A Comcast cable J-box is also known as a cable junction box. As the name suggests the box houses a "junction" inside where multiple cable subscriber connections connect to a central board that is connected to a bigger junction nearby. To prevent vandalism, tampering, and damage the box is locked and a special tool is required to open the box. This prevents easy access by a screwdriver or other standard tools. However, if you merely want to check your connection or see the internal workings of the cable box you can use a cable termination tool to open most cable boxes.
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Inspect the outside of the box. Locate the small "keyhole" that has a small inset bolt inside that can have a tool inserted into it. The color of this keyhole may be either silver or gold.
Insert the metal end of the cable termination tool into the opening in the center of the "keyhole." Ensure that it is fully inserted.
Grasp the handle end of the cable termination tool and turn it clockwise. You will feel the bolt loosening. If you don't, apply more force to the handle or try going counterclockwise in the event the keyhole is installed upside down.
Swing open the cable box door if the door is hinged. If you do not see any door mechanism, lift the entire box cover up from the top to reveal the internal connections.