[R-390] R-390A 2-diode AGC SSB fix update R3

[Larry H]

A couple months ago, we had a discussion on the list about this fix, the
additional agc clamp circuit and what diode to use.  This change improves
the c/l meter linearity and reduces the negative effects on the agc from
noise on the antenna.  I was not able to put it on our website then, but it
is out there now at: R390A Fix Lankford 2 diode SSB AGC.pdf (r-390a.net)
<https://www.r-390a.net/R390A%20Fix%20Lankford%202%20diode%20SSB%20AGC.pdf>
.

Regards, Larry