Searching on google I found this where the user [Bimpelrekkie] tips about voltage drop on vout of LM350K is due thermal protection, I admit I overstimated that and iniatially I reuse a intel CPU heatsink to mount the LM350K... without the cooling fan powered.... and without thermal grease.....

Well after powering the cooling fan and using thermal grease for CPU the problem of voltage drop on LM350K was solved (or at least diminished at deprecable levels). So this means that I can continue and finish the power stage board, I only need to solder connections of the second L298H needed for the third VCM.

For my ToDo list: Learn more about IC thermal managment.