Zenith Advanced System 3 help needed

[zenith2134]

SONY KV2729R, circa 1987, had an optional outboard multiplexer for MTS Stereo broadcasts. otherwise it was a 4" monaural speaker. We had this set when i was a kid, with MPX unit fed into a Pioneer SX-838 stereo receiver feeding Advent bookshelf speakers. MUTING Could be enabled by the Sony TV handset.