Something very odd !
There is obviously a major noise source around tone 300, or something that is killing the signal.
Above that it looks OK - so why?
Can I suggest that you power off, then disconnect everything, wait 30 minutes or so. Power up, let it stabilise then reconnect - using the "test socket" initially and no filter.
Just wondering where about you are and if there is a high power AM radio transmitter close by? That notch is around 1.5 MHz, so a 1548 kHz radio tx could casue it. I see wuit a few on mine at 693, 720, 909, 1215 kHz which nicely align with AM stations.
M H C
taurus excreta cerebrum vincit
Edited by MHC (Fri 03-Apr-15 20:14:11)