[R-390] AGC mod

[Jim Haynes]

I haven't studied the one in the Navships manual, but there is one
in the September 2006 issue of Electric Radio.  And there are some
others that have been done with the intention of making the R-390A
good for SSB without an external SSB converter.  The general
principle of these is to make the AGC fast on attack and slow on
decay, so it holds the gain down in response to the envelope of
the SSB signal.  Then they greatly increase the BFO injection
level.  If you have strong enough BFO in relation to signal you can
detect SSB fairly undistorted with a diode detector.  This is a
lot simpler to do than putting a product detector into the R-390A.

Some guy used to sell an SSB modification kit that required no internal
changes.  Consisted of a rectifier connected to the line audio output,
and feeding into the AGC line.  The idea was you would listen with
local audio, and turn up the line audio gain to get some audio AGC.
This was fairly ineffective.