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

[Jacques Fortin]

Hello Larry,

Thank you again for providing improvement ideas for our beloved receivers.
I have two questions, however.
I understand the purpose of the Mod. 3: there is a similar arrangement used
in some other receivers, the Racal RA17C12, for example.
Question 1: a 8Meg. ohms resistor is not a part of "standard" value, but
8.2Meg. ohms is.
I then presume that there will be no difference in the Mod. 3 implementation
if a 8.2Meg. ohms part is used. Please confirm.
Question 2: did you recommend using those 200 PIV, 400ma rectifiers for a
special reason, like a super-low reverse leakage ??
They are all Silicon rectifiers of old manufacture, so they will be hard to
find.
Could a "good old" 1N4004 be used instead ???
Thinking about another trick, does the B-C junction of a high voltage
silicon transistor can be used (like a 2N3440, or similar) ??

73, Jacques, VE2JFE

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
part de Larry H
Envoyé : 26 août 2021 23:43
À : R-390 Forum <r-390 at mailman.qth.net>
Objet : [R-390] 2 diode agc ssb mod c/l meter linearity

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