You could check the input to the tuner with an ohmeter. See if there is a DC path between the inner and outer of the RF connector.
If you wish to be really safe why not open up the tuner and put a 0.1uF disc ceramic capacitor in series between the center pin of the input RF connector, and the lead that was originally connected to it that goes to the main PCB ?
It would be very easy to restore the tuner back to its original condition if you ever wanted to sell it.
I use an LDG Z11Pro in conjunction with a PRC 320 and it works very well.
Martin - G8JNJwww.g8jnj.webs.com