Hi Jomi, they say that in the poor man's house, the joy lasts little and it has been, because when I have placed it in its place, it has not worked again.
I have checked with the multimeter that the SPEAKER A and SPEAKER B switches make contact and in fact yesterday I let it work and today I have assembled everything and it still sounds only by headphones.
Jomi, I already fixed it. The amplifier was fine.
It turns out that the speaker with which to do the tests, had been cut and therefore no sound came out.
It occurred to me that the speaker could have broken down, I checked and that was it.
Sometimes one goes crazy over the accumulation of breakdowns.
Thanks for everything.