Logic analyzer project 2. - first "repair"

After thinking a lot (2 sec) and reading thoroughly the service manual (5 min) with a long hard work (30 sec) I repaired the bigger fault of the Logic Analyzer. The soft key button error:

And the high tech advanced tool used for the repair:

You see correctly it is a cosmetic cotton pad (a dirty one).
Explanation:
The unit has touch screen. This called "soft keys". Because of the age of the unit it is not a capacitive or resistive touch screen but an optical grid created with infrared LEDs and phototransistors.

1. When I first seen the error I think that one of the LEDs may failed (2 sec)
2. Then I read the manual what the error code is about (5 min)
3. When it proved that my theory is close to the possible cause, first cleared the frame of the display (30 sec)