After doing multiple cleanings of switches, buttons and rotary knobs, it's clear that the problems are more extensive than just dirty controls. While I can get all four channels in some modes, I get varying degrees of the four channels in other modes. Also, I discovered that when left on and warmed up, more channels come to life in some of the modes where they were not working when first turned on. 4 Channel discrete Tape input works fine. In the decoder options, HALL works fine, SURROUND only has rear channels when first tuned on, but after getting warm, the R front channel returns. Same symptom on SQ and QS. CD-4 is a different variation and the only one where both rear channels don't work. So, it definitely looks like it is repairs, not just cleaning, that will be needed to get everything functioning again.
