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. The 2 wire cable is for track power and the B unit does not have any track power pickups thus this is needed for the B unit sound. 4 wire cable is to be used between A and B units only, NEVER BETWEEN 2 B UNITS!!!! This is the speaker multiplexing cable so the B unit can feed in an AB configuration or a ABA with the B unit not having sound. You need ABBA FOR ALL TO HAVE SOUND FROM THE 4 SPEAKERS but no 4 wire cable between the B units. You should have the 2 wire cable between all units. Note the new Marklin has sound in the A unit and the B is just a speaker. And the cables are different than the LGB manufactured AB units. On another note, many LGB sound systems will not work properly unless the chuff sensor can see the wheel turning.