OBERHEIM SYNTH

My Matrix 6R has a strange problem - no matter which preset you 
select, with both Stereo option ON and OFF, only Left/Mono channel 
works.
On headphones, on the other hand, you can hear both channels. 

I am trying to find out whether this is an electronic problem or if 
there's something wrong in the setting.

I tried the following to no avail so far:

1. Full Reset ("Calibration")

2. Replaced the 3 ICs at the final amplification stage, which are a 
741, 4082 and 4053. When disconnecting the "Left/Ground/Ground/Right"
 wire going from the main voice board to the headphones/out board, 
and checking the output jack on the voice board with an audio probe, 
only left channel plays, so I guess there is nothing to do with 
neither the output jack board nor the headphones board.

I can't see any component the might have an influence on the right 
channel alone, beside those 3 ICs which I've replaced...

Any suggestions ?

Thanks