Vintage Synth Repair

Hi, I'm new to this group, and I recently picked up a Korg M1 for cheap.

Replaced the internal battery, reloaded all the patches, everything is
just fine.  Except...the sound only comes through on the left channel. 

I tested this both with the L&R out, as well as with the headphone jack.
Same thing.  There might be a little bit of signal coming through, but it
is hard to tell. 

What should be the first thing to check?  Is anyone familiar with this
particular problem? 

Thanks. 

Owen