Vintage Synth Repair

While we're on the subject of Odysseys, I have one on the bench with a 
high-pass filter that has no response for 4/5ths of the slider travel -- 
then at the top position it finally cuts off.
This is one of the most simple circuits in the world-- I've replaced the 
slider, cap & resistor with no effect.
Can anyone with a Mark II tell me if this is normal, or is there 
something going on with this particular unit?

Thanks.
~GMM