Well now I am -- I've been out most of the day. With a problem like this, there's only a few things that it can result from:
1. The composite signal must be present, undistorted, and of sufficient amplitude.
2. The sub-carrier must be present with sufficient amplitude, and of the proper phase relative to the composite signal, as presented to properly functioning demodulator diodes.
3. The matrix output stage must be functioning properly to match the amplitude of the L+R audio signal and the L-R sub-carrier signal so as to achieve individual +2L and +2R output signals.
For one channel to not work properly, something has gone wrong at some point in this process. That usually implies something in the demodulators or the matrix stage, but phase issues can cause these types of issues as well.
So what did you find?
Dave