LGB F7 Right now, just a few pictures of a Massoth decoder in an LGB F7. This F7 is the one with the clear shell. Per Dan Pierce: All F7B units had the speed sensor, reeds, and sound board in the B unit, not the A. The A just had a speaker. The sound board in the B has 2 audio amps and can drive the A and B in the AB setup, or just drive 2 A's in the ABA setup. This is why the 4 pin audio speaker cable can never be used between 2 B units, it would tie 2 audio amps together. Original F7A/B 's did not have MTS and needed 2 55021's in the A unit (one for each motor) and another 55021 in the B unit for horn blasts for direction. On another note, many LGB sound systems will not work properly unless the chuff sensor can see the wheel turning.