Transceivers RX Performance on the 600m Band

I have been confused since my very first listening sessions on 500kHz by the carrier at the top of our allocation on 504kHz. Most screen captures and On-line Grabbers show this at varying strengths.
My Kenwood TS870 shows very little of the carrier using a long wire antenna, using my tuned 600m TX antenna on RX there is no sign of the signal.

Finally I have conducted some comparison test with the Kenwood TS870, Yaesu FT897, and Icom IC706. The screen shots show the three rigs using my 40m Doublet antenna with a Ferrite Slab coil in series roughly tuned to 500kHz. The pictures can be viewed full size by, right click on picture -view image

First the FT897. (the wide near vertical lines are from my LCD monitor)
The 504kHz carrier shows red.
Many odd wide horizontal lines are evident.
Reception of in band WSPR signals is good

Next the TS870, same antenna, no carrier on 504kHz, no horizontal crud. (again he wide, near vertical lines are from the LCD monitor) In band WSPR signals are good, maybe a dB or so up on the small rigs.

The IC706 MK nothing same antenna.(the monitor crud is almost horizontal)
The 504kHz carrier is bright green.
There are wide horizontal crud lines, not strong.
in band WSPR signals are good.

The IC706 again to prove a point. First half of the display is with the Doublet, second half is with the tuned 600m TX antenna. In band WSPR signals are stronger using the purpose 600m antenna, the 504kHz carrier is reduced.

My conclusions are that contrary to popular belief there is no radiated harmonic on 504kHz, it is a mixer product generated within the receiver, particularly the small portable toy rigs.

The short wide horizontal noise lines also experienced on these rigs are mixer products too.

As a final test I tuned my doublet to 252kHz and fed the TS870 with the signal.
Looks familiar the rig can be forced into the same poor performance as the toys.

The wide crud lines have a similar frequency distribution as seen in many screen shots.