[R-390] Dr. Lankford 2 diode agc ssb mod

[Larry H]

On the last R-390A I refurbished (a few months ago), I decided to install a
minor variation of the Dr. Lankford 2 diode SSB fix to the AGC.  I did the
Mod 3 version that I sent out to you all a couple weeks ago.  And, I did
install the additional cap across the BFO injection cap to increase the
level of BFO signal fed to the audio detector.  As Dr. Lankford points out,
this is usually  necessary to reduce distortion due to the IF signal level
being much higher than the BFO signal level in the detector.

However, I did not install his complete rework of the AGC speed circuit.
The reason is that while testing the rx on AM, I found that the original
unmodified 'slow' and 'med' speeds are very helpful in improving audio
quality on certain AM fade patterns.  More testing on SSB  reception
revealed that I got the best audio using his 'med' speed.  That was a .47
mfd cap from the AGC line to ground through the switch.  Since I normally
don't use the original unmodified 'fast' speed, I decided to use it for SSB
reception.  I found that adding a .6 mfd cap (because it's in series with
C551, a 2 mfd cap) to ground on the AGC speed switch 'fast' contact (switch
contact 10) provided very good SSB reception quality.  I went through the
same testing scenario on my daily driver (which has a product detector
installed) and found that the same AGC speed (a .6 mfd cap) was very good
there, also.   Now if you have the 'moment of silence' fix on the speed
switch, then the .47 will probably be optimal.

Happy listening.

Regards, Larry