I have a samsung ht-z310 system and started experiencing the protected-off problem after about 5 years.
Uncabeling all apart from the power cable made no difference.
I opened the the box and noticed the fan was working properly and no dust prevented the cooling functionality.
But on the mother board one IC circuit equipped with an aluminium cooling device was very hot compared with the others.
After mouting a block of cupper on top of it to help it getting rid of the generated heat the problem has disapperad.
Protected-off is now gone.
Odd A.
June 2014