All keyboard dead on PSR-410

I have been gifted this keyboard from the original owner bought new - 30 years ago. Everything works except the keyboard itself. I have tried a general reset. I have the Service manual and have tried the tests as follows: Mode 1 - test 1 - Er1 displayed and unable to continue tests. Mode 2: test 1 -Er1.

This acts like the internal keyboard is not connecting to the MKS board or the Main CPU. Everything works if I use an external MIDI controller keyboard.

Oddly it did seem to respond to the two and three right white keys test when powering on, which is haw I saw the error codes. The lady at Yamaha service suggested that the sub cpu on MKS board was bad. Seems like one is available here. But would like 2nd opinion and a real schematic.

CPU (DM board) talks to the scanner (MKS board) as a serial data stream. MKS scanner gets info from MK board.

If the CPU cannot connect to the scanner (MKS board) then you either got a cable problem (MKS ↔ DM) or the scanner is dead (MKS board)