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

[2002tii]

Drew wrote:

>I've seen a similar scheme which put the resistor in series with the 
>time constant cap.  I think that would allow the full AGC voltage to 
>reach the gain controlled stages (relatively) immediately upon 
>reception of a large signal transient, instead of being dragged down 
>while the cap is charging. The effect might be to reduce the "pop" 
>often heard on the leading syllable of a strong SSB transmission.

The two achieve different objectives.  The resistor in series with 
the diode slows down the AGC attack.  The resistor in series with the 
cap does as you suggest -- speeds up the attack for an instant, then 
lets it recover quickly.  This can have the disconcerting effect of 
blunting the leading edge of a vocal plosive, with a louder following 
vowel sound.  This inversion of normal speech dynamics can sound quite odd.

>do you see a disadvantage to using the original AGC integrator 
>(tube) circuit with a good cap as opposed to disabling it and using 
>larger caps as you did? Is disabling the AGC integrator tube mainly 
>to facilitate gaining flexibilty in selecting time constants?

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).  As I 
said, "If you can live with the 'audio disappearing' problem when 
switching to and from Slow AGC, you can omit mod (2)."

>If adding capacitance to increase BFO injection, it has been 
>recommended to readjust the neutralization of the last IF amplifier 
>stage to keep the added BFO injection from finding its way back 
>through that stage and into the AGC IF amp/detector, where it would 
>drive up the AGC voltage and desense the receiver.

It's good practice to adjust C525 after any work on the BFO, but IME 
even if you don't adjust it you are very unlikely to notice whatever 
small desensitization may be introduced.

Best regards,

Don