Well, I found the fault and want to share it, so other people with other units could fix theirs (even if not the same model, as you will see!)
I have measured all power supply voltages, and all where fine and noise-less. So it seemed as if the CPU didn’t boot. When that happens, I usually try to RESET the CPU using the /RES or similar signal. But I also use to check the clock oscillators… And this time, just measuring with the DVM, the main CPU was OK about 1.8Vdc (that measure is just indicating there is some activity there) but then, while touching CL1, the 4MHz ceramic resonator of the keyboard scanning IC (IC2 YM6633) (hint, it generated false key presses when working!), the unit has come to life!. And, as soon as I removed the test probe, it freezed…
So I have taken out the DM board for closer inspection on my lab. The board seemed basically fine but the ceramic oscillator CL1 was somewhat dull looking. So I have resoldered it and cleaned carefully with contact cleaner to remove any dust below it… and now the CLP-155 powers up fine AND does not read false keys!!!
So, take note, as my guess is that the main CPU asks to the YM6633 for a reading and, if it is not working, the CPU stays waiting for that missing data. So this fix could be useful for other YAMAHA keyboards which may use a similar system.
I hope this helps someone in the future!. If so, please, let us know here so we can add info on other affected models.