[R-390] Resistors, SSB

[Drew Papanek]

Hello all,

I have found the 2 diode (Lankford) modification to be extremely effective 
for SSB reception on the R-390A.  The addition of these diodes is not an 
alteration to the detector as some might suspect.   Instead, diodes are used 
to shunt  2 AGC time constant determining resistors.  The result is a short 
negative going time constant (fast attack) and the original positive going 
time constant (slow decay).  Fast attack enables AGC to capture signal peaks 
and slow decay "remembers" (for a short time) those peaks so that gain is 
appropriate for future peaks.  Result is a tolerable signal to BFO ratio at 
the detector (minimizes distortion).  That time constant alteration accounts 
for most of the mod's effectiveness.

The Lankford modification also increases BFO coupling cap for more 
injection, and adds capacitance to AGC line.  The mod works well without 
these capacitor changes.

For good AGC action on all modes using medium and slow AGC switch settings 
verify that the 2 uF oil filled AGC cap is not electrically leaky.

On replacing leaded caps/resistors in those crowded spaces: Trying to undo 
the connection from a tube socket pin or other terminal often does damage.  
Cutting out old component leaving a stub at the terminal and splicing in new 
component using small copper tubing sleeves was a good suggestion.  
Alternatively, the new component's leads can  be coiled using a piece of the 
old component's lead as a form, and these coils slipped over the 
aforementioned stubs and soldered.  This works especially well if new 
component has smaller diameter leads than old.  Such would be the case when 
using 1/4 watt carbon film or metal film resistors to replace old 1/2 watt 
units (calculate dissipation to determine suitability of replacement).

Unless they're cooked, leave those 2.2K decoupling resistors alone.  As Bob 
pointed out, the variation in voltage drop across these (due to drifting out 
of tolerance)  is  miniscule compared to changes brought about by line 
voltage variation and normal gm tolerance range of tubes.  Of more 
importance would be screen and cathode resistor values.  For IF stages after 
"mechanism a philharmonic" (that funky fractured translation of "mechanical 
filter"), resistor values become less critical.  This is because that 
section of the IF chain has much more gain than needed and we reduce it 
anyway by tweaking IF gain pot for best S+N/N ratio.

Drew




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