here I am a photographer and my flash cobra 580 exII of which I attach the photo has almost no more power

I would like to fix it because on the one hand I do not like the mess but also by what a baby like that costs relatively expensive but voila! yet a good handyman in many areas, electronics is not part of it so mounting / dismounting will be on my ropes but finding the breakdown is something else.
can you help me knowing that i have a tester and i am ready to open the beast ..
thank you all.