[R-390] R-390/R-390A AGC for SSB, fix to Dr. Lankford's 2 diode mod

[Larry H]

I added a product detector to my R-390A daily driver many years ago - thank you Capt. P.H. Lee.  Worked great, but the AGC was not good for SSB.  When I heard of Dr. D Lankford's 2 diode mod, I tried it right away, and it worked great.  Thank you Dallas!  SSB  is now great.  But, it did remove the 'delay' in the AGC.  I was not aware of it right away, but did notice some side affects:

1. Weak signals did not come in as well as before.
2. Audio output was reduced.
3. Noise limiter did not work quite as well.

I had a good idea what the AGC was supposed to do, but could not find any measurements anywhere, so I took a few measurements without the mod on, so I could see what it was suppose to do on weak signals:

    uv in balanced     AGC det pin 1 V     AGC TB102 p3 V

            0               -0.4               -.15
           .5               -1.7               -.18
            1               -3.2               -.23
            2               -4.9               -.65
            3               -7.6               -1.25
            4               -9.2               -1.6
            5               -11.1              -1.9
            10              -15.2              -3.2

            20              -17.4              -3.6
It looks like the delay is overcome at about -3.5 V.  It took me a little while to put it all together and even longer to come up with a fix.  I wanted something simple and easy to remove.  I finally decided on the following circuit:

https://s26.postimg.org/hvmo7yhzd/IMG_7794s.jpg 

After reinstalling Dr. Lankford's mod and my fix, I took the following readings:

    uv in balanced         AGC det pin 1 V         AGC TB102 p3 V 

            0                  -0.4                  -.15 
            .5                 -1.6                  -.18 
            1                  -3.0                  -.25 
            2                  -4.8                  -.7 
            3                  -7.4                  -1.3 
            4                  -9.0                  -1.7 
            5                  -10.8                 -2.0 
            10                 -15.0                 -3.3 
            20                 -17.0                 -3.7

As can be seen, the AGC voltages are very similar and it is working very well for weak and strong SSB, CW and AM signals.

Regards, Larry