[R-390] Lankford AGC Mod ?

[DJED1@aol.com]

The AVC is half of the problem with SSB on the R-390s; the other half is the 
product detector.  I would estimate that fixing the AVC is the most important 
part, because to detector in the radio does OK if yyou just reduce the IF 
signal enough.  The classic way to do this is to go to manual gain control, 
and reduce the IF signal enough to have the detector operate in a linear 
manner.  As we know, the SSB signals sound pretty good when the radio is 
operated in manual mode.   So... I believe much of the problem is caused by 
the fact that the AVC doesn't respond fast enough to the varying SSB siignal, 
resulting in distortion at the detector.  The Lankford mod just allows the 
AVC to increase quickly by changing  the attack time constant (it basically 
shorts the timing resistors in one current direction).  Similarly, an audio 
AVC will help the fast attack requirement.  I went the whole way and built an 
external adapter which provides both product detection and fast AVC- works 
quite well except my BFO is not nearly as stable as the internal one.  Hope 
this helps
Ed