Below is a picture of the BT I Plate PCB, at the top you can see the RF filter that is designed to remove RF interference from the line(similar concept to a BT 80A RF2 as seen here, although a BT 80A RF3 is the SAME filter )



At the bottom is the main feature-the bell wire filter(or inductor).

You can see the same filter fitted to our NTE5A's here :-