[Graham G3ZVT]

I once went to commission an SiS TV system in a betting shop. It was two shops knocked through into one, and building work was ongoing.

I pretty much sussed out what the problem was as soon as I walked through the door. There was an almighty hum on the speakers and asynchronous hum-bars on all the TV monitors.

The satellite receiver and decoder was plugged in to the supply of the original shop, and the audio amp, video & audio distribution amps and all the TVs were on the "new" side, on a different phase. I ran an extension lead to power the satellite kit temporarily, and left them to sort things out.