it´s been a while, since i posted, but anyway, igot some schematics from different places, and figured my way out of this; the problem resulted in several places: had to change the speaker proteccion IC from the bad channel, and solder reinforce some tracks under the PS caps as they were bad and rotten, maybe from cap fluid from the originals that were changed by someone else. There was some corrosion on the connector from the transformer to the PCB too.
anyway just that and some minor electr caps and transistors around the spkr protection circuit. It is playing loud and nice since then at my friends rehearsing studio and no problem even at clipping it with 4ohm speakers!
uruapan michoacan mex