I am not sure if this will help you, as I am not familiar with the Sat500, but
I had a very similar problem with a Sangean ATS818CS (large portable
w/cassette) a few years back. The symptoms exhibited were the same (display,
backlight, power and FM were fine, but no AM or SSB, just rushing noise and
occassional off-frequency garble that was 'almost' a station coming in)...and
it turned out that the pot that controlled the BFO had cracked off of the front
PC board. It took a bit to get to the site of the break, but once there, I
resoldered the pot to the board via some flat bar-wire, and the radio was back
to normal.
Perhaps there is something wrong in the BFO circuitry (if the '500 sports a BFO
or SSB feature), and could be checked out by a local tech.
Good luck, the problem can't be too terrible to be fixed.
Linus