Hi can use the manual but you’ll have to work out the circuit path from the reistor supplying the power for the notes (keys) in question. The wiring to the decoder is 'shared' by the other notes so it is alright as they are working. From there it is multipled through all the E keys and 3rd black keys and then out to the decoder. The circuit that you should test is from the DMLCD board Resistor 838 resistors pins 5 & 7 to connector CN831 wires 11&12 which go to 61H Board (on the keyboard itself) connector CN1 wires N24 & N14. You need to check that the 3.3V DC is available at pins 5 & 7 of Resistor 838 on the DMLCD board and that the appropriate wires on the CN831 connector and CN1 connector are not loose and that the connectors are inserted properly, especially as the wires are at one end of the connectors so it might not be inserted 'straight'. The circuit path through to the keyboard is the most likely area of the problem as it passes through two connectors to get to the keyboard. This power feeds into the MicroProcessor (CPU) and also through the keyboard and back into the decoder.

They are both 'powered' by the same 3.3V DC supply rail fed from 10K Resistor(s) R838 pins 5 and 7. I think, not being a musician, that it also affects the '3rd' black key as well as they share the exact same wiring path up to the keys. Here is an image of the relevant part of the circuit which affects the E key.

Yamaha psr e403 not working