[R-390] Best approach for SSB mod on R-390A

[2002tii]

Tom wrote:

>What is the suggested approach for SSB modification for the R-390A.
>
>Is the 2 diode modification worth attempting?   I know it has been 
>covered here more than once just wanted to solicit the current 
>thinking  on the matter

Personally, I would either (i) just modify the AGC to work properly 
and increase the BFO injection, or (ii) build an outboard IF/detector 
box that has some IF filtering and AGC, will do both SSB and 
selective-sideband synchronous AM detection, and has 2 or 3 notch 
filters.  The former is quite easy, the latter a fair amount of work 
(I've been noodling at a design for 20 years, but it has never come 
to the top of my "to do" list).

Here are the AGC and BFO mods I've been doing for 35 years:

(1)  Parallel R546 and R547 with solid-state signal diodes (1N914 or 
equivalent), with their cathodes toward V509A.  This will produce 
asymmetrical attack and release times.  (Attack will be very quick, 
release will not change.)  To slow down the attack, if desired, put a 
resistor in series with the diode across R546 -- I often use 1 to 5 kohms.

(2)  Install a new 20 uF film capacitor with one end to the junction 
of C548 and C551 (i.e., the grid of V506A).  Disconnect the plate of 
V506A from J512 pin 15, and connect the free end of the new 20 uF 
capacitor to J512 pin 15 instead.  Switch the wires from S107, 
terminals 9 (ground) and 7 (C551) -- ground to terminal 7, C551 to 
terminal 9.  Install two, 2.2 Mohm resistors at S107 -- one each from 
terminal 9 (C551) to terminal 7 (ground) and from terminal 8 (new 
capacitor) to terminal 7 (ground).  The AGC switch will then add C551 
in parallel with C548 for Medium AGC and the new 20 uF film capacitor 
in parallel with C548 for Slow.  The new 2.2 Mohm resistors bleed any 
residual charge from C551 and the new capacitor to reduce any pops 
when switching between AGC time constants.  You can adjust the AGC 
release times by changing the values of C547 and 548 (Fast), C551 
(Medium), and the new capacitor (Slow).  I often use as much as 50 uF 
for the Slow AGC capacitor.

Mod (2) fixes the "audio disappearing" problem when switching to and 
from Slow AGC.  (Note that this is a design "feature" [i.e., problem] 
-- If your 390A does NOT have this problem, it's because C551 is not 
a capacitor anymore.)  If you can live with the "audio disappearing" 
problem when switching to and from Slow AGC, you can omit mod (2).

(3)  To increase the BFO injection, replace C535 (12 pF) with 47-75 pF.

These changes will make the 390A perfectly serviceable for CW and SSB 
reception, as well as improving AM peformance substantially.

Best regards,

Don