is the SH-2000 what MultiVox cloned as the MX-2000?
(lloks just like it on the VSE archive pages)
if so, the MV version has a Hall-effect sensor to trigger the aftertouch. looks like a flat transistor. the keyboard has a metal bar that moves during aftertouch and a magnet that moves toward and away from the sensor. IIRC i'vs seen them in Jameco electronics
found a nasty copy of the SH-2000 schematic on the net, can barely read it, but this synth DOES use the Hall sensor.
there is a 'range' and 'offset' pot also that MIGHT be part of the problem...but my money is on the sensor. Find a qualified tech.
better yet, get a real copy of the service manual from MuzikElektronix (above) and maybe even contact Steph about taking care of the problem for you.
Go ahead! Skin that smoke-wagon, and see what happens...