Thank you for the quick reply, Dieter.

Yes, I want to try the repair myself, for I have some experience
in electronics and soldering.

I'll follow your advice-steps, but if those do not solve the prob, I would rather buy another A-108. 

So I'll let you know.

Oh yes, the thing about turning off the system when you remove, install or swap modules is, that sometimes you just want to check little things online (= system on) in between!

Or sometimes you think, that installing just one module would take only a minute and you will be extra carefully.

But now I have learn my lesson, for I experienced a pretty big force, when the sparks appeared. There also occurred a very small flame for a very short time. I could see it clearly, for it was in the evening with low light in the room.

When I remove the damaged potentio meter after some weeks, I saw the resistance-material inside it was greatly scorched .