Is there any flashing lights or indication of power before it turns off (usually number and rate of blinking power light describes the problem)? Generally speaking these have a lot of protection circuits and is likely a true fault (something is broken). 15 secs is quite a while for it to be a direct short, like an output transistor, so it might be something like an earlier stage failing and slowly causing an imbalance and the amp powers down to save itself.
First step is to "Carefully" take the cover off and see if you can measure some voltages (using multi-meter) before it turns off. Mainly want to make sure the transformer is working, otherwise it is a boat anchor. If the power supply is working, then you can try and unplug the amplifier board. If it stays powered on at that point, then you know the issue in with the amplifier board. If not, then you are looking for either a low-voltage (<12V) problem, op-amp or some other issue on one of the input/output boards.
I'm not good at telling what to do with words (more hammer than scalpel guy), but start there and then at least you will have more information before deciding if it's junk.
First step is to "Carefully" take the cover off and see if you can measure some voltages (using multi-meter) before it turns off. Mainly want to make sure the transformer is working, otherwise it is a boat anchor. If the power supply is working, then you can try and unplug the amplifier board. If it stays powered on at that point, then you know the issue in with the amplifier board. If not, then you are looking for either a low-voltage (<12V) problem, op-amp or some other issue on one of the input/output boards.
I'm not good at telling what to do with words (more hammer than scalpel guy), but start there and then at least you will have more information before deciding if it's junk.