PA0O ham radio

To be able to receive at 500kHz  I had to kill the 500kHz  QRM source. Yesterday  I used my modified to 475-500kHz "RANEX"  pocket radio  to search the  the noise at 500kHz.  That QRM was covering 500kHz for more than a week.  Constantly  50kHz wide and stable in frequency. This time I searched at neighbours home  but nothing found. This MW receiver is a great noise sniffler. There are lot's of noise sources in a home to be found. I found an new one a CD player in standby mode. Back at the radio the 500kH noise  was  gone suddenly.. I switched the CD player back on but that was not it. Nothing found but QRM was gone. Then replaced the 1:9 transformer back to the original in the K9AY rx antenna loop. This one has a small core with high AL.  I tested a transformer with low Al   not trifilar but "normal" wound 1:9   In the hope getting better result but it was no improvement. The K9AY does not work at 137kHz it starts to work at 500kHz the signa