Vintage Synth Repair

Hello, 

Maybe someone can help or point me in the right direction.
This does not exactly concern a synth but a late 60s early 
70s 
GEM Jumbo 61 R6
http://www.combo-organ.com/Gem/index.htm

Problem is with the tone of the C key (3rd octave).
Also this C is the first key just after the highest bass key
of the two lower octaves.

The 3rd C sounds weak in tone compared to all other keys. It is in
tune and all other keys from top to bottom are in tune as well. Also
all other keys have the correct tone. 

But for some reason the 3rd C is not getting the full tone like the
others. Its like it is attenuated or filtered. It is not as full or
fuzzy or fat sounding as the other keys. 

- I cleaned the CONTACT (J-WIRE & BUS BAR) - Its OK
- I checked the FREQUENCY DIVIDER chips (SAJ 110s & SAJ 210s) - OK 
  They all work. I swapped them around to see if I could transfer the    
  problem  but no changes. 

- It seems to me that this is something to do with a component or a 
connection in relation to the circuit related to the tones & filters
not the oscillators, coils or dividers. Because again everything is in
tune from top to bottom.  

- I don't have a schematic and tracing the lead from the C's j-wire to
the board is difficult because of wad of wires & many are the same
color. I will have to unwrap the wad of wires but I want to avoid it.
Also I tried to trace it with a continuity meter, but I get a reading
on many points of the board. So with out a diagram I will have to
unwrap the group of wires... 


Any advice to point me in the right direction would be appreciated.

Br
Allan