merlynski
Curmudgeon Electronicist
Suggest you start your own new thread in the Pioneer forum, you will probably get a better response . .I am having a similar issue and have isolated it to the phono jack. It seems like a bad connection to the circuit board. If I gently wedge something between the jacks and the board both speakers work.
I have zero experience working on these, though I currently have the side, rear and bottom panels off.
Are the jacks just held on by solder?
And Welcome to AK!