I have one of these (it was my first SS processor). Its long since retired, and I don't have a schematic, but I am going to toss something out on the odd chance it may help.
My unit also had a problem with the center channel, it would cut out on occasion. After some experimentation, I determined that the issue was mechanical...I could give the unit a good whack, and it would sometimes come back on.
Here is the punchline...
There was a capacitor on the board that had a faulty solder joint. Apparently, this component was somehow in the signal path for the center channel.
I found it by taking the cover off the processor, and putting pressure on the board in various spots. It was actually very easy to find and repair once I determined what area of the board seemed to be affected.
Not sure this would help, but these units are well over 10 years old, and may not be worth investing any $$$$ to fix.
Good luck.
Brian