[R-390] AGC Slow - Moment of silence

[Charles Steinmetz]

Larry wrote:

>  It looks like the problem with the circuit is the charge time of C551 
>  when switching the AGC into the slow position.  This causes the agc 
>  line to be pulled + for a second while C551 charges.  Even if it had 
>  zero leakage, one end is switched from ground to B+ potential, 
>  causing the problem.
>  I came up with a fairy simple solution to this problem.  [etc.]


A more comprehensively engineered version of the same basic solution was posted back in December 2009, in a thread about SSB reception (this version removes V506A from the circuit entirely -- it's not quite as simple, but makes it easy to create whatever release times you want very easily):

The "nether" end of C551 gets switched between ground and the plate 
of V506A, which sits at about 30 volts. Every time you switch from 
Slow to Medium AGC, you put a -30 volt step onto the AGC line, which 
turns all the gain-controlled stages hard off. You don't get your 
audio back until C551 discharges through the AGC release 
resistance. When you switch from Medium to Slow, you get a blast of 
distortion (but it doesn't last as long as the silence, because the 
cap discharges relatively quickly through the grid of V506A). 

Mod (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 from Slow to Medium AGC, you can omit 
Mod (2).


Best regards,

Charles