I noticed that I have the 7180-7182 KHz VCO stability problem.
Discovered that when I began loosening the screws on the bottom cover,
the buzzing sound disppeared. Experimented with this for a while and
found that if I remove the screw that holds the cover on the right side
of the chassis viewed with the unit placed upside down on the table,
front panel facing you. The problem goes away. I used a stable signal
from my HP8640B sig gen and moved the frequency between 7170 to 7190 and
no longer observed the instablity.
On my unit the problem appears to be caused by a ground loop around the
VCO through the bottom cover right side circuit board stud.
I am going to try using a piece of Kapton tape placed over the inside of
the cover where the stud makes contact and a nylon screw to keep this