Two weeks ago I was working on an 800V power supply when it suddenly started burning input fuses. It would directly burn the fuse just as i connect it to power.
Of course the usual diagnostics to checking shorts, DMM checks and then general components checks, no good, then I went and checked all the components (not so many) still all were looking ok. This was when I was really getting frustrated, it was an old linear regulator, so what I did was connect an external DC power to the regulation circuit with current limit to see if I can check the problem live, the bloody thing was working ok, of course I was working on 20V and it originally has an 850V input, then one of my colleagues suggested that it might be insulation problems, I tried to check it without power I was not able to, then I took the gamble of having the board directly in my view then plugging it in, I saw a small spark, it was in a middle point between a transistor and a resistor, so I instantly thought it might be sparking between the leads, nope, after actually having to remove both the transistor and the resistor I found out that the insulation layer above the resistive film in the resistor has decayed in an area as big as a hair pin, and this has happened only on the bottom side of the resistor, with the ground plain underneath all the components 850V on the resistor was enough to create a spark there.
Simple problem of changing the resistor
took one whole day