Oh so now I’m giving myself RFI. I pinched a pair of powered speakers from one of the domestic PCs yesterday and wired them to the mixer. One speaker is powered from a USB adapter and it is then wired to the mixer via a 3.5mm stereo jack and to the other speaker. No idea if the wires are screened. There is an amp in the powered unit.
The YT-1200 upsets the speakers when it is operating, most so on 20m (which is the closest match for the loft antenna). But only the non-powered speaker is affected. A truckload of ferrite on the cable between the speakers has quietened it down for normal operation but it is still upset when tuning. The wire is currently wrapped multiple times through two decent ferrites, so much so that the speakers are now next to each other! Hmmm. Looks like I need to take these things apart to see what the cables are actually like.