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.
When the wise man points to the stars, the fool looks at the finger.