Quote:
Originally Posted by rambo1152
Modulation hum is a phenomenon of mains operated receivers, ...
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But it could also arise during transmission.
If the Spitfire has a dodgy earth, so that the RF earth path is partly through a diode which is also handling 50Hz, then the radiated field will be different according to whether that diode is conducting or non-conducting. So the emission will be modulated by 50Hz, even though the Spitfire itself is pushing out a steady CW.
Similarly, if the Spitfire aerial is a bit dodgy, so that there is capacitive coupling to some wiring with a diode in it, switching at 50Hz, then again the radiated field will be modulated at 50Hz.
However, reading the posts, you seem to have eliminated these.
As the hum is really strong, can you 'scope the Spitfire output to see if the generated signal is modulated? That should give a clue. At least with a severe fault, it should be that much easier to find!