Yes I'm not messing up inside the camera without knowing where I'm going. The fault is occurring both in manual mode (when I select myself the shutter speed) and in mode "auto" (but this time it started at 1/15s which was like 4-5s in real). I did talk about the light meter because it's also part of the electronic board and it seems like it's working correctly. The problem is I don't have an analog camera repair guy in my area and I believe it's going to be too expensive.
I didn't mention that I know how to solder properly (SMD),also I did dozens of electronic devices unit replacements but I'm not familiar with testing a board (I can always learn with good material and advices). So what should I do?