did you look at it with thermal camera?...see if anything getting hot...there is a good therm cam you can buy for £150...ask me and i will tell you part number.
otherwise, strip it down and see what you can get working....eg disconnect components and give the FAN6754 a dc bias voltage from external supply then see.
....take off output diode....shove vout on it with an ext psu...and see if their is output short circuit...etc etc...
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maybe one of the aux caps is dry...replace all.