Vintage Synth Repair

Hi all - 

I recently acquired an RMI 368X Electra Piano. The lowest note on 
the keyboard does not sound at all. All other keys sound perfectly. 
I'm thinking this is not simply a contact issue. Anyone have any 
suggestions as to what this might be or how I might begin to 
troubleshoot this.

Also, does anyone have any idea where I might get a service manual 
for the 368 series, preferrably on-line?

Thanks very much.

JQ