Well, I guess I could use more of your advice..
I twisted the cables as Shock mentioned. The thing is now that when I put the head back to its cabinet the noise begins again ( the cabinet has a metallic paper shield inside).
I took the head out again and I placed a metallic plate over it, near but without touching anything. And the noise came again!
Well, I guess that, as oldway said, the inputs of the transistor are not sufficiently shielded? So the transformer has gone bad and needs replacement?
And also, is it possible that the transformer, because of the burnt transistors, was got very hot and the shield gone bad? Because this wasn't a problem before.
Thanks again!