Thanks Paul, for the explanation on how to use a scope. I agree regarding the good ground, that's why I was using the two copper lids on the motherboard, that I surely wrongly called "main" ground in the previous message. I need from you, if possible, confirmation on where to measure the DAC, is it R47 pins a good place? If that's OK, I read a pulse, but pretty weak and not well defined.
I attach a couple of pictures.
Many thanks!