I was offered a JX3P in "as new" conditions, really I think that the owner never used it

Well I tested it and it looks like everything is working, except for the led of some buttons (from 8 to 16) which don't light up, when I push the buttons they work, the patches are changing, but the little red led lights don't light up, even though when we left the synth on for a few hours, 5-6, they suddenly began to light
I'm wondering if it could be an easy issue to fix or if it could be a more problematic scenario, my doubt is that there is an "hidden" problem and the led that are not lighting are only the top of an "iceberg"

Any suggestions will be greatly appreciated, thank you!