How do I diagnose this motherboard?

I also made sure to use a 9V AC power supply.

@epnskj277 yes, symptoms like this are usually related to the power rail. Can you take a more focused picture of the 3 blue capacitors and the area of where the power jack is? It could be related to a bad cap(acitor). Also, try a factory reset

1. Power off the SR-16 by disconnecting the power adapter.

2. Press and hold the "ERASE" and "PLAY" buttons simultaneously.

3. While holding these buttons, reconnect the power adapter to turn on the SR-16.

4. Continue holding the "ERASE" and "PLAY" buttons until the display shows "FACTORY RESET."

5. Release the buttons and press the "STORE" button to confirm the factory reset.

6. The SR-16 will now reset to its original factory settings.

Looks like the SR16 has a 9V and a 12V version. Double check which one you have. Also, make sure that your 9V power adapter outputs 1amp (1000mA) or more (if yours is a 9V version).

Even so there is no exact data available on which one is the 12V version, it appears that those before 2012 were 9V and after 2012 are 12V

@oldturkey03

I think that I already tried to factory reset it a few weeks ago already. I already double-checked that I used the right power supply.

The capacitors also do not look damaged to me. However, I noticed that there is a lot of corrosion but I do not know where it could come from.

I just checked the resistance of the capacitors and two of them charge until 2000 kΩ and then reset. The other one is somewhat charging but very slowly. Should I therefore replace them?

I still have not resolved this issue and it would be great if someone could suggest another solution.