Distorted sound on Hall effect Korg M1

[shirreff]

Hi, I have a Korg M1 that works fine apart from the Hall effect which does not sound right at all.
I have reloaded the presets and it is still not right, felt and room etc is fine and if I turn off the effect the patches are clear. So does anyone know how to identify the effects chips on the main board? I have a few broken m1 mother boards so I could possibly swap some parts of I knew where to start.