2 channel mode just duplicates stereo on the rear channels , matrix 1,2 and 3 sends the signal through the decoder in receiver , and discreet-CD4 sends a separate signal to each channel by way of RCA jacks on back , for hooking up a 4 channel tape deck or CD4 decoder . a lot of quads strapped the front and rear channels together when run in 2 channel mode , to increase the output . hope this helps a bit , sounds like you have more than one problem going on (wonkey left rear channel and tired decoder ?) all these old quads are pushing 40+ years now and getting tired , I got more than a few that have age related problems of their own.