I found in my parents home attic an old Marantz Quadraphonic amplifier.
The Marantz 4070.
In perfect condition.
I use it with a garrard Zero 100 turntable an a few SQ records.
I hear only 3 channels.
In Stereo mode it sounds good. Also in "Vario Matrix" mode.
But in SQ... only 3 channels.
I do not know what happens.
But, I open a "Box" under the amplifier and, there is a SQ module. The SQ-A2 (Full Logic Module).
I take it out and, with a cotton and isopropil alcohol, I carefully clean the multiple plain contacts.
Then, I return the module in it's box and close it.
Now, I hear full quadraphonic sound with an amplifier and turntable 40 years old.
I put this Forum topic only to be seen by those members that has a similar Marantz Quad amplifier and one of the channels does not sound.