3 notes are not working suddenly

Three notes on my Korg monologue are suddenly not working. I've seen other similar repair threads on Korg keyboards where 3 or more keys stop working that are the same distance apart. I don't know the correct terms but I imagine there is a component that these notes - G,B ,D# are all attached to. my electronic skills are limited. I tried cleaning some connections gently with isopropyl so far to no success.

I include images with bad notes circled on red and another photo further up the scale where there appears to be corrosion (green arrow) but would it have an affect on those notes? I could test with a multimeter if I knew what connection I was testing for.