But why 200 Hz? The fact that 200 Hz is a harmonic of 50 Hz sounds significant. (Pun not intended!
) However, the fact that it is only detectable on the higher 'ends' of MW is intriguing. Perhaps there are two sources!
An H.V. transformer and an R.F. device that is radiating a strong enough signal to overload your receiver - which takes me back to my last post . . . .
And then the rather complex matter of intermodulation could be a factor.
Like many other members here, I've had - and tried to help with - diagnosing and fixing strange faults in the past - but this one is probably the most puzzling I have ever read!
Methinks your best tool is sheer persistence!
Good luck!
Al.