Philco 38-7 tuning mechanism

[comradesvox]

I noticed something else strange with this tuning mechanism. Sometimes when I touch the metal top of the tuning knob I get a loud pop. It may have something to do with it being in between the intermediate and out position? FYI I do not have a grounding wire attached to the back terminal. From what I have read if you are just using a straight wire antenna you hook that to the red terminal and then connect the black and ground terminals together. Don't know if it is related to the popping problem but thought I would mention it.