[Drake] T-4X mystery knob

[Darryl Trump]

I am in the process of getting my first pair of "twins" on line.  The 
R-4A is alive now, and many thanks again to the group for your help 
with the bad oscillator transistor.  I'm working to get the T-4X 
purring.  I may have trouble with the T/R relay circuit.  It doesn't 
sound like it clicks very positively when I switch to TUNE.  I have 
found an unlabeled control pot on the rear of the chassis, right behind 
the Carrier Balance control, and beside the KEY jack at the rear 
corner.  It is a 15 Meg pot serving as  a variable resistor.  It seems 
to be connected to the grid circuit of the T/R relay driver tube.  In 
fact it may be replacing R95, a 3.3 meg fixed resistor in my schematic. 
  This appears to be a factory change that isn't covered in my manual.  
The chassis photos in my manual don't show anything where this pot is.  
Can anyone tell me what  this control is, how is it used, and what is 
the proper method for adjustment?

Thanks in advance,

KB8BKU, Darryl