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

[Roger Ruszkowski]

Larry Haney, Thank you for the PDF file with schematic included.We will be looking forward to seeing the information included onthe R390 CD for future reference. I see your up dated postwith links for the PDF and R390 CD for all the readers. The text in the PDF with the AGC voltage measurementswhen a good receiver is driven from reasonable signalgenerator levels is an often asked for reference. AGC is a problem in the receivers and old leaking caps arethe most likely causes, followed by an oxidized ground lugpoint. Thank you for a table of values that we can compareour receivers to and make some better maintenance choices. It is nice to know what good AGC operation measures asand what we can expect from our receivers in four differentconfigurations. Stock, with .3 diode, with .7diode, and with better AGCand trim values on a timing cap for AGC attack toenhance SSB signal quality. Larry your receiver modification as described in the PDF fileto modify the AGC behavior of the receiver to improve SSBperformance is a simple change to install or include with theLankford SSB diode modification. The detailed notes in the PDF explain what is going on in the receiver system and how the choice of diode switchvoltage and capacitor values improve the audio performanceof the receivers.  You make a compelling argument that the Lankford SSB diodemodification and your capacitor to shape the AGC attack fora better SSB receiver performance should be installed inevery receiver. Your analysis of the circuit explains why someFellows have reported poor results from the Lankford SSB diode modification. And we have another lesson in parts and part specifications.The faster diodes do make a difference you can hear and measure.  The idea of tuning a resistor diode capacitance circuit for best audioresponse. The guy with the Fender Amp wants the distortion. Yesbut we want to hear it cleanly. Research and bench test time willcontinue on this thread for a long time to come. Thank you, Respectfully, Roger Ruszkowski AI4NI 33C4H  -----Original Message-----
From: Larry H <larry41gm at gmail.com>
To: R-390 Forum <r-390 at mailman.qth.net>
Sent: Sun, Dec 22, 2019 12:42 am
Subject: [R-390] R-390/R-390A AGC for SSB, fix to Dr. Lankford's 2 diode mod update

I originally announced my fix to *Dr. Lankford's 2 diode AGC mod *on Sept
2, 2017.  It contained a 68 mfd cap, but recent testing showed me that a
better value is 10 mfd.  The reason is that the 68 mfd cap would charge up
quickly and slowly discharge thus changing the agc level for a few
seconds.  The 10 mfd cap is large enough to keep the fast attack for ssb
still working.  My apologies for not getting this right the first time.
I'm attaching the schematic showing the change.  I'll be sending a complete
updated document to Al to replace the one on our 390 disk.

Regards, Larry