Hi @onlaf ,
Couldn’t find the service manual for a P-45 keyboard, but looking at the P-45B service manual the 12V DC input is applied to the control processor circuitry and then to a 3.3V regulator once the power has been turned by the operation of the Power switch in the keyboard. There is no separate internal power supply in the keyboard.
Hopefully the P45-B is similar to the P45 as far as the power supply is concerned, so since you have switched power adapters and it still doesn’t turn on you may have to start tracing the power input circuit to find out what is wrong.
Here’s an cropped compilation image showing the 12V power input circuit. There is a fuse in the circuit on the JK board which you may want to check first and then the power goes from the JK board to the DM board through a cable harness, so maybe one of the cable connectors at either end may have come a bit loose etc.
If the power is OK going to the DM board then you may have to check if the Power switch is OK
The service manual gives details on how to dis-assemble the keyboard as well as the board layout etc
(click on image to enlarge for better viewing)
Hopefully a start.
Be patient if you go to download the service manual as it takes a bit of time to change from processing to download after you have passed the recaptcha test.