Show me a tidy shack and I'll show you the shack of someone who doesn't spend much time tinkering with equipment, or someone who has been nagged by his XYL to make the shack tidy and curses every time he needs to connect or disconnect something - which at G4ILO seems to be several times a week.
When I switched on the computer this morning there appeared to be a Sporadic-E opening building on 2m to the south east so I switched on the IC-910H and turned the SuperMoxon in that direction. Nothing was heard, so I switched to the vertical instead and was puzzled as to why the band noise was so much greater on the beam than the Slim Jim.
The opening passed, as all such openings have this year with nothing heard or worked, and I decided to investigate. I switched the computer off and the noise fell to the same level as on the vertical. Eventually, after much trial and error and cursing because the cables can only be reached by groping blind (with the aid of a shaving mirror or a picture of the back of the rig) behind the equipment I established that the problem was the serial cable providing PTT control to the interface connecting the 910H to the computer. Even when it was disconnected from the radio, plugging it in to the serial port on the computer caused the noise level to jump up.
This was a cheap interface I'd bought some time ago for the FT-817. The serial cable supplied wasn't screened, but I'd shortened it to use with a USB to serial adapter without a lot of spare cable lying around. This had resulted in it being too short to use with a real serial port, so I had replaced the cable. I had used screened cable for the replacement, but although I had connected the copper shield to the GND pin of the RS-232 port, it wasn't connected to the body or shell of the plug, which in any case was plastic.
I managed to solder to the metal flange of the plug, the bit that pushes on to the socket on the PC, and connect a short wire from that to the GND pin and the shield. That seemed to do the trick, and the noise level is now the same when the beam is pointing towards the shack as it is on the vertical. Problem solved, for once! Nevertheless I do sometimes wonder if computers are worth all the trouble they cause to us radio enthusiasts.