Yamaha cs15d note repair

Hi all
i recently picked up a yamaha cs15d, which I love and it was working great until recently. But recently I lost a couple of notes…play a G and hear an Ab (Ab key plays an Ab as it should), play a C# and hear a D (D plays a D). Something about the key assignment is fouled up. Anyone have any idea what might be causing this? I see parts listed on the syntaur site for yamaha CS, like the key contact board, could that be something to try replacing? I’m handy with a soldering iron, but pretty rusty with schematic reading. thanks! (this still occurs after letting it warm up for a while as well)