You must measure all output voltage levels of the source, as well as revolving electrolytic capacitors (there are two or three of 1000uf that know how to fail) voltage level of the output diode that powers the stby circuit. Then if the memory does not fail me, there is a protection zener (z18v) that does not appear in the diagram or in the serigraphy of the plate, but this and if you see it from above where the components are, it is located near the exit of the source and half position in diagonal attached to the c847, is usually put in short after an electrical departure and povoca what it does to you)