitter cost so much to replace? I have 3 receivers and 3 transmitters where all 3 receivers work fine, and only 1 transmitter is working ok. I had to buy used sets just to get another transmitter. (it was just a little more than buying a new TPB12!) Hard to believe after so long and multiple generations the transmitters are still the weak link. Now the latest version which is on sale now for 149 is not compatible with the G50/G90??????!!! WTFF! Not good.