Vintage Synth Repair

I wonder if anyone could help with this one:

On my OBXa, one voice card sound thinner than the others. When 
playing high notes this is less noticable but as going down the 
keyboard it goes and sound thinner, sort of like a high pass filter 
(no low frequencies). I tried to replace the CEMs but it's not in 
there...

Does anyone have any idea what could that be ? I thought it might be 
a calibration problem, would any of that trimpots effect the sound in 
that way has it not been set correctly ?

VCO1 Volts Per Octave
VCO1 High Track
VCO1 Initial Frequency
VCO1 Pulse Width
VCO2 Volts Per Octave
VCO2 High Track
VCO2 Initial Frequency
VCO2 Pulse Width
Filter Envelope Modulation
Filter Resonance
Filter Volts Per Octave (4-Pole)
Filter Volts Per Octave (2-Pole)
Filter Initial Fequency (4-Pole)
Filter Initial Frequency (Both Filters)
VCA Offset

I must say that the tinner sound is general on this card, i.e with 
any preset or setting used, no certain VCO...

Thanks!