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?

I personally find the HPF almost useless; and to make matters worse it even
affects the sound slightly when the slider is in the bottom position (you
can see this with an oscilloscope).
Anyway, the important thing to keep in mind for the HPF is that it can only
work on material that contains suitable frequencies. Compare it to the LPF:
If the source material is reasonably "buzzy" you will always get some
filtering result because the harmonics continue upwards anyway.
With the HPF it's different: If you play a somewhat middle to high note the
HPF will have nothing to pass for quite some time.
So please test this once again with very low notes (transposition two
octaves down) and check if the response now starts earlier.