1) First I marked the keys which are not responding by marker pen.
2) I opened up the keyboard from back side.
3) I observed the working of the mechanism by playing the keys in open condition, When you hit the key it presses the rubber key
pad. ( the other end of the rubber key pad has has projection on which carbon film is deposited for electrical contact) (Image-6)
4) When we press key it makes Rubberized carbon film contact with the PCB tracks and shorts electrically. (Image-4)
5) I found in most of the keys, carbon layer was vanished (Image-9)and in some cases PCB tracks are not clean hence not
making electrical contact.(Image-5)
6) Those PCB tracks are cleaned by brush and Isopropyl alcohol.
7) I got the old synthesizer, I removed its rubberized contact strip.(Image-7) made small round pieces (Dia 3 mm). (Image-8)
8) Then these pieces are fixed over damaged Rubber contact by instant Adhesive. (Image-9)
After reassembly found all the keys are working fine
Its a great pleasure but requires patience, It took 3 hours to fix 6 non working keys.