Switch Mode power supplies usually are left on permanently and as a result the start up circuit is never used. When the power in the house goes off, the start up circuit is not working because the start up capacitor has "dried up" and lost its value. Unfortunately some times it half works and takes the output stage out.
Check for a semiconductor fuse in line with the transformer. It may have failed.
Firstly ensure the main large capacitors have totally discharged otherwise when removing the components, there is a chance that they will damage other components.
The components to look out for are, on the mains side, small electrolytic capacitors that when you measure them with a capacitance bridge, they are low value. The components must be removed to test them.
On the power side of the board, there are a few large capacity capacitors, they usually show physical signs of aging and can be replaced.
Alswo to look for are any very high value resistors. Sony love to use 22Mohm resistors that go open circuit as do other manufacturers.
I hope that gives you an insight into switched mode power supply repair.
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