traktor Kontrol S8, no output

Not my unit, im not a dj, however have an issue with a friends machine, who is a dj

While operating there was a few crackles from the monitors and then all of a sudden, 0 output

i think it may be some kind of hardware failure.

Controls are all operational.

isolating the above, with no software, not connected to pc etc, there is no output using a simple source 'in' to the rear rca's & 'out' to a set of headphones

all fuctions appear as per normal, input levels showing up green, no sound from headphones