Disassemble it and use a cell phone to check the transmitters. They will show up in a cell phone. If the phone shows it glowing then you have a receiver problem. I found in my Dr. Who that after replacing a bad one, another one went out shortly after that and then another after that and so on and so on. I finally replaced the whole transmitter and receiver board. They aren't that expensive and it's worth not having to continually take it apart to keep replacing individual LEDs.