[Collins] R-390A frequency measurement - in service

[frankshughes at aim.com]

Hi,
I am searching for a way to connect my HP 5316A frequency counter to the R-390A so I can get an accurate 
measurement of what frequency it is tuned to while in operation.

I searched around and it looks like I need to connect to the PTO output. 
If this is the best way, what should I use to make the connection w/o effecting the circuit operation? 

73, Frank
KJ4OLL