Synthesizer Key Repair

I have Yamaha PSR E 403 Synthesizer ( Image-1) using for the last 4 years. I found some keys are not working. So decided to repair at home.

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.