Monoblock K8081

Hi Forum,
I assamblied two monoblocks from Velleman K8081. After that I controled the whole thing, everything was on his place. The tests are wright. They are both not working.
The first one has the following problem, When I put it on he gives sound after 5 minutes he goes off. After some waiting I put him on and there is sound but short after that he goes off. The yellow led stays off.
The secund one; The leds burnig as in the discription but there is no sound, only a sort of humming or some rustle.
Who can tell me how I can find the solution of this problem.
It is mine first audio job at a age of 72.
Mister Velleman can you give me a hint, please

Just a guess
The one that shuts down after 5 minutes.
Are you sure you put heat sink compound on the back of the transistors?
Are they tight against the heat sink?
It sounds like the unit is hot and shutting down.

I had place some compound, but I will put some more behind the transistors. I put compound behind the isolator and behind the transistor also around and above the transistors, so `I will check it. thanks for your idea.

I’have done it, enough compund behind en around the transistors. The problem stays.
Is there an other option?