hi, i recently got this heathkit IO-12 from my father and was cleaning the pots when i noticed a wire had come loose. it is a black wire coming from the horizontal gain knob. i do not know where i can find this on the diagram or where it may go. i have attached 3 pictures in hopes that someone can help me. i have also attached a diagram of the area nearby the pot and the whole diagram.
edit: i have learned that this device has been open possibly multiple times as it came from a school/college.