Multimix 12R Only One Channel Ouput

Trying to understand how to troubleshoot an issue with my Multimix 12R. Output is only on the Left Channel for ALL inputs. The LED Meter also shows output on Left Channel only as well. I have taken apart the mixer looking for an easy to find problem such as soldering issue or component damage (by visual inspection). Ribbon cables seem to be fine.

I wonder if there are schematics or a block diagram on this mixer to facilitate following the signal to understand where is the issue. Or maybe, someone could point me in the right direction to understand the issue.
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