I noticed that the same channel when loaded, feeding sine wave, I noticed that the THERM protection LED turns on quick, even the heat sink is not so hot, I even have the fans running on top of the heat sink for testing purposes.
I conducted a test on the working channel, forcing it to over heat, no fans used, THEN!!!! the working channel started to show the same effect!!! as of the faulty channel!!!
Image from the WORKING channel when loaded and forced to over heat