Following on from my last post it seems I have found the main source of QRM that was caused by one of my PCs at least. In the pic with my previous post you may have noticed the "Black box" Icom PCR1000 receiver, sitting on top of my little HF linear amp. That receiver is connected to the main pc-the one I have moved onto another desk.Having moved that PC I found that the S9 of noise on 10 metres which was always there when the PC was on had disappeared. Today I decided to reconnect the Icom receiver. Since the Icom uses a serial port connector I use a USB to Serial lead out of the PC which connects to serial lead and then to the Icom.
Plugging the lead into my USB hub, Windows 7 automatically installed the USB to serial driver and then............S9 of noise on 10 meters. Moving the lead directly into a USB on the PC reduced this to S6 but unplugging altogether reduced the noise to nil.
I don't know why this lead should be radiating noise like this, but I will try and move the Icom receiver to be controlled by the other PC. Perhaps winding the lead around a ferrite ring would help? I think I have one here and will give it a try.If that doesn't work I think I will be moving the Icom on.