[Drake] T4XC Won't operate PTT

[Ka9p at aol.com]

On the off chance I'm making a silly mistake I thought I'd check and see if 
 there is a reason for not being able to make a T4XC operate PTT when it 
will  operate VOX.
 
I've switched out the 6EV7 and the 12AX7's just in case that might be the  
cause.  Other than that there are no clues except for a note in the manual  
that an 18k resistor was removed in the mic circuit.
 
Curious if that resistor was an impedance-related mod or could be messing  
up the PTT, and hoping for good ideas as to what to check next.
 
Thanks, Scott ka9p