I am aware of a 777 incident quite a few years ago where all VHF, HF and Satcom communications were lost. 7600 was then selected on the transponder. On every frequency that was selected there was an unusual background hum in both pilots' headset. The same hum was there even when no frequency was selected. After 10 minutes of trying numerous different frequencies on all three boxes and Satcom the copilot could smell electrical burning and then saw a few wisps of smoke.
The smoke was coming from the copilot's hand mic on the right fuselage stowage. The side stowage mics are very rarely used.(Anyone remember on the 747 they used to be so emphatic in the scan checks that the mic had to be stowed facing the right way round?) Anyway this one was too hot to touch and the plastic surround was actually melting. When the mic jack was unplugged from the socket all the radios worked normally and contact with ATC was resumed.
It was an interesting lesson that what was clearly a massive short from the copilot's hand mic took out all the radios and not just the frequency that the copilot's transmit selector was set to. This would suggest some interrelationship between the bus bars of different radios.