I don't mean to be argumentative, but, that's not the entire schematic. I noticed that your posting shows "Gbox schematic B" so I'm assuming that there's a "Gbox schematic A" somewhere. For example, my board has a 7812 VR that feeds it's output to the 7805 VR. There's no 7812 shown in the schematic that you posted. Nor are any of the power rectifier diodes shown or the power transformer. Anyway, the bridge rectifier circuit produced 27 VDC output which got passed through the shorted 7812 which put 27 VDC across the 100 uf electrolytic capacitor (rated for 25 WVDC) that is on the output of the 7812. I "guess" that's why the capacitor overheated and exploded.