04-03-2014, 02:34 PM
Quote:With the modulation input switched off then what I call intermod hum isn't really. Its certainly not 50Hz or 100Hz rather like a gritty noise. Its inaudible sitting in an armchair, at decent listening volume, and 12 feet away even in the quite passages.
I had the same problem with a mains powered mini tx on MW. It was modulation hum. It came and went. Turned out to depend on what was plugged into the mains that had a semiconductor somewhere. Dimmer switch, electric blanket, heating pump - neighbours' stuff too. When some or a certain combination of these came on, the TX signal had hum.
I may be wrong, but I believe it's RF getting into the mains and getting roughly modulated with 50 Hz in the power supply. I significantly reduced it by careful decoupling all diodes in the power supply of both tx and radios.
It's much less of a problem with battery-operated gear, both tx and rx, but still possible on rare occasions.
Ian
