[sdiy] Odd keyboard on Ensoniq

[rkmoore at memphis.edu]

Hey y'all,

I've got an old ensoniq EPS that was one of my first 'real' pieces of 
equipment and it has served me well.  For some reason whenever the 
polyphonic aftertouch is assigned to modulate a parameter it seems to 
return the maximum value constantly.  

I pulled it apart and found that the keyboard contact system is rather 
strange.  Instead of leaf switches or carbon contacts connected to a 
diode matrix,there are metal plates on the underside of each key and 
the circuit board has a spiral of traces underneath the plate for each 
key.  When the key is played it gets closer to, but never touches, the 
spiral on the contact board.  My only guess is that the spiral is like 
an inductor and moving the plate closer to or farther away from it 
induces a small current.

What I really want to know is how the aftertouch is supposed to work 
and if there is a way to get my EPS fully functional again.

-Richard Moore