[R-390] 2 diode agc ssb mod c/l meter linearity

[Larry H]

Hi all,  The c/l meter linearity may be skewed if you have the Lankford 2
diode agc mod on for ssb reception.  The reason is that the original
'suppressor grid' clamping at the junction of R544, R546 and R547 is not
effective enough to overcome the increased voltage from internal noise, an
RF signal or the antenna noise.

The clamping circuit provided by this fix (Mod 3), improves the situation
considerably, but does not completely solve it for a high noise antenna.
This problem has been there all along, but with the original design, it was
not very noticable. With Mod 2 and Mod 3 on, it looks like the c/l meter
noise tolerance operation is a little better than with none of these AGC
mods on. Mod 3 also improves the c/l meter linearity close to what it was
without these mods on. How does this affect the c/l meter linearity, you
say? Well, with mod 3 on, the meter 0 set point is at a better place. This
is what improves the linearity. The diode needs to be 200 volts at 400 ma
with a .6 volt forward drop or less because of the high voltage pulses
created when switching the AGC speed.  There were a number of diodes made
that meet the requirements in the 60's and 70's, but very few available
now. Ones I found are: 1N1693, 1N1694, 1N1695, or 1N2092.

However, if you have the 'moment of silence' fix on for the AGC speed, then
a lower voltage one (such as a 1N63) can be used.

I have attached the updated schematic for this fix.  If you don't receive
it from the list server and would like a copy, please let me know.

Regards, Larry
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